Biology Chapter 53: Community Ecology
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Biology Chapter 53: Community Ecology Species Interaction −
A biological community consists of interacting species, usually living within a defined area
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To study species interaction, biologists focus on analyzing the effects on the fitness of individuals involved
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Four categories of interaction
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-/- relationship known as competition
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+/- relationship known as consumption and parasitism
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+/+ relationship termed mutualism
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+/0 relationship termed commensalism
Ex. Birds that follow moving army ants in the tropics
As ants march along forest, they hunt insects and small vertebrates
As they do, birds follow and pick off prey species that fly or jump out of the way of the ants
The birds are commensals that benefit from the association (+), but have no measurable impact on the ants (0)
Section focuses on three key themes o
Species interaction may affect the distribution and abundance of a particular species
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Changes in species interactions often explain short-term changes in population size and distribution
Species act as agents of natural selection when they interact
Deer are fast and agile in response to natural selection exerted by major predators •
Speed and agility of deer, in turn, promote natural selection that favours wolves and cougars that are fast and have superior eyesight and senses of smell
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Changes in species interactions lead to long-term changes in characteristics of populations, a phenomenon called coevolution, in addition to having short-term impacts on population size
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Outcome of interactions among species is dynamic and conditional
Consider relationship between army ants and bids that follow them, which is usually commensal
If bird attacks start to force other insects into path of ants, then both benefit and relationship becomes mutualistic
If birds begin to steal prey that would otherwise be taken by ants, relationship becomes parasitic
Competition o
Competition is a -/- interaction that occurs when different individuals use the same resources and when those resources are limiting
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Competition that occurs between members of same species is called intraspecific competition
Intraspecific competition for space, sunlight, food and other resources intensifies as population’s density increases
A major cause of density-dependent growth
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Interspecific competition arises when individuals from different species use the same limiting resources
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There are many types if interspecific competition
Consumptive competition occurs when individuals consume the same resources (ex. trees competing for same water and nutrients)
Pre-emptive consumption exists when one species makes space unavailable to other species
Overgrowth competition happens when one species grows above another (ex. large fern overgrown other individuals and is shading them)
Chemical competition takes place when one species produces toxins that negatively affect another species
Territorial competition arises when a mobile species protects its feeding or breeding territory against other species (ex. grizzly bears drive off black bears)
Encounter competition occurs when two species interfere directly for access to specific resources (ex. spotted hyenas and vultures fight over kill)
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Using the Niche Concept to Analyze Competition
A niche can be thought of as the range of resources that the species is able to use or the range of conditions it can tolerate
Interspecific competition occurs when niches of two species overlap
What Happens When One Species Is a Better Competitor
G.F. Gause claimed it is not possible for species with the same niche to coexist
This hypothesis is called the competitive exclusion principle and was inspired by a series of experiments Gause did with Paramecium
When Gause placed small populations of P. caudatum and P. Aurelia in separate laboratory cultures, both specie exhibited logistic growth
But Gause showed that when two species are put in the same culture together, only the P. Aurelia population exhibits a logistic growth pattern, P. caudatum is driven to extinction
His results are a product of asymmetric competition
When asymmetric competition occurs, one species suffers a much greater fitness decline than the other species does
Under symmetric competition, each of the interacting species experiences a roughly equal decrease in fitness
If asymmetric competition occurs and the two species have completely overlapping niches, then the stronger competitor is likely to drive the weaker competitor to extinction
But if the niches do not overlap completely, then the weaker competitor should be able to retreat to an area of non-overlap
In cases like this, an important distinction arises between a species’ fundamental niche, which is the combination of resources or areas used or conditions tolerated in the absence of competitors and its realized niche, which is the portion of resources or areas used or conditions tolerated when competition occurs
Experimental Studies of Competition
Joseph Connell observed that there were two species of barnacles with distinctive distributions in an intertidal rocky shore
Barnacle larvae are mobile, but adults live attacked to rocks
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The adults of one species, Chthamalus stellatus, occurred in an upper intertidal zone, while the adult of the other species, Semibalanus balanoides, were restricted to a lower intertidal zone
The upper zone is a more severe environment for barnacles because it is exposed to air for longer periods at low tide each day
The young of both species were found together in the lower intertidal zone
To explain observations, Connell hypothesized that adult Chthamalus were competitively excluded from the lower intertidal zone
The alternative hypothesis is that adult Chthamalus are absent from the lower intertidal zone because they do not thrive in the physical conditions there
Connell tested these hypotheses by removing a number of rocks that had been colonized by Chthamalus from the upper intertidal zone and transplanting them into the lower intertidal zone
He screwed the rocks onto place and allowed Semibalanus larvae to colonize them
Once the colonization period was over, Connell divided each rock into two groups
In one half he removed all Semibalanus that were in contact with or next to a Chthamalus
Allowed Connell to document Chthamalus survival in absence of competition with Semibalanus and compare it with survival during competition •
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Common experimental strategy (one competitor is removed and response by remaining species is observed)
Connell’s results support hypothesis of competitive exclusion
In unmodified areas, Semibalanus killed many of the young Chthamalus by growing against them and lifting them off the substrate
Chthamalus survival was much higher when all Semibalanus were removed
Mechanisms of Coexistence: Fitness Trade-Offs and Niche Differentiation
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Why haven’t Semibalanus and other superior competitors taken over the world
The key here is that the ability to compete for a particular resource is only one aspect of an organism’s niche
If individuals are extremely good at competing for a particular resource, then they are probably less good and enduring drought conditions, warding off diseases or preventing predation
In the case of Semibalanus and Chthamalus, fitness trade-off is rapid growth and success in competing for space versus ability to endure harsh physical conditions of upper intertidal
Semibalanus are fast-growing and large
Chthamalus grow slowly, but can survive long exposures to air and to intense sun and heat
Neither species can do both things well so fitness trade-offs limit the ability of superior competitors to spread
Because competition is a -/- interaction, strong natural selection on both species to avoid it
An evolutionary change in traits reduces the amount of niche overlap and thus the amount of competition
This change in resource use is called niche differentiation or resource partitioning
The change in species’ traits is called character displacement
The fundamental idea is that competition exerts natural selection and that the characteristics of species change in a way that reduces competition
Consumption o
Occurs when one organism eats another
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Three major types of consumption
Herbivory takes place when herbivores consume plant tissue
Parasitism takes place when parasites consume relatively small amounts of tissue or nutrients from another individual, called the host •
Often occurs over long period of time
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Predation occurs when a predator kills and consumes all or most of another individual •
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Not all parasitism involves consumption, social parasites in birds and insects lay their eggs in other species’ nests and induce them to raise the young
Consumed individual is called the prey
How Do Prey Defend Themselves
Standing or constitutive defences are always present •
Prey may hide, run, swim away when they sense predator
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Many species find safety in numbers
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Other prey species spray toxins or employ weaponry
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Mimicry o
Occurs when one species closely resembles another species
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When harmful prey species resemble each other, Mullerian mimicry is said to occur
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To explain existence of Mullerian mimics, biologists propose that the existence of similar-looking dangerous prey in the same habitat increases the likelihood that predators will learn to avoid them
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In this way, Mullerian mimicry should reduce likelihood of dangerous individuals being attacked
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Batesian mimicry occurs when predators avoid the harmless mimics because they mistake them for a dangerous prey
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Key point here is that prey have adaptations that reduce their likelihood of becoming victims
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These adaptations are responses to natural selection exerted by predators
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Constitutive defences are expensive in terms of energy and resources that must be devoted to producing and maintaining them however
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Therefore, many prey species have inducible defences, traits that are produced only in response to presence of a predator
Inducible defences are efficient energetically, but are slow
Ex. Blue mussels •
Predation on mussels by crabs was high in an area of the estuary with relatively slow tidal currents, but low in an area of the estuary with relatively rapid tidal currents
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Researchers hypothesized that if blue mussels possess inducible defences, then heavily defended prey individuals should occur in low-flow area where predation is higher, but not in high-flow area, where water movement reduces the number of crabs present
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To test this hypothesis, biologists had two tanks, both containing seawater, but one with a crab that ate on fish, but not mussels and one with no crab
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The tank with the crab allowed researchers to measure shell growth in mussels that were “downstream” of the crab
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As predicted, the mussels exposed to a crab in this way developed significantly tougher shells than did mussels not exposed to crab
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These results suggest that even without direct contact, mussels can sense the presence of crabs and increase their investment in defences
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In a similar experiment, investigators compared mussels that were exposed to water running through broken mussel shells versus intact, but empty mussel shells
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They recorded a significant increase in shell thickness in the tank downstream from the broken shells
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This result supports the hypothesis that mussels can detect presence of predators from molecules released by broken shells
Are Animal Predators Efficient Enough to Reduce Prey Populations
Research supports hypothesis that species interactions have a strong impact on evolution of predator and prey populations
Prey are typically smaller than predators, have larger litter sizes and tend to reproduce at a younger age
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As a result, they have a much larger intrinsic growth rate (r max)
If prey reproduce rapidly and are also well defended, it is not clear whether predators should be able to kill enough of them to reduce prey population significantly
Some results suggest that they can (wolf vs. moose)
Other experiments support the hypothesis that predators play a role in density-dependent growth of prey populations
Data available indicate that in many instances, predators are efficient enough to reduce prey population
Why Don’t Herbivores Eat Everything
Top-down control hypothesis •
States that herbivore populations are limited by predation and disease
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Predators and parasites remove herbivores that eat plants
Poor-nutrition hypothesis •
Contends that plants are a poor food source in terms of nutrients they provide
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Plant tissue have less than 10% of nitrogen found in animal tissues
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If growth and reproduction of herbivores are limited by availability of nitrogen, then their populations will be low and impact of herbivory relatively slight
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Herbivores could eat more plant material to gain nitrogen, but at a cost – they would be exposed to predation and expend more energy processing food
Plant-defence hypothesis •
Holds that plants defend themselves effectively enough to limit herbivory
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Most plant tissues are defended by weapons such as thorns, prickles or hairs, or by potent poisons
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No animal species, without help from protists or bacteria, digest cellulose or lignin, which are components of wood
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All three factors are important in limiting the impact of herbivory, although the particular mix of factors will vary from plant species to plant species and from habitat to habitat
Adaptations and Arms Race
When predators and prey or herbivores and plants interact over time, coevolutionary arms races result •
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Natural selection should favour plants that evolve an everchanging suite of compounds to deter the ever-changing array of herbivores they face
Consumers evolve traits that increase their efficiency, in response, prey evolve traits that make them unpalatable or elusive, which leads to selection on consumers for traits that counter the prey adaptation and so on
Can Parasites Manipulate their Hosts
To thrive, parasites have to be transmitted to new hosts
To a parasite, an uninfected host represents uncolonized habitat, teeming with resources
Consider species of land snails that are parasitized by flatworms, specifically by flukes in the genus Leucochloridium
When the flukes have matured and are ready to be transmitted to their next hose, a bird, they burrow into the snail’s tentacles and wriggle
In addition, infected snails become attracted to light, even though uninfected snails avoid sunlit areas and prefer dark, shady environments
When infected snails move out of the shade into the open and glide about with wriggling tentacles, they are more easily spotted and consumed by bids
Biologists suggest that the worms manipulate the behaviour of the snail and that the change in snail behaviour makes the parasite more likely to be transmitted to a new host
Mutualism o
Mutualisms are +/+ interactions that involve a wide variety of organisms and rewards
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Ex. Many species of bees visit flowers to harvest nectar and pollen
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Bees benefit because nectar is used as a food source for adult bees and pollen is fed to larvae
Flowering plants also benefit because in the process of visiting flowers, foraging bees carry pollen from one plant to another and accomplish pollination
Ex. Ants in the genus Crematogaster, which live in acacia trees
Ants in live in bulbs at the base of acacia thorns and feed on small structures that grow from tree branches
These ants protect the tree by attacking and biting herbivores and by cutting vegetation from the ground below the host tree
Plants have high protein bodies that ants can feed on
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Important to note that even though mutualism benefits both species, the interaction does not involve individuals from different species being altruistic to each other
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Judith Bronstein described mutualism as “a kind of reciprocal parasitism; each partner is out to do the best it can by obtaining what it needs from its mutualist at the lowest possible cost to itself”
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Not surprising that some species cheat on mutualistic systems
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Ex. Deceit pollination occurs when certain species of plants produce a showy flower, but no nectar reward
Pollinators have to be deceived to make a visit and carry out pollination
Evolutionary studies show that deceit pollinators evolved from ancestral species that did provide a reward
Over time, a +/+ interaction evolved into a +/- interaction
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Because the costs and benefits of species interaction are fluid, an interaction between the same two species may range from parasitism to mutualism to competition
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Table 53.1
Community Structure −
How Predictable are Communities o
Frederick Clements promoted the view that biological communities are stable, integrated and orderly entities with a highly predictable composition
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Hypothesis was that species interaction are so extensive and coevolution is so important that the groups of species called communities have become highly integrated and interdependent units of nature
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Argued that communities develop by passing through a series of predictable stages dictated by extensive interactions among species and that this development culminates in a stable final stage known as a climax community
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According to Clements, nature of the climax community is determined by the area’s climate and does not change over time
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Henry Gleason, in contrast, contended that the community found in a particular area is neither stable nor predictable
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He claimed that plant and animal communities are ephemeral associations of species that just happen to share similar climatic requirements
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Mapping Current and Past Species’ Distributions
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If communities are predictable assemblages, then the ranges of species that make up a particular community should be congruent
The same group of species should almost always be found growing together
However, biologists found independently of each other
Also, an important pattern emerged in data of fossils
that
species
came
and
went
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Species do not come and go in the fossil record in tightly integrated units
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Instead, the ranges of individual species tended to change independently of one another
Experimental Tests
Biologists constructed 12 ponds and put in enough chlorine to kill an pre-existing organisms
If community structure is predictable, then each pond should develop the same community of species once the chlorine vaporized and made the water habitable
If community structure is unpredictable, then each pond should develop a different community
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Results showed that a total of 61 species inhabited all the ponds, but individual ponds only had 31 to 39 species
A number of species occurred in most or all of the 12 ponds, but each pond had a unique species assemblage
Researchers contended that some species are particularly good at dispersing and are likely to colonize all or most of the available habitats
Other species disperse more slowly and tend to reach only one or a few of the assembly and composition are somewhat contingent and difficult to predict
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Overall message is that Clement’s position is too extreme and Gleason’s view is closer to being correct
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Although both biotic interactions and climate are important in determining which species exist at a certain site, chance and history also play a large role
How Do Keystone Species Structure Communities o
Experiments have shown that, in some cases, the structure of an entire community can change drastically if a single species of predator or herbivore is removed from a community or added to it
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Ex. Seat star Pisaster ochraceous is an important predator
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When research removed Pisaster from experimental areas, what had been diverse communities of algae and invertebrates became overgrown with solid stands of the California mussel Mytilus californianus
Although M. californianus is a dominant competitor, its populations had been held in check by sea-star predation
When this predator was gone, the species richness and structural complexity of the habitat changed radically
Robert Paine coined the term “keystone species”
A keystone species has a much greater impact on the surrounding species than its abundance and total biomass would suggest
Community Dynamics −
Disturbance and Change in Ecological Communities
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Community composition and structure may change radically in response to changes in abiotic and biotic conditions
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A disturbance is any event that removes some individuals or biomass from a community
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Forest fires, windstorms, floods, the fall of a large canopy tree, disease epidemics all qualify as disturbances
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These events alter light levels, nutrients, unoccupied space or some other aspect of resource availability
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Biologists have come to realize that the impact of disturbances is a function of three factors
The type of disturbance
Its frequency
Its severity
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Most communities experience a characteristic type of disturbance and in most cases, disturbances occur within a predictable frequency and severity
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Biologists refer to a community’s disturbance regime
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How do Researchers Determine a Community’s Disturbance Regime
Two strategies
First is based on inferring long-term patterns from data obtained in a short-term analysis •
Ex. An observational study might document that 1% of all boreal forest on Earth burns in a given year
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Assuming that fires occur randomly, researchers project that any particular piece of boreal forest has a 1/100 chance of burning each year
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According to this reasons, fires will recur in that particular area every 100 years on average
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Straightforward to implement, but has important drawbacks o
In boreal forests, fires do not occur randomly in either space or time
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More likely to occur in some areas than in others and tend to occur in particularly dry areas
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Unless sampling is extensive, difficult to avoid errors caused by extrapolating from particularly disturbanceprone or disturbance-free areas or years
Second approach is based on reconstructing history of a particular site •
Ex. Forest fires often leave a layer of burned organic matter and charcoal on surface of ground
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Researchers can dig a soil pit, find charcoal layers and use radioisotope dating to establish when the fires occurred
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Also possible to date the death of trees killed by fire by comparing their growth rings with those of living trees
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Further, trees that are not killed by fire are often scarred
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These fire scars occur most often at the tree’s base, where dead leaves and twigs accumulate and furnish fuel
Why is it Important to Understand Disturbance Regimes
Can tell researchers if disturbances are severe or not
Some “disturbances” may be beneficial to organisms
Succession: The Development of Communities after Disturbance o
Severe disturbances remove all or most of the organism from an area
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The recovery that follows is called succession
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Primary succession occurs when a disturbance removes the soil and its organisms as well as organisms that live above the surface
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Glaciers, floods, volcanic eruptions and landslides often initiate primary succession
Secondary succession occurs when a disturbance removes some or all of the organisms from an area, but leaves the soil intact
Fire and logging are examples of disturbances that initiate secondary succession
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Early successional communities are dominated by species that are short lived and small and that disperse their seeds over long distances
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Late successional communities are dominated by species that tend to be long lived, large and good competitors for resources such as light and nutrients
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Theoretical Considerations
Biologists focus on three factors to predict outcome of succession in a community •
Particular traits of species involved
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How species interact
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Historical and environmental circumstances (size of area involved and weather conditions)
Species traits, such as dispersal capability and ability to withstand extreme dryness, are particularly important early in succession
Recently disturbed sites tend to be colonized by plants and animals with good dispersal ability
When these organisms arrive, however, the often have to endure harsh environmental conditions
These pioneering species tend to be adapted for growth in disturbed soils
Early sucessional species devote most of their energy to reproduction and little to competitive ability •
They have small seeds, rapid growth and a short life span and begin reproducing at an early age
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As a result, they have a high reproductive rate
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They can also tolerate severe abiotic conditions, such as high light levels, poor nutrient availability and drying
Once colonization is under way, the course of succession tends to depend less on how species cope with aspects of abiotic environment and more on how they interact with other species
This change occurs because plants that grow early in succession change abiotic conditions in a way that makes the conditions less severe
Because plants provide shade, they reduce temperatures and increase humidity
Their dead bodies also add organic material and nutrients to the soil
As abiotic conditions improve, biotic interactions become more important
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During succession, existing species can have one of three effects on subsequent species (facilitation, tolerance or inhibition)
Facilitation takes place when presence of an early arriving species makes conditions more favourable for the arrival of a certain later species by providing shade or nutrients
Tolerance means that existing species do not affect the probability that subsequent species will become established
Inhibition occurs when the presence of one species inhibits the establishment of another •
In addition to species traits and species interaction, the pattern and rate of succession depend on the historical and environmental context in which they occur •
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Ex. A plant species that requires high light levels to germinate may be inhibited late in succession by the presence of mature trees that prevent sunlight from reaching the forest floor
Ex. Researchers found that communities that developed after forest fires disturbed Yellowstone National Park depended on the size of the burned patch and how hot the fire had been at that location
Succession is also affected by the particular weather or climate conditions that occur during the process
A Case History: Glacier Bay, Alaska
A rapid and extensive glacial recession is occurring at Glacier Bay
Several plant communities found in the area
The oldest sites are dense forests of Sitka spruce and western hemlock
Areas that have been deglaciated for 100 years are inhabited by scattered spruce trees and dense thickets of a Sitka alder
Sites that have been deglaciated for 45-80 years are also covered with dense alder thickets, but emergent trees are primarily cottonwood
Locations that have been ice free for 20 years or less do not have a continuous plant cover
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Instead, they host scattered individuals of willow and a small shrub called Dryas
Observations inspired a hypothesis for the pattern of succession in Glacier Bay •
With time, the youngest communities of Dryas and willow succeed to alder thickets, which subsequently become dense spruce-hemlock forests
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There is a single sucessional pathway throughout the bay
A recent study has challenged this hypothesis
Researchers found that three distinct successional pathways have occurred •
In the lower part of the bay, soon after the ice retreated, Sitka spruce began growing and quickly formed dense forests o
Western hemlock arrived after spruce and is now common in the understory
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At middle-aged sites in the upper part of the bay, alder thickets were dominant for several decades and spruce is just beginning to become common
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The youngest sites in uppermost part of bay may be following a third pathway o
Alder thickets become dominant fairly early, but spruce trees are scarce
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Instead, cottonwood trees are abundant
Species traits may be especially important in explaining certain details about the sucessional pathways
Ex. Western hemlock is abundant at older sites, but largely absent from young ones •
Logical because its seeds germinate and grow only in soils containing a substantial amount of organic matter and because the young trees can tolerate deep shade, but not bright sunlight
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It’s intolerance of early successional conditions explains why none of the three pathways began with colonization by this species
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Species interaction has been important in all three pathways
Ex. Research has shown that alder facilitates the growth if Sitka spruce •
Facilitating effect occurs because symbiotic bacteria that live inside nodules on the roots of alder convert atmospheric nitrogen to nitrogen-containing molecules that alder use to build proteins and nucleic acids
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When alder leaves fall and decay or roots die, the nitrogen becomes available to spruce
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Although spruce trees are capable of invading and growing without presence of alder, they grow faster when alder stands have added nitrogen to soil
Competition is another important species interaction
Ex. Shading by alder reduces growth of spruce until spruce trees are tall enough to protrude above alder thicket •
Once spruce trees breach alder canopy, alder dies out because it is unable to compete with spruce trees for light
Historical and environmental context also influences succession •
Ice was very thick in upper part of Glacier Bay
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Therefore, glacier eliminated all of existing forests
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In lower part of bay, ice was substantially thinner
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As a result, some forest remained on mountain slopes beyond the ice
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As glacier retreated from these areas, forests on the slopes provided a source of spruce and hemlock seeds and set a different successional pathway in motion
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In this way, environmental context (distance to existing forests) helped determine how the community developed
Species traits and species interaction tend to make succession predictable, while history and chance events contribute a degree of unpredictability to succession
Species Richness in Ecological Communities −
Species richness is a simple count of how many species are present in a given community
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Species diversity is a weighted measure that incorporates a species’ relative abundance as well as its presence or absence
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Predicting Species Richness: The Theory of Island Biogeography o
Strong pattern that larger patches of habitat contain more species than do smaller patches of habitat
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Observation is logical because large patches should contain more niches and thus support higher number of species
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Another observation was that islands in the ocean have smaller number of species than do areas of same size on continents
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One hypothesis was that speciation occurs so slowly that number of species present on an island is a product of just immigration and extinction
Rates of both processes should vary with number of species present on an island
Immigration rates should decline as the number of species on an island increases because individuals that arrive are more likely to represent a species already present
Extinction rates should increase as species richness increases because niche overlap and competition for resource will be more intense
Result is an equilibrium, a balance between arrival of new species and extinction of existing ones
Immigration and extinction should also vary as a function of island size and how close island is to continent or other source of immigrants
Immigration rates should be higher on large islands that are close to mainlands because immigrants are more likely to find large islands that are close to shore than small ones that are far from shore
Extinction rates should be highest on small islands that are far from shore because fewer resources are available to support large populations and because fewer individuals arrive to keep population going
As a result, species richness should be higher on larger islands than smaller islands and on nearshore islands vs. remote islands
This model, called the theory of island biogeography is important because
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It is relevant to a wide variety of island-like habitats such as alpine meadows, lakes and ponds and caves
Made specific predictions that could be tested
Relevant to metapopulation dynamics and can help inform decisions about the design of natural preserves •
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Measuring Species Diversity o
To measure species diversity, biologist could count number of species present in a community
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Problem is that simple counts provide an incomplete picture of diversity
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Relative abundance of species is also important to consider
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Shannon Index
S
H =−∑ p i ln pi '
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In general, most-species rich reserves should be ones that are relatively large and located close to other relatively large habitat areas
i=1
Pi is the proportion of individuals in the community that belong to species “I”
Index is summed over all of the species in the study
Global Patterns in Species Richness o
Data compiled in intervening years have confirmed existence of a strong latitudinal gradient in species diversity, for communities as a whole as well as for many taxonomic groups
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To explain why species diversity might decline with increasing latitude, biologists have to consider two fundamental principles
Causal mechanism must be abiotic because latitude is a physical phenomenon produced by Earth’s shape •
Explanation must be a physical factor that varies predictably with latitude and that could produce changes in species diversity
Species diversity of a particular area is the sum of four processes: speciation, extinction, immigration and emigration
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One hypothesis is that high productivity in the tropics promotes high diversity by increasing speciation rates and decreasing extinction rates
Idea is that increased biomass production supports more herbivores and thus more predators and parasites and scavengers
Speciation rates should increase when niche differentiation occurs within populations of herbivores, predators, parasites and scavengers
Although this high-productivity hypothesis is supported by globalscale correlation between productivity and diversity, experimental studies challenge it
Ex. Researchers who add fertilizer to aquatic or terrestrial communities routinely observe significant increases in productivity, but decreases in diversity
Energy hypothesis
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Latitudinal gradient must be caused by an abiotic factor that affects rate of those factors in a way that would lead to more species in the tropics and fewer near the poles
High temperatures increase species diversity by increasing productivity and the likelihood that organisms can tolerate the physical conditions in a region
Third hypothesis is that tropical regions have had more time for speciation to occur than other regions have
Recent data suggest, however, that tropical forests were dramatically reduced in size by widespread drying trends during the ice ages
Existing forests may be much younger than originally thought
Intermediate-disturbance hypothesis
Holds that regions with a moderate type, frequency and severity of disturbance should have high species richness and diversity
Logic is that with intermediate levels of disturbance, communities will contain pioneering species as well as species better adapted to late-successional conditions
Ex. Recent studies have confirmed that tree falls and canopy gaps occur regularly in tropical forests and that fires occur in these biomes occasionally
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However, as of yet, no data showing that intermediate levels of disturbance are more likely to occur in tropics than at higher latitudes
Biology Chapter 53: Community Ecology Species Interaction −
A biological community consists of interacting species, usually living within a defined area
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To study species interaction, biologists focus on analyzing the effects on the fitness of individuals involved
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Four categories of interaction
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-/- relationship known as competition
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+/- relationship known as consumption and parasitism
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+/+ relationship termed mutualism
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+/0 relationship termed commensalism
Ex. Birds that follow moving army ants in the tropics
As ants march along forest, they hunt insects and small vertebrates
As they do, birds follow and pick off prey species that fly or jump out of the way of the ants
The birds are commensals that benefit from the association (+), but have no measurable impact on the ants (0)
Section focuses on three key themes o
Species interaction may affect the distribution and abundance of a particular species
o
Changes in species interactions often explain short-term changes in population size and distribution
Species act as agents of natural selection when they interact
Deer are fast and agile in response to natural selection exerted by major predators •
Speed and agility of deer, in turn, promote natural selection that favours wolves and cougars that are fast and have superior eyesight and senses of smell
•
Changes in species interactions lead to long-term changes in characteristics of populations, a phenomenon called coevolution, in addition to having short-term impacts on population size
2 o
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Outcome of interactions among species is dynamic and conditional
Consider relationship between army ants and bids that follow them, which is usually commensal
If bird attacks start to force other insects into path of ants, then both benefit and relationship becomes mutualistic
If birds begin to steal prey that would otherwise be taken by ants, relationship becomes parasitic
Competition o
Competition is a -/- interaction that occurs when different individuals use the same resources and when those resources are limiting
o
Competition that occurs between members of same species is called intraspecific competition
Intraspecific competition for space, sunlight, food and other resources intensifies as population’s density increases
A major cause of density-dependent growth
o
Interspecific competition arises when individuals from different species use the same limiting resources
o
There are many types if interspecific competition
Consumptive competition occurs when individuals consume the same resources (ex. trees competing for same water and nutrients)
Pre-emptive consumption exists when one species makes space unavailable to other species
Overgrowth competition happens when one species grows above another (ex. large fern overgrown other individuals and is shading them)
Chemical competition takes place when one species produces toxins that negatively affect another species
Territorial competition arises when a mobile species protects its feeding or breeding territory against other species (ex. grizzly bears drive off black bears)
Encounter competition occurs when two species interfere directly for access to specific resources (ex. spotted hyenas and vultures fight over kill)
3 o
o
o
Using the Niche Concept to Analyze Competition
A niche can be thought of as the range of resources that the species is able to use or the range of conditions it can tolerate
Interspecific competition occurs when niches of two species overlap
What Happens When One Species Is a Better Competitor
G.F. Gause claimed it is not possible for species with the same niche to coexist
This hypothesis is called the competitive exclusion principle and was inspired by a series of experiments Gause did with Paramecium
When Gause placed small populations of P. caudatum and P. Aurelia in separate laboratory cultures, both specie exhibited logistic growth
But Gause showed that when two species are put in the same culture together, only the P. Aurelia population exhibits a logistic growth pattern, P. caudatum is driven to extinction
His results are a product of asymmetric competition
When asymmetric competition occurs, one species suffers a much greater fitness decline than the other species does
Under symmetric competition, each of the interacting species experiences a roughly equal decrease in fitness
If asymmetric competition occurs and the two species have completely overlapping niches, then the stronger competitor is likely to drive the weaker competitor to extinction
But if the niches do not overlap completely, then the weaker competitor should be able to retreat to an area of non-overlap
In cases like this, an important distinction arises between a species’ fundamental niche, which is the combination of resources or areas used or conditions tolerated in the absence of competitors and its realized niche, which is the portion of resources or areas used or conditions tolerated when competition occurs
Experimental Studies of Competition
Joseph Connell observed that there were two species of barnacles with distinctive distributions in an intertidal rocky shore
Barnacle larvae are mobile, but adults live attacked to rocks
4
The adults of one species, Chthamalus stellatus, occurred in an upper intertidal zone, while the adult of the other species, Semibalanus balanoides, were restricted to a lower intertidal zone
The upper zone is a more severe environment for barnacles because it is exposed to air for longer periods at low tide each day
The young of both species were found together in the lower intertidal zone
To explain observations, Connell hypothesized that adult Chthamalus were competitively excluded from the lower intertidal zone
The alternative hypothesis is that adult Chthamalus are absent from the lower intertidal zone because they do not thrive in the physical conditions there
Connell tested these hypotheses by removing a number of rocks that had been colonized by Chthamalus from the upper intertidal zone and transplanting them into the lower intertidal zone
He screwed the rocks onto place and allowed Semibalanus larvae to colonize them
Once the colonization period was over, Connell divided each rock into two groups
In one half he removed all Semibalanus that were in contact with or next to a Chthamalus
Allowed Connell to document Chthamalus survival in absence of competition with Semibalanus and compare it with survival during competition •
o
Common experimental strategy (one competitor is removed and response by remaining species is observed)
Connell’s results support hypothesis of competitive exclusion
In unmodified areas, Semibalanus killed many of the young Chthamalus by growing against them and lifting them off the substrate
Chthamalus survival was much higher when all Semibalanus were removed
Mechanisms of Coexistence: Fitness Trade-Offs and Niche Differentiation
5
−
Why haven’t Semibalanus and other superior competitors taken over the world
The key here is that the ability to compete for a particular resource is only one aspect of an organism’s niche
If individuals are extremely good at competing for a particular resource, then they are probably less good and enduring drought conditions, warding off diseases or preventing predation
In the case of Semibalanus and Chthamalus, fitness trade-off is rapid growth and success in competing for space versus ability to endure harsh physical conditions of upper intertidal
Semibalanus are fast-growing and large
Chthamalus grow slowly, but can survive long exposures to air and to intense sun and heat
Neither species can do both things well so fitness trade-offs limit the ability of superior competitors to spread
Because competition is a -/- interaction, strong natural selection on both species to avoid it
An evolutionary change in traits reduces the amount of niche overlap and thus the amount of competition
This change in resource use is called niche differentiation or resource partitioning
The change in species’ traits is called character displacement
The fundamental idea is that competition exerts natural selection and that the characteristics of species change in a way that reduces competition
Consumption o
Occurs when one organism eats another
o
Three major types of consumption
Herbivory takes place when herbivores consume plant tissue
Parasitism takes place when parasites consume relatively small amounts of tissue or nutrients from another individual, called the host •
Often occurs over long period of time
6 •
Predation occurs when a predator kills and consumes all or most of another individual •
o
Not all parasitism involves consumption, social parasites in birds and insects lay their eggs in other species’ nests and induce them to raise the young
Consumed individual is called the prey
How Do Prey Defend Themselves
Standing or constitutive defences are always present •
Prey may hide, run, swim away when they sense predator
•
Many species find safety in numbers
•
Other prey species spray toxins or employ weaponry
•
Mimicry o
Occurs when one species closely resembles another species
o
When harmful prey species resemble each other, Mullerian mimicry is said to occur
o
To explain existence of Mullerian mimics, biologists propose that the existence of similar-looking dangerous prey in the same habitat increases the likelihood that predators will learn to avoid them
o
In this way, Mullerian mimicry should reduce likelihood of dangerous individuals being attacked
o
Batesian mimicry occurs when predators avoid the harmless mimics because they mistake them for a dangerous prey
•
Key point here is that prey have adaptations that reduce their likelihood of becoming victims
•
These adaptations are responses to natural selection exerted by predators
•
Constitutive defences are expensive in terms of energy and resources that must be devoted to producing and maintaining them however
7
o
Therefore, many prey species have inducible defences, traits that are produced only in response to presence of a predator
Inducible defences are efficient energetically, but are slow
Ex. Blue mussels •
Predation on mussels by crabs was high in an area of the estuary with relatively slow tidal currents, but low in an area of the estuary with relatively rapid tidal currents
•
Researchers hypothesized that if blue mussels possess inducible defences, then heavily defended prey individuals should occur in low-flow area where predation is higher, but not in high-flow area, where water movement reduces the number of crabs present
•
To test this hypothesis, biologists had two tanks, both containing seawater, but one with a crab that ate on fish, but not mussels and one with no crab
•
The tank with the crab allowed researchers to measure shell growth in mussels that were “downstream” of the crab
•
As predicted, the mussels exposed to a crab in this way developed significantly tougher shells than did mussels not exposed to crab
•
These results suggest that even without direct contact, mussels can sense the presence of crabs and increase their investment in defences
•
In a similar experiment, investigators compared mussels that were exposed to water running through broken mussel shells versus intact, but empty mussel shells
•
They recorded a significant increase in shell thickness in the tank downstream from the broken shells
•
This result supports the hypothesis that mussels can detect presence of predators from molecules released by broken shells
Are Animal Predators Efficient Enough to Reduce Prey Populations
Research supports hypothesis that species interactions have a strong impact on evolution of predator and prey populations
Prey are typically smaller than predators, have larger litter sizes and tend to reproduce at a younger age
8
o
As a result, they have a much larger intrinsic growth rate (r max)
If prey reproduce rapidly and are also well defended, it is not clear whether predators should be able to kill enough of them to reduce prey population significantly
Some results suggest that they can (wolf vs. moose)
Other experiments support the hypothesis that predators play a role in density-dependent growth of prey populations
Data available indicate that in many instances, predators are efficient enough to reduce prey population
Why Don’t Herbivores Eat Everything
Top-down control hypothesis •
States that herbivore populations are limited by predation and disease
•
Predators and parasites remove herbivores that eat plants
Poor-nutrition hypothesis •
Contends that plants are a poor food source in terms of nutrients they provide
•
Plant tissue have less than 10% of nitrogen found in animal tissues
•
If growth and reproduction of herbivores are limited by availability of nitrogen, then their populations will be low and impact of herbivory relatively slight
•
Herbivores could eat more plant material to gain nitrogen, but at a cost – they would be exposed to predation and expend more energy processing food
Plant-defence hypothesis •
Holds that plants defend themselves effectively enough to limit herbivory
•
Most plant tissues are defended by weapons such as thorns, prickles or hairs, or by potent poisons
•
No animal species, without help from protists or bacteria, digest cellulose or lignin, which are components of wood
9 •
o
All three factors are important in limiting the impact of herbivory, although the particular mix of factors will vary from plant species to plant species and from habitat to habitat
Adaptations and Arms Race
When predators and prey or herbivores and plants interact over time, coevolutionary arms races result •
o
−
Natural selection should favour plants that evolve an everchanging suite of compounds to deter the ever-changing array of herbivores they face
Consumers evolve traits that increase their efficiency, in response, prey evolve traits that make them unpalatable or elusive, which leads to selection on consumers for traits that counter the prey adaptation and so on
Can Parasites Manipulate their Hosts
To thrive, parasites have to be transmitted to new hosts
To a parasite, an uninfected host represents uncolonized habitat, teeming with resources
Consider species of land snails that are parasitized by flatworms, specifically by flukes in the genus Leucochloridium
When the flukes have matured and are ready to be transmitted to their next hose, a bird, they burrow into the snail’s tentacles and wriggle
In addition, infected snails become attracted to light, even though uninfected snails avoid sunlit areas and prefer dark, shady environments
When infected snails move out of the shade into the open and glide about with wriggling tentacles, they are more easily spotted and consumed by bids
Biologists suggest that the worms manipulate the behaviour of the snail and that the change in snail behaviour makes the parasite more likely to be transmitted to a new host
Mutualism o
Mutualisms are +/+ interactions that involve a wide variety of organisms and rewards
o
Ex. Many species of bees visit flowers to harvest nectar and pollen
10
o
Bees benefit because nectar is used as a food source for adult bees and pollen is fed to larvae
Flowering plants also benefit because in the process of visiting flowers, foraging bees carry pollen from one plant to another and accomplish pollination
Ex. Ants in the genus Crematogaster, which live in acacia trees
Ants in live in bulbs at the base of acacia thorns and feed on small structures that grow from tree branches
These ants protect the tree by attacking and biting herbivores and by cutting vegetation from the ground below the host tree
Plants have high protein bodies that ants can feed on
o
Important to note that even though mutualism benefits both species, the interaction does not involve individuals from different species being altruistic to each other
o
Judith Bronstein described mutualism as “a kind of reciprocal parasitism; each partner is out to do the best it can by obtaining what it needs from its mutualist at the lowest possible cost to itself”
o
Not surprising that some species cheat on mutualistic systems
o
Ex. Deceit pollination occurs when certain species of plants produce a showy flower, but no nectar reward
Pollinators have to be deceived to make a visit and carry out pollination
Evolutionary studies show that deceit pollinators evolved from ancestral species that did provide a reward
Over time, a +/+ interaction evolved into a +/- interaction
−
Because the costs and benefits of species interaction are fluid, an interaction between the same two species may range from parasitism to mutualism to competition
−
Table 53.1
Community Structure −
How Predictable are Communities o
Frederick Clements promoted the view that biological communities are stable, integrated and orderly entities with a highly predictable composition
11 o
Hypothesis was that species interaction are so extensive and coevolution is so important that the groups of species called communities have become highly integrated and interdependent units of nature
o
Argued that communities develop by passing through a series of predictable stages dictated by extensive interactions among species and that this development culminates in a stable final stage known as a climax community
o
According to Clements, nature of the climax community is determined by the area’s climate and does not change over time
o
Henry Gleason, in contrast, contended that the community found in a particular area is neither stable nor predictable
o
He claimed that plant and animal communities are ephemeral associations of species that just happen to share similar climatic requirements
o
Mapping Current and Past Species’ Distributions
o
If communities are predictable assemblages, then the ranges of species that make up a particular community should be congruent
The same group of species should almost always be found growing together
However, biologists found independently of each other
Also, an important pattern emerged in data of fossils
that
species
came
and
went
•
Species do not come and go in the fossil record in tightly integrated units
•
Instead, the ranges of individual species tended to change independently of one another
Experimental Tests
Biologists constructed 12 ponds and put in enough chlorine to kill an pre-existing organisms
If community structure is predictable, then each pond should develop the same community of species once the chlorine vaporized and made the water habitable
If community structure is unpredictable, then each pond should develop a different community
12
−
Results showed that a total of 61 species inhabited all the ponds, but individual ponds only had 31 to 39 species
A number of species occurred in most or all of the 12 ponds, but each pond had a unique species assemblage
Researchers contended that some species are particularly good at dispersing and are likely to colonize all or most of the available habitats
Other species disperse more slowly and tend to reach only one or a few of the assembly and composition are somewhat contingent and difficult to predict
o
Overall message is that Clement’s position is too extreme and Gleason’s view is closer to being correct
o
Although both biotic interactions and climate are important in determining which species exist at a certain site, chance and history also play a large role
How Do Keystone Species Structure Communities o
Experiments have shown that, in some cases, the structure of an entire community can change drastically if a single species of predator or herbivore is removed from a community or added to it
o
Ex. Seat star Pisaster ochraceous is an important predator
o
When research removed Pisaster from experimental areas, what had been diverse communities of algae and invertebrates became overgrown with solid stands of the California mussel Mytilus californianus
Although M. californianus is a dominant competitor, its populations had been held in check by sea-star predation
When this predator was gone, the species richness and structural complexity of the habitat changed radically
Robert Paine coined the term “keystone species”
A keystone species has a much greater impact on the surrounding species than its abundance and total biomass would suggest
Community Dynamics −
Disturbance and Change in Ecological Communities
13 o
Community composition and structure may change radically in response to changes in abiotic and biotic conditions
o
A disturbance is any event that removes some individuals or biomass from a community
o
Forest fires, windstorms, floods, the fall of a large canopy tree, disease epidemics all qualify as disturbances
o
These events alter light levels, nutrients, unoccupied space or some other aspect of resource availability
o
Biologists have come to realize that the impact of disturbances is a function of three factors
The type of disturbance
Its frequency
Its severity
o
Most communities experience a characteristic type of disturbance and in most cases, disturbances occur within a predictable frequency and severity
o
Biologists refer to a community’s disturbance regime
o
How do Researchers Determine a Community’s Disturbance Regime
Two strategies
First is based on inferring long-term patterns from data obtained in a short-term analysis •
Ex. An observational study might document that 1% of all boreal forest on Earth burns in a given year
•
Assuming that fires occur randomly, researchers project that any particular piece of boreal forest has a 1/100 chance of burning each year
•
According to this reasons, fires will recur in that particular area every 100 years on average
•
Straightforward to implement, but has important drawbacks o
In boreal forests, fires do not occur randomly in either space or time
o
More likely to occur in some areas than in others and tend to occur in particularly dry areas
14 o
o
−
Unless sampling is extensive, difficult to avoid errors caused by extrapolating from particularly disturbanceprone or disturbance-free areas or years
Second approach is based on reconstructing history of a particular site •
Ex. Forest fires often leave a layer of burned organic matter and charcoal on surface of ground
•
Researchers can dig a soil pit, find charcoal layers and use radioisotope dating to establish when the fires occurred
•
Also possible to date the death of trees killed by fire by comparing their growth rings with those of living trees
•
Further, trees that are not killed by fire are often scarred
•
These fire scars occur most often at the tree’s base, where dead leaves and twigs accumulate and furnish fuel
Why is it Important to Understand Disturbance Regimes
Can tell researchers if disturbances are severe or not
Some “disturbances” may be beneficial to organisms
Succession: The Development of Communities after Disturbance o
Severe disturbances remove all or most of the organism from an area
o
The recovery that follows is called succession
o
Primary succession occurs when a disturbance removes the soil and its organisms as well as organisms that live above the surface
o
Glaciers, floods, volcanic eruptions and landslides often initiate primary succession
Secondary succession occurs when a disturbance removes some or all of the organisms from an area, but leaves the soil intact
Fire and logging are examples of disturbances that initiate secondary succession
o
Early successional communities are dominated by species that are short lived and small and that disperse their seeds over long distances
o
Late successional communities are dominated by species that tend to be long lived, large and good competitors for resources such as light and nutrients
15 o
Theoretical Considerations
Biologists focus on three factors to predict outcome of succession in a community •
Particular traits of species involved
•
How species interact
•
Historical and environmental circumstances (size of area involved and weather conditions)
Species traits, such as dispersal capability and ability to withstand extreme dryness, are particularly important early in succession
Recently disturbed sites tend to be colonized by plants and animals with good dispersal ability
When these organisms arrive, however, the often have to endure harsh environmental conditions
These pioneering species tend to be adapted for growth in disturbed soils
Early sucessional species devote most of their energy to reproduction and little to competitive ability •
They have small seeds, rapid growth and a short life span and begin reproducing at an early age
•
As a result, they have a high reproductive rate
•
They can also tolerate severe abiotic conditions, such as high light levels, poor nutrient availability and drying
Once colonization is under way, the course of succession tends to depend less on how species cope with aspects of abiotic environment and more on how they interact with other species
This change occurs because plants that grow early in succession change abiotic conditions in a way that makes the conditions less severe
Because plants provide shade, they reduce temperatures and increase humidity
Their dead bodies also add organic material and nutrients to the soil
As abiotic conditions improve, biotic interactions become more important
16
During succession, existing species can have one of three effects on subsequent species (facilitation, tolerance or inhibition)
Facilitation takes place when presence of an early arriving species makes conditions more favourable for the arrival of a certain later species by providing shade or nutrients
Tolerance means that existing species do not affect the probability that subsequent species will become established
Inhibition occurs when the presence of one species inhibits the establishment of another •
In addition to species traits and species interaction, the pattern and rate of succession depend on the historical and environmental context in which they occur •
o
Ex. A plant species that requires high light levels to germinate may be inhibited late in succession by the presence of mature trees that prevent sunlight from reaching the forest floor
Ex. Researchers found that communities that developed after forest fires disturbed Yellowstone National Park depended on the size of the burned patch and how hot the fire had been at that location
Succession is also affected by the particular weather or climate conditions that occur during the process
A Case History: Glacier Bay, Alaska
A rapid and extensive glacial recession is occurring at Glacier Bay
Several plant communities found in the area
The oldest sites are dense forests of Sitka spruce and western hemlock
Areas that have been deglaciated for 100 years are inhabited by scattered spruce trees and dense thickets of a Sitka alder
Sites that have been deglaciated for 45-80 years are also covered with dense alder thickets, but emergent trees are primarily cottonwood
Locations that have been ice free for 20 years or less do not have a continuous plant cover
17 •
Instead, they host scattered individuals of willow and a small shrub called Dryas
Observations inspired a hypothesis for the pattern of succession in Glacier Bay •
With time, the youngest communities of Dryas and willow succeed to alder thickets, which subsequently become dense spruce-hemlock forests
•
There is a single sucessional pathway throughout the bay
A recent study has challenged this hypothesis
Researchers found that three distinct successional pathways have occurred •
In the lower part of the bay, soon after the ice retreated, Sitka spruce began growing and quickly formed dense forests o
Western hemlock arrived after spruce and is now common in the understory
•
At middle-aged sites in the upper part of the bay, alder thickets were dominant for several decades and spruce is just beginning to become common
•
The youngest sites in uppermost part of bay may be following a third pathway o
Alder thickets become dominant fairly early, but spruce trees are scarce
o
Instead, cottonwood trees are abundant
Species traits may be especially important in explaining certain details about the sucessional pathways
Ex. Western hemlock is abundant at older sites, but largely absent from young ones •
Logical because its seeds germinate and grow only in soils containing a substantial amount of organic matter and because the young trees can tolerate deep shade, but not bright sunlight
•
It’s intolerance of early successional conditions explains why none of the three pathways began with colonization by this species
18
Species interaction has been important in all three pathways
Ex. Research has shown that alder facilitates the growth if Sitka spruce •
Facilitating effect occurs because symbiotic bacteria that live inside nodules on the roots of alder convert atmospheric nitrogen to nitrogen-containing molecules that alder use to build proteins and nucleic acids
•
When alder leaves fall and decay or roots die, the nitrogen becomes available to spruce
•
Although spruce trees are capable of invading and growing without presence of alder, they grow faster when alder stands have added nitrogen to soil
Competition is another important species interaction
Ex. Shading by alder reduces growth of spruce until spruce trees are tall enough to protrude above alder thicket •
Once spruce trees breach alder canopy, alder dies out because it is unable to compete with spruce trees for light
Historical and environmental context also influences succession •
Ice was very thick in upper part of Glacier Bay
•
Therefore, glacier eliminated all of existing forests
•
In lower part of bay, ice was substantially thinner
•
As a result, some forest remained on mountain slopes beyond the ice
•
As glacier retreated from these areas, forests on the slopes provided a source of spruce and hemlock seeds and set a different successional pathway in motion
•
In this way, environmental context (distance to existing forests) helped determine how the community developed
Species traits and species interaction tend to make succession predictable, while history and chance events contribute a degree of unpredictability to succession
Species Richness in Ecological Communities −
Species richness is a simple count of how many species are present in a given community
19 −
Species diversity is a weighted measure that incorporates a species’ relative abundance as well as its presence or absence
−
Predicting Species Richness: The Theory of Island Biogeography o
Strong pattern that larger patches of habitat contain more species than do smaller patches of habitat
o
Observation is logical because large patches should contain more niches and thus support higher number of species
o
Another observation was that islands in the ocean have smaller number of species than do areas of same size on continents
o
o
One hypothesis was that speciation occurs so slowly that number of species present on an island is a product of just immigration and extinction
Rates of both processes should vary with number of species present on an island
Immigration rates should decline as the number of species on an island increases because individuals that arrive are more likely to represent a species already present
Extinction rates should increase as species richness increases because niche overlap and competition for resource will be more intense
Result is an equilibrium, a balance between arrival of new species and extinction of existing ones
Immigration and extinction should also vary as a function of island size and how close island is to continent or other source of immigrants
Immigration rates should be higher on large islands that are close to mainlands because immigrants are more likely to find large islands that are close to shore than small ones that are far from shore
Extinction rates should be highest on small islands that are far from shore because fewer resources are available to support large populations and because fewer individuals arrive to keep population going
As a result, species richness should be higher on larger islands than smaller islands and on nearshore islands vs. remote islands
This model, called the theory of island biogeography is important because
20
It is relevant to a wide variety of island-like habitats such as alpine meadows, lakes and ponds and caves
Made specific predictions that could be tested
Relevant to metapopulation dynamics and can help inform decisions about the design of natural preserves •
−
Measuring Species Diversity o
To measure species diversity, biologist could count number of species present in a community
o
Problem is that simple counts provide an incomplete picture of diversity
o
Relative abundance of species is also important to consider
o
Shannon Index
S
H =−∑ p i ln pi '
−
In general, most-species rich reserves should be ones that are relatively large and located close to other relatively large habitat areas
i=1
Pi is the proportion of individuals in the community that belong to species “I”
Index is summed over all of the species in the study
Global Patterns in Species Richness o
Data compiled in intervening years have confirmed existence of a strong latitudinal gradient in species diversity, for communities as a whole as well as for many taxonomic groups
o
To explain why species diversity might decline with increasing latitude, biologists have to consider two fundamental principles
Causal mechanism must be abiotic because latitude is a physical phenomenon produced by Earth’s shape •
Explanation must be a physical factor that varies predictably with latitude and that could produce changes in species diversity
Species diversity of a particular area is the sum of four processes: speciation, extinction, immigration and emigration
21 •
o
o
One hypothesis is that high productivity in the tropics promotes high diversity by increasing speciation rates and decreasing extinction rates
Idea is that increased biomass production supports more herbivores and thus more predators and parasites and scavengers
Speciation rates should increase when niche differentiation occurs within populations of herbivores, predators, parasites and scavengers
Although this high-productivity hypothesis is supported by globalscale correlation between productivity and diversity, experimental studies challenge it
Ex. Researchers who add fertilizer to aquatic or terrestrial communities routinely observe significant increases in productivity, but decreases in diversity
Energy hypothesis
o
o
Latitudinal gradient must be caused by an abiotic factor that affects rate of those factors in a way that would lead to more species in the tropics and fewer near the poles
High temperatures increase species diversity by increasing productivity and the likelihood that organisms can tolerate the physical conditions in a region
Third hypothesis is that tropical regions have had more time for speciation to occur than other regions have
Recent data suggest, however, that tropical forests were dramatically reduced in size by widespread drying trends during the ice ages
Existing forests may be much younger than originally thought
Intermediate-disturbance hypothesis
Holds that regions with a moderate type, frequency and severity of disturbance should have high species richness and diversity
Logic is that with intermediate levels of disturbance, communities will contain pioneering species as well as species better adapted to late-successional conditions
Ex. Recent studies have confirmed that tree falls and canopy gaps occur regularly in tropical forests and that fires occur in these biomes occasionally
22
However, as of yet, no data showing that intermediate levels of disturbance are more likely to occur in tropics than at higher latitudes
