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Chapter 6

BIO120H1 Chapter 6: Struggle for Existence Part VI

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Spencer Barret

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Monday, November 21, 2016
Struggle for Existence!
Part VI: Population Ecology
-Population ecology (also called population size): study of the number of
individual organisms in a population!
Population size: represented by symbol “N”!
estimating population sizes in nature is very difficult!
-difficult to find and enumerate the organisms (animals are mobile)!
-difficult to determine boundaries of populations (wide habitats)!
-not easy to distinguish individuals (for plants especially)!
genet (single genetic individual) can grow into large clone of identical plants
ramets sometimes become independent physiological units!
-Mathematical models and theory!
2 cases when scientists try to estimate population sizes in continuous habitats in
-pest species in agriculture/ forestry/ epidemiology!
economic costs of population outbreaks gives a rationale for research!
don’t have to know exact number, just know if its a big year or small!
makes predication for the method of control before it’s too late!
-dwindling populations of endangered species (typically vertebrates)!
legal responsibilities to prevent extinction!
exact numbers is important!
question intertwined with political issues (ex. preserving habitat vs developing
-Direct empirical studies!
very idiosyncratic (definition: peculiar/ individual)!
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Monday, November 21, 2016
outcome is dependent come biological detailed of species and its habitat’s
-Modelling population changes through time!
classical approach: deterministic models with calculus — differential equation!
-dN/ dt = some function!
-produces a prediction of population’s growth trajectory (how N changes through
-outcome is determined by started conditions and fixed parameters (constants)!
-only one result for any set of initial conditions!
-ex. density-independent growth — logistic equation!
-two-species variants: interspecific competition + predator-prey interactions
(usually too simple give an accurate description)!
recent approach: simulation models!
-don’t calculate for one single outcome!
-a computer program is written to encapsulates rules for population growth;
program is then ran to make a growth trajectory!
-models are stochastic rather than deterministic!
-models are often more complex and realistic; richer and more informative!
-collection of trajectories can be found: average population size, maximum,
-Nt+1: population size one year in the future!
-Nt: population size now!
-B: number of births that occur during the interval!
-D: number of death that occur during the interval!
-Geometric growth: Nt+1 = Nt + B - D!
-Simplified equation — difference equation: Nt+1 = Nt λ!
-Allee effects: Inverse density dependence!
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