C HAPTER 51 Population Ecology

E COLOGY B ASICS Terms to know… Ecology Branch of biology Relatively new science Biotic factors Abiotic factors Environmental science ecology + human interactions Population same species, same area, same time Population ecology numbers + changes

F EATURES OF POPULATIONS Properties that individuals lack: Population density Population dispersion Birth/death rates Growth rates Survivorship rates Age structure Properties that communities lack: Common gene pool Reproductive success Evolution Economic importance (crops, forests, game animals…)

P OPULATION DYNAMICS Density – the number of individuals per unit of area or volume at a given period of time Affected by: Habitat Season Other populations Weather Limiting factors : the number of individuals in a population is controlled by the ability of the environment to support it Density-dependent factors – the effect increases as population density increases Examples: Density-independent factors – affects the size of a population but is not influenced by changes in population density; typically abiotic Examples:

P OPULATION DYNAMICS … Dispersion – spacing in relation to other members of the population Three basic varieties: 1. Clumped (aggregated distribution, patchiness) Individuals are concentrated in specific areas Reasons: distribution of resources, social behavior of animals (herds, family groups), reproduction Advantages: reduced chance of predation 2. Uniform Individuals are fairly evenly spaced Reasons: social behavior of animals (territories), high levels of competition between individuals Advantages: reduced competition 3. Random Individual spacing is unrelated to others in the population Does not occur often in nature

C HANGES IN POPULATION SIZE Per capita – per individual Natality – average per capita birth rate ( b ) Mortality – average per capita death rate ( d ) Immigration – individuals entering a local population ( i ) Emigration – individuals leaving a local population ( e ) Population growth rate ( r) : r = ( b + i ) – ( d + e ) If r = positive number  population is increasing If r = negative number  population is decreasing If r = zero  population is staying the same Examples:

I NTRINSIC RATE OF INCREASE Maximum rate of increase when: Conditions are ideal Resources are abundant Population density is low r max Factors which influence this: Age at which reproduction begins The fraction of the life span devoted to reproduction The number of reproductive cycles The number of offspring produced each cycle Different species have different intrinsic rates… Small organisms have high rates (bacteria); large species have low rates (elephants)

E XPONENTIAL POPULATION GROWTH Optimal conditions allow a constant per capita population growth ( r max ) The larger the population gets, the faster it grows J shape curve: Organisms cannot reproduce this way indefinitely because of increased: competition, predation, disease, wastes

L OGISTIC POPULATION GROWTH Population growth rate nears zero Occurs near the environment’s limits to support the population Carrying capacity (K) – the largest population that an area can maintain indefinitely, assuming no changes in the environment S shape curve: