1 Population Ecology

2 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.

3 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

4 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.

5 Population Dispersion

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7 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

8 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.

9 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

10 Survivorship Curves

11 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.

12 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.

13 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.

14 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 = r i N

15 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 = r i N

16 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.

17 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

18 The Carrying Capacity K

19 The Carrying Capacity K

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