Population Ecology Chapter 27

Population Ecology Certain ecological principles govern the growth and sustainability of all populations Human populations are no exception

Limits to Growth A population’s growth depends on the resources of its environment Deer introduced to Angel Island –Population outstripped resources

Human Population Problems Over 6 billion people alive About 2 billion live in poverty Most resources are consumed by the relatively few people in developed countries

Population A group of individuals of the same species occupying a given area Can be described by demographics –Vital statistics such as size, density, distribution, and age structure

Population Age Structure Divide population into age categories Population’s reproductive base includes members of the reproductive and pre- reproductive age categories

Density & Distribution Number of individuals in some specified area of habitat Crude density information is more useful if combined with distribution data clumped nearly uniform random

Determining Population Size Direct counts are most accurate but seldom feasible Can sample an area, then extrapolate Capture-recapture method is used for mobile species

Capture-Recapture Method Capture, mark, and release individuals Return later and capture second sample Count the number of marked individuals and use this to estimate total population

Assumptions in Capture-Recapture Marking has no effect on mortality Marking has no effect on likelihood to being captured There is no immigration or emigration between sampling times

Changes in Population Size Immigration adds individuals Emigration subtracts individuals Births add individuals Deaths subtract individuals

population growth per unit time net population growth rate per individual per unit time number of individuals =X in-text, p. 688

Zero Population Growth Interval in which number of births is balanced by number of deaths Assume no change as a result of migration Population size remains stable

Per Capita Rates Rates per individual Total number of events in a time interval divided by the number of individuals Per capita birth rate per month = Number of births per month Population size

r Net reproduction per individual per unit time Variable combines per capita birth and death rates (assuming both constant) Can be used to calculate rate of growth of a population

Exponential Growth Equation G = rN G is population growth per unit time r is net reproduction per individual per unit time N is population size

Exponential Growth Population size expands by ever increasing increments during successive intervals The larger the population gets, the more individuals there are to reproduce

Effect of Deaths Population will grow exponentially as long as per capita death rates are lower than per capita birth rates 25% mortality between divisions

Biotic Potential Maximum rate of increase per individual under ideal conditions Varies between species In nature, biotic potential is rarely reached

Limiting Factors Any essential resource that is in short supply All limiting factors acting on a population dictate sustainable population size

Carrying Capacity (K) Maximum number of individuals that can be sustained in a particular habitat Logistic growth occurs when population size is limited by carrying capacity

Logistic Growth Equation G = r max N (K-N/K) G = population growth per unit time r max = maximum population growth rate per unit time N = number of individuals K = carrying capacity

Logistic Growth As size of the population increases, rate of reproduction decreases When the population reaches carrying capacity, population growth ceases carrying capacity Time

animation Click to view animation.

Overshooting Capacity Population may temporarily increase above carrying capacity Overshoot is usually followed by a crash; dramatic increase in deaths

Density-Dependent Controls Logistic growth equation deals with density-dependent controls Limiting factors become more intense as population size increases Disease, competition, parasites, toxic effects of waste products

Density-Independent Controls Factors unaffected by population density Natural disasters or climate changes affect large and small populations alike

Fig. 40.6, p. 690 initial carrying capacity new carrying capacity