Population Ecology
Population Ecology; Nnow = Npast + (B + I) – (D+E) Population – discrete group of individuals of the same species A single genetic entity may be made up of several modules or repeated units (especially plants) hard to determine individuals * definition of a population depends on the research question and the organisms under investigation Modulesnow = modulespast + modulesborn – moduledeaths Goal of population ecology not only describe changes in population size but also understand and predict them study patterns of birth, growth and death of populations Life tables – summaries of mortality and survivorship ▪ age specific (cohort) life tables – eg. important in fisheries for example to know ages at certain sizes and avoid overfishing for example follow group of individuals throughout their lifetime (often by tagging and monitoring individuals) lx = nx/no (proportion surviving to age x) dx = (nx – nx+1)/no (proportion dying) qx= (dx/lx)/number of days (mortality on a given day) allows periods of greater death/birth to be detected opportunities for conservation allow survivorship curves to be made Pearl’s Classic Survivorship curves;
▪ Time-Specific (Static) life tables – snap shot of how many individuals of each age class are living in the population at a single point of timeused for longer lived species (trees, elephants) need representative samples can work out life expectancy based on age class (and make survivorship curves and life history) - assumptions of time specific life tables; - birth and death rates do not vary from one year to the next
Population Growth Models; age specific fecundity = av. number of female offspring produced per female at each age class – important to know what age organisms are when they reproduce (important in management of fisheries etc.,) Ro – net reproductive rate (or the number of females produced per female per generation) Ro > 1 : population increases Ro = 1 : population stable Ro < 1 : population decreases *Ro not suitable for comparison between species as different species have different generation times (only suitable for comparison within species) generation – av. time period between a females birth and when a female gives birth (averaged across females within a population)
Predicting population growth using population growth models; ▪ Deterministic models – geometric growth: - need to know; - Ro - initial population size (i.e. population size at time t) Nt+1 = RoNt gives net reproductive rate (rate at which population will multiply per generation) r = instantaneous rate of increase when Ro = 1, r = 0 r = (lnRo)/Tc Tc = generation time
Constraints on geometric growth; - shortage of key resources (food, space, breeding sites, etc.,) - attack from predators - epidemics
▪ logistic growth models – occurs in populations where resources are or can be limiting
▪ Time-Specific (Static) life tables – snap shot of how many individuals of each age class are living in the population at a single point of timeused for longer lived species (trees, elephants) need representative samples can work out life expectancy based on age class (and make survivorship curves and life history) - assumptions of time specific life tables; - birth and death rates do not vary from one year to the next
Population Growth Models; age specific fecundity = av. number of female offspring produced per female at each age class – important to know what age organisms are when they reproduce (important in management of fisheries etc.,) Ro – net reproductive rate (or the number of females produced per female per generation) Ro > 1 : population increases Ro = 1 : population stable Ro < 1 : population decreases *Ro not suitable for comparison between species as different species have different generation times (only suitable for comparison within species) generation – av. time period between a females birth and when a female gives birth (averaged across females within a population)
Predicting population growth using population growth models; ▪ Deterministic models – geometric growth: - need to know; - Ro - initial population size (i.e. population size at time t) Nt+1 = RoNt gives net reproductive rate (rate at which population will multiply per generation) r = instantaneous rate of increase when Ro = 1, r = 0 r = (lnRo)/Tc Tc = generation time
Constraints on geometric growth; - shortage of key resources (food, space, breeding sites, etc.,) - attack from predators - epidemics
▪ logistic growth models – occurs in populations where resources are or can be limiting
