CHAPTER 51Population Ecology

ECOLOGY BASICS

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

FEATURES 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…)

POPULATION 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:

POPULATION 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

CHANGES 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:

INTRINSIC RATE OF INCREASE

Maximum rate of increase when: Conditions are ideal Resources are abundant Population density is low

rmax

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)

EXPONENTIAL POPULATION GROWTH

Optimal conditions allow a constant per capita population growth (rmax)

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

LOGISTIC 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: