BIO120H © Lisa Zhao 2011
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chapter 19: ECOLOGICAL SUCCESSION AND COMMUNITY DEVELOPMENT
Krakatau island (Indonesia) exploded on Aug. 27, 1883 all life was obliterated it was a great chance for scientists to study the development of biological communities on a newly formed, raw terrain of volcanic ash nearest sources of colonists were from Sumatra and Java (40 km away) sea-dispersed plants were first to show up (made up 10 out of 24 species by 1886) wind-dispersed grasses and ferns – seeds and spores could be blown across ocean wind-dispersed tree species arrived; closed forest developed across the island by 1920s pioneering species were pushed to marginal habitats or disappeared birds and bats were attracted to the trees (fruit-eating species) brought seeds of animal-dispersed trees and shrubs animal-dispersed outnumber sea- and wind-dispersed plants now island fragments are constantly changing due to continuing eruptions island will continue to change communities exist in a state of continuous flux, but indiv. are replaced by their own kind disturbance: an event that causes rapid or marked change in a population or community, often thought of as displacing an ecological system from its equilibrium after a habitat is disturbed, pioneering species adapted to disturbed habitats are successively replaced by other species as the community attains its former structure and composition species successively replace one another in the process of succession succession: a sequence of changes in the species composition of a community initiated by a disturbance, progresses to a stable state climax community: the ultimate association of species achieved after succession the end point of a successional sequence, or sere; a community that has reached a steady state under a particular set of environmental conditions Frederic E. Clements
THE CONCEPT OF THE S ERE INCLUDES ALL THE STAGES OF SUCCESSION AL CHANGE any new habitat attracts colonizers (adapted to be good pioneers) rates of succession slow as slower-growing plants appear transition from abandoned field to mature forest takes only one of several successional sequences that may lead to similar climax communities w/in a given biome sere: a series of stages of community change in a successional sequence leading toward a stable state although initial stages of the sere depend on the beginning habitat, with time, seres converge on similar climax communities seres have been classified into 2 types according to origin Primary Succession
primary succession: succession in a newly formed or exposed habitat devoid of life ex. natural conversion of ponds in north-temperate and boreal climates to dry lands deep ponds (kettle holes) left by glaciers and those formed by beaver dams undergo bog succession rooted aquatic plants establish at the edge of the pond some species of sedges form mats on the water surface and eventually cover pond
BIO120H © Lisa Zhao 2011
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produces a firm layer of vege. “quaking bog”
detritus produced by sedge mat accumulates at pond bottom (anaerobic, thus no microbial decomposition occurs down there) as a bog accumulates sediments and detritus, sphagnum moss and shrubs establish along edges add to development of a soil black spruce and larch birch, maple, fir over thousands of years, aquatic habitat becomes terrestrial as organic detritus accumulates and soil rises above water table
Secondary Succession
ecologists in the field
secondary succession: succession in a habitat that has been disturbed, but in which some aspects of the community remain even when reseeding initiates a secondary sere, the type of disturbance and size of gap it creates influences the first species to establish disturbance and recovery occur quickly in marine habitats marine invertebrates have inversely related colonizing and competitive abilities created bare patches on rock occupied by sessile invertebrates and; filled in quickly by tunicates and sponges (highly successful competitors) gap size didn’t influence succession bryozoans and polychaete larvae attempted to colonize but were overgrown on artificial hard surfaces, ex. ceramic tiles, placed in sand away from colonists gap size had huge effect on pattern of colonization tunicates and sponges don’t disperse well, thus, didn’t Michael Keough colonize them (influence of gap bryozoans and polychaetes had chance size/isolation on larger gaps = larger targets – colonized by T & S succession in smaller gaps – colonized by B & P (once established, can marine habitats) prevent the colonization of T & S) B & P are disturbance-adapted species weed a.k.a. ruderals: an organism, generally having strong powers of dispersal, that is capable of living in highly disturbed habitats colonize open patches quickly mature and produce offspring at an earlier age eliminated by slowly colonizing superior competitors require frequent disturbances to stay in sys. size of gap also influences whether predators & herbivores will be active there some feed in large gaps – easy to find others are vulnerable to predation and need the cover of intact habitat, from whose edges they venture to feed – graze small gaps and gap edges than the centre of large gaps
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The Climax Community
succession leads to the climax community (ultimate expression of community dvlpmt) each sere develop under a particular set of envmtal conditions, but all progress toward similar climax states Clements believed that climate alone determined the nature of the local climax and that diff. climax states were discrete, recognizable, and separate from one another (closed community) communities are more commonly viewed as open sys. today alternative “climax” communities are possible, resulting from factors ex. intensity of a disturbance, size of the gap it produces, physical conditions during early succession continuum index: the scale of an environmental gradient based on changes in physical characteristics or community composition along that gradient
SUCCESSION ENSUES AS COLONISTS ALTER ENVI RONMENTAL CONDITIONS a species’ presence in a sere is determined by: 1. how it readily invades a newly formed/ disturbed habitat 2. how it responds to changes that occur in the envmt over the course of succession those that disperse and grow rapidly dominate the early stages of a sere they sometimes modify envmts that allow later-stage species to become established o ex. herbs shad soil to help it retain moisture others inhibit others into a sere by competing more effectively or by direct interfrnc. Joseph Connell and R. O. Slatyer classified the processes governing the course of succession into 3 categories: facilitation, inhibition, tolerance describes the effect of one species on the prob. of a second becoming established and whether that effect is +ve, -ve, or neutral facilitation: a process by which one species increases the probability of another species becoming established, particularly during early succession ex. alder trees harbour N-fixing bacteria in roots which provide soil w/ N, facilitates N-limited plants i.e. spruce, which eventually replace them T. A. Dean and L. E. Hurd (facilitation in marine sys.) o hydroids facilitate settlement of tunicates, and together facilitate mussels inhibition: the suppression of one species by the presence of another, esp. during a successional sequence by eating them , reducing their resources, attacking them w/ chemicals ore antagonistic behaviour inhibition in early states of a sere can prevent mvmt towards climax priority effect: the result of an interaction between two species during a successional sequence whose outcome depends on which becomes established first o outcome of interaction depends on which established first colonists are often seeds or larvae and may not even become established in the presence of adults of the other succession then will depend on precedence o may be random (depending on which reaches site first) tolerance: a lack of influence by one species on the presence or absence of other species depends on its dispersal ability, and tolerance of physical conditions of envmt.
BIO120H © Lisa Zhao 2011
ecologists in the field
Henry J. Oosting Catherine Keever
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tolerance and inhibition combine to influence the early stages of plant succession in old agricultural first 3-4 years: species of crabgrass, horseweed, ragweed, aster, broomsedge replaced each other in rapid sequence (stress tolerant) their indiv. life cycle determines its place in the successional seq. crabgrass (rapidly growing annual) – prominent during year when field is abandoned horseweed (winter annual; seeds germinate in autumn) – disperses well, develops rapidly, dominates 1yro fields, seedlings require full sunlight; quickly replaced by shadetolerant species early succession is dominated by tolerance (colonizing species disperse and cope w/ harsh conditions) inhibition then comes into play decaying horseweed roots stunt seedling growth (selflimiting) – growth inhibitor; it is a by-product of adaptations that increase fitness during year 1 of succession ragweed (summer annual) – dominates fields that are plowed under late autumn (instead of horseweed) aster, broomsedge (biennials, germinate in spring and early summer, exist through winter and bloom for first time in their second autumn); broomsedge persists and flowers during the following autumn, overgrows aster and other early colonizers, dominates sere until the arrival of shrubs and trees
Facilitation, Inhibition, and Invasive Species
invasive species often dominate during early succession their success is most influenced by interactions w/ fungi and other soil org. that either facilitate or inhibit their establishment mutualistic symbioses w/ plant roots (mycorrhizae) help them extract mineral nutrients from soil plants provide them w/ CHO NRG some fungi are parasitic“ly” or pathogenic“ly” associated w/ plants John Klironomos (studied how 10 diff. species of mycorrhizal fungi influenced the growth of 10 species of plants) amount of inhibition/facilitation depended on the comb. of plant and fungal species o not consistent w/ “home-home” and “home-foreign” comb. of the same 2 species interactions depend on local evolutionary adaptation inhibiting/facilitating effects were strongest in “home-home” combinations introduced species might escape the strongest inhibiting effects of soil org. but also fail to benefit from facilitating effects some believe that species have become invasive b/c they escaped their native predators ex. black cherry trees are self-limiting in native range (NA) cherry seedlings close to parent rarely survive seedlings can grow close to the parent in Europe sterilizing the soils has shown that soil pathogens are responsible for this selfinhibition
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ex. maple trees (native in NA, non-native in Europe) seedlings from both grown on native soil were inhibited on soil from non-native growth increased thus, soil org. associated w/ native species facilitate the establishment of non-native maples in native soils, facilitating interactions of this kind are overshadowed by the inhibitory influence of natural enemies in the soil
The Differing Adaptations of Early and Late Successional Species
early and late species have. diff. growth and reproduction strategies early-stage: dispersal ability, tolerate stress climax species: disperse and grow slowly, shade-tolerant seedlings, large size when mature; thus, more competitive than early thus, there is a shift in the balance b/w adaptations promoting dispersal, rapid growth, early reproduction, and competitive ability enhancement early have small, wind-dispersed seeds which can remain dormant seed bank: dormant seeds in the soil that can germinate when conditions are favourable climax have nutrient-filled seeds
SUCCESSION BECOMES S ELF-LIMIT ING AS IT APPRO ACHES THE CLIMAX succession continues until the addition of new species to the sere and the exclusion of established species no longer change the envmt of the developing community conditions change more slowly after the vegetation achieves the largest growth form the envmt can support final biomass of climax community is climate-dependant succession time to climax community depends on the nature of the climax and the quality of the habitat climax target point also changes with long-term climate change loss of keystone consumers transform communities continuously communities have continuous dynamic response to changing conditions Climax Communities Under Extreme Environmental Conditions
soil nutrients, moisture, slope, and exposure determine the composition of a climax community fire, grazing pressure (heavy grazing of one species may stimulate the growth of plant that another prefers)
Transient and Cyclic Climaxes
succession doesn’t always lead to a stable climax transient climax: a climax community that develops in an ephemeral (short-lived) habitat, such as a temporary pond ex. seasonal changes that dry/freeze ponds destroy the established communities each year (restocked from larger, permanent bodies of water in the spring, restarting succession) succession recurs whenever a new envmtal opportunity appears ex. excreta and dead org. provide resources for scavengers and detritus feeders ex. carcasses on African savannas: devoured by a succession of vultures the first are large, aggressive species that eat the largest pieces smaller species then follow finally, a vulture that cracks open bones to feed on marrow
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scavenging mammals, maggots, and microorg. then enter the sequence at diff. points
ensuring that nothing remains this succession has no local climax; all scavengers disperse when the feast concludes the scavengers form part of a larger climax: the entire savanna community
cyclic climax: a self-perpetuating, repeating sequence of stages produced by ongoing succession, none of which by itself is stable, but which together constitute a persistent pattern ex. plant species A can germinate only under species B, B only under C, C only under A creates a regular cycle of species dominance (length of each stage determined by life span of dominant species) usually driven by stressful envmtal conditions deaths of indiv. alter the envmt ex. open spaces in canopies cyclic succession may involve a sequence of damage and regeneration