Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Population Ecology

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Intro to Population Ecology A.Population ecology – investigates the quantitative dynamics of how individuals within a species interact B.Populations differ in factors such as their distribution, density, age structure, sex-ratio, and numbers or size. C.Population dynamics is a study of how these characteristics of populations change in response to changes in environmental conditions (including temperature, disease, resource availability, and competing species).

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings I. Population Density and Distribution A. Population density = the number of individuals within a population per unit area 1) High densities make it easier to find mates, but increase competition, and vulnerability to predation 2) Low densities make it harder to find mates, but individuals enjoy plentiful resources and space I can’t find a date, but at least there are a lot of zebras to eat!

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings I. Population Density and Distribution B. Population distribution (dispersion) is the spatial arrangement of organisms within an area. There are three general patterns of population distribution within a habitat 1) Random dispersion 2) Uniform dispersion 3) Clumping

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings C. Random dispersion – haphazardly located individuals, with no pattern D. Uniform dispersion – individuals are evenly spaced due to territoriality E Clumped– arranged according to availability of resources -Most common in nature

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings I. Population Density and Distribution F. Why is clumping both so common and so valuable to species? 1)Organisms tend to cluster where the resources are available – cottonwood trees along streams, fish along upwellings, ect. 2)Individuals in groups tend to have a better chance of encountering clumps of resources. 3)Living in groups protects some animals from predators (schools of fish) 4)Hunting in packs gives predators better chances of catching prey.

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings A. Sex ratio = proportion of males to females -In monogamous species, a 50/50 sex ratio maximizes population growth B. Age Structure = the relative numbers of organisms of each age within a population -Age structure diagrams (pyramids) = show the age structure of populations II. Population Age Structure and Sex Ratio

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings II. Population Age Structure and Sex Ratio C. A population’s age structure can have a strong effect on how rapidly it increases or decreases in size. D. Age structures are usually described in terms of three groups: 1)Pre-reproductive stage – organisms not mature enough to reproduce 2)Reproductive stage – Reproduction physically possible 3)Post reproductive stage – those too old to reproduce

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings II. Population Age Structure and Sex Ratio E. The size of a population is likely to increase if it is made up mostly of individuals in their reproductive stage or soon to enter this stage. F. In contrast, a population dominated by individuals past their reproductive stage will tend to decrease over time. G. Excluding emigration and immigration, populations with fairly even distribution among the three age groups trends to be relatively stable, because births balances deaths.

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings III. Population Size A. Population size = the number of individual organisms present at a given time -Numbers can increase, decrease, cycle or remain the same

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings III. Population Size B. Four variables govern changes in population size – births, deaths, immigration, and emigration. C. Populations increase by birth and immigration (arrival of individuals from outside the population) and decrease through death and emigration (departure of individuals from a population)

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings III. Population Size D. Population factors 1) Natality births within the population 2) Mortality = deaths within the population 3) Immigration = arrival of individuals from outside the population 4) Emigration = departure of individuals from the population E. Population Change = (Births + immigration) – (deaths + emigration)

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings IV. Population Growth – J and S curves A.Species vary in their biotic potential, or capacity for population growth under ideal conditions. B.Generally, populations of species with large individuals, such as elephants and blue whales, have low biotic potentials. Small species, such as bacteria and insects, have high biotic potential. C.The intrinsic rate of increase (r) is the rate at which the population of a species would grow if it had unlimited resources.

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings IV. Population Growth – J and S curves D. Individuals in populations with a high intrinsic rate of growth typically reproduce early in life, have short generation times, can reproduce many times, and have many offspring each time they reproduce. E. However, research reveals that no population can grow indefinitely because of limits on resources and competition with other species for resources.

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings F. Exponential population growth 1) Growth by a certain percentage - growth starts slowly but then accelerates as the base population increases -Something increases by a fixed percent -Graphed as a J-shaped curve 2) Exponential growth cannot be sustained indefinitely -It occurs in nature with a small population and ideal conditions

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings G. Limiting factors restrain growth 1) Limiting factors = physical, chemical and biological characteristics that restrain population growth -Water, space, food, predators, and disease 2) Environmental resistance = All limiting factors taken together

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings H. Population density affects limiting factors Density-dependent factors = limiting factors whose influence is affected by population density - Increased risk of predation and competition for mates occurs with increased density Density-independent factors = limiting factors whose influence is not affected by population density -Events such as floods, fires, and landslides

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings I. Carrying capacity 1) Carrying capacity (K) = the maximum population size of a species that its environment can sustain -An S-shaped logistic growth curve -Limiting factors slow and stop exponential growth

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings Perfect logistic curves aren’t often found

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings V. Biotic potential and reproductive strategies vary A. K-selected species = animals with long gestation periods and few offspring -Have a low biotic potential -Stabilize at or near carrying capacity -Good competitors B. r-selected species = animals which reproduce quickly -Have a high biotic potential -Little parental care

Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings

K-selected vs. r-selected species