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Population Ecology u Study of the factors that affect population size and composition.
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Population u Individuals of a single species that occupy the same area.
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Important Characteristics 1. Density 2. Dispersion
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Density u Number of individuals per unit area or volume. u Ex: u Diatoms - 5 million/m 3 u Trees - 5,000/km 2 u Deer - 4/km 2
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Dispersion u Pattern of spacing among individuals. u Types: 1. Clumped 2. Uniform 3. Random
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Clumped Dispersion u May result form a patchy environment. u May increase chances for survival. u Ex: u Schooling behavior u Flocks of birds
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Uniform Dispersion u Often the result of antagonistic interactions between individuals. u Ex: u Territories u Spacing between desert plants
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Random Dispersion u Often the result of the absence of strong attractions or repulsions between individuals. u Not a common pattern.
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Demography u The study of the vital statistics that affect population size. u Ex: Birth and Death rates
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Demographic Factors u Age structure of the population. u Birth and Death rates. u Generation time. u Sex ratio and reproductive behavior.
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Life Tables u Mortality summary for a cohort of individuals. u First developed from life insurance studies.
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Life Tables Show u Mortality rate per year. u Life span of the organism. u Fecundity (birth rate).
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Survivorship Curve u Plot of the numbers of a cohort still alive over time. u Curve Types: u Type I u Type II u Type III
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Type I u Low early deaths. u High late deaths. u Ex: u Humans u Other large mammals
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Type II u Constant death rate. u Ex: u Annual plants u Many invertebrates
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Type III u High early deaths. u Low late deaths. u Ex: u Trees u Oysters
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Comment u Curve type may change between young and adults. u Ex: Nestlings - Type III Adult Birds- Type II
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Life History Strategies 1. "r" or Opportunistic species 2. "k" or Equilibrial species
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"r" Species u Increase fitness by producing as many offspring as possible. u Do this by: u Early maturation u Many reproductive events u Many offspring
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Result u Maximize reproduction so that at least a few offspring survive to the next generation. u Most offspring die (Type III curve).
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"k" Species u Increase fitness by having most offspring survive. u Do this by: u High parental care u Late maturation u Few reproduction events u Few offspring.
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Result u Maximize survivorship of each offspring. u Few offspring, but most survive (Type I curve).
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What is the strategy u For a weed? u For an endangered species? u For Garden Pests?
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Population Growth N/ t = b - d u Where: u N= population size u t = time u b = birth rate u d = death rate
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Rate of Increase u r = difference between birth rate and death rate. u r = b - d
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Equation: N/ t = rN u N = population size u t = time u r = rate of increase
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From Calculus The equation N/ t = rN becomes: u dN/dt = r max N u r max = intrinsic rate of increase
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Exponential Growth u dN/dt = r max N u Characteristic of "r" species. u Produces a “J-shaped” growth curve. u Only holds for ideal conditions and unlimited resources.
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Logistic Growth u dN/dt = r max N K-N K u K = carrying capacity
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Result u “S-shaped” growth curve. u Characteristic of “k" species. u Common when resources are limited.
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Comment u K is not a constant value. u Populations often oscillate around “K” as the environment changes.
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Additional Comments u Populations often overshoot “K”, then drop back to or below “K”. u AP Exam rarely asks you to work the equations, but you should be able to give them.
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Regulation of Population Size 1. Density- Dependent Factors 2. Density- Independent Factors
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Density-Dependent u Affect is related to N. u As N increases, mortality increases. u Ex: Food, nesting space, disease
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Density-Independent u Affect is not related to N. u Mortality not related to population size. u Ex: Weather and climate
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Population Cycles u Cyclic changes in N over time. u Often seen in predator/prey cycles. u Ex: Snowshoe Hare - Lynx
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Causes u Density dependent factors. u Chemical cycles. u Saturation strategy to confuse predators.
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Age Structure Diagrams u Show the percent of a population in different age categories. u Method to get data similar to a Life Table, but at one point in time.
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Importances u Can be used to predict future population growth trends, especially for long lived species.
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Exponential Growth u Produces age structures that are a triangle or pyramid shape.
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Logistic Growth u Produces age structures that have even sizes between most age categories.
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Declining Populations u Produce age structures with a narrow base and wider middles.
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Summary u Know density and dispersion patterns. u Know Life Tables and survivorship curves. u Be able to contrast and compare “r” and “k” strategies.
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Summary u Know exponential and logistic growth curves and equations. u Know density and density- independent growth factors.
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