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Population Ecology

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Environmental Variation

• Key elements of an organism’s environment include:

– temperature– water– sunlight– Soil– Classical limiting factors

• Many organisms actively employ mechanisms to maintain homeostasis, while others conform to their environment.

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Populations• A population consists of individuals of a given species

living together at the same place at the same time.• Populations have characteristic features -

Geographic location density distribution demography

• Population distributions– Most species have a limited geographic range.– Population ranges change through time.

environment changes organisms circumvent previously inhospitable

habitat

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Population Dispersion

• Randomly spaced– Individuals are randomly spaced within a

population.• Uniformly spaced

– Uniform spacing within a population often results from resource competition.

• Clumped spacing– Individuals clump into groups or clusters in

response to uneven distribution of resources in their immediate environment.

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Population Dispersion

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Population Dispersion

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Demography

• Demography - statistical study of populations• Factors affecting growth rates

– sex ratio effect also determined by mating habits

– generation time average interval between birth of an

individual and the birth of its offspring

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Demography

• Age structure– cohort - individuals of the same age

fecundity - number of offspring produced in a given period of time

mortality - number of deaths in a given period of time

– Age structure determined by the relative number of individuals in each cohort.

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Demography

• Life tables– constructed by following the fate of a cohort

from birth until death• Survivorship curves

– Survivorship is the percentage of an original population that survives to a given age.

Type I - full life span Type II - mortality unrelated to age Type III - early susceptibility

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Survivorship Curves

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Cost of Reproduction

• A life history is the complete life cycle of an organism.

– all involve significant trade-offs Due to limited resources, increased

reproduction may decrease survival and chances of future production.

cost of reproduction Natural selection will favor the life

history that maximizes lifetime reproductive success.

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Cost of Reproduction

• Investment per offspring– Key reproductive tradeoff concerns the

amount of resources to invest in producing any single offspring.

number of offspring versus size of each offspring

In many species, offspring size critically affects chances of survival.

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Cost of Reproduction

• Reproductive events per lifetime– semelparity - organisms focus all

reproductive efforts on a single, large event– iteroparity - organisms produce offspring

several times over many seasons• Age at first reproduction

– Longer-lived animals tend to reproduce later, and provide more parental care than shorter-lived animals.

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Population Growth

• Darwin recognized that organisms can out reproduce their environmental resources

• Biotic potential - the rate at which a population of a species will increase when there are no limits on its rate of growth

• Mathematical terms -

dN/dt = riN

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Where:• N = number of individuals in the population• dN/dt = the rate of change of population

numbers over time• r = intrinsic rate of increase for that

population (intrinsic capacity for growth)

r is difficult to calculate and is often considered as the difference between birth rate and the death rate

dN/dt = riN

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Biotic Potential• Exponential growth model

– Rate at which a population of a given species will increase when no limits are placed on the rate of growth.

Innate capacity for growth of any population is exponential.

Even when rate remains constant, actual increase in number accelerates as the population size grows.

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Biotic Potential

• Carrying capacity– Carrying capacity (K) is the maximum

number of individuals the resources in a given area can support.

• Logistic growth– As a population approaches carrying

capacity, its growth rate slows as resources become depleted.

sigmoidal growth curve

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The Carrying Capacity

K

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The Carrying Capacity

K

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