Population Ecology

Chapter 53

Population Ecology

•Population = individuals of a given species living together at the same place

•Distributions

–species have varying geographic ranges.

–ranges change through time.

•Habitat changes

•species invades previously inaccessible habitat

–Size of range & number of individuals associated w/ genetic drift & bottleneck effect

Population Dispersion

•Randomly Spaced

•Uniformly Spaced

–resource competition.

•Clumped Spacing

–uneven distribution of resources in immediate environment.

Population Dispersion

•Dispersal Mechanisms

–Plants = Light Seeds, Hooks, Fruits

–Animals = floating larvae, parasitic larvae, long dispersal flights, aggressive social interaction

•Human Effect

–Habitat alteration allows some species to expand ranges

–Causes others to be reduced

Metapopulations

•Metapopulation = distinct populations interacting with moderate to low gene flow

–Degree of interaction depends on dispersal.

–Must have corridors for dispersal/gene flow

–Repopulation of areas from which species was extirpated by individuals from neighboring populations

Demography

•Demography - study of population growth and composition.

•Factors Affecting Growth Rates

–Generation Time

•Interval between birth of an individual and birth of its offspring.

–Sex Ratio

•Proportion of males : females

•Influence associated w/ reproductive strategies

Demography (cont.)

•Age Structure – describes proportion of population at each given life stage

–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.

Demography (cont.)

•Life Table – tabulation of complete mortality data for population with respect to age

–Describes probability of death throughout full lifespan of population

•Survivorship Curves

–Survivorship is the percentage of an original population that survives to a given age.

•Type I - Full Life Span

•Type II - Mortality un-related to age

•Type III - Early Susceptibility

Survivorship Curves

Cost of Reproduction

•Life history = life cycle of an organism.

–Different “strategies” have different advantages

•Due to limited resources, increased reproduction may decrease survival and chances of future production.

–Cost of Reproduction

»Predict that selection will favor life history that maximizes lifetime reproductive success.

Fecundity and Mortality Correlation in Birds

Cost of Reproduction (cont.)

•Investment Per Offspring

–Reproductive tradeoff balances amount of resources in each offspring versus number of offspring.

•offspring size critically affects chances of survival.

Cost of Reproduction

•Reproductive Events Per Lifetime

–Semelparity - single, large reproductive event.

–Iteroparity – reproduce several times over many seasons.

•Age at First Reproduction

–Longer-lived animals generally reproduce later, provide more parental care than shorter-lived animals.

“Normal” Populations

•More offspring born than will survive to reproduce

–Generally balance of several variables:

•Age at first reproduction

•Periodicity of reproduction

•Fecundity

•Mortality

•Immigration/emigration

–Generally stable population size from generation to generation

Exponential Growth Model

•Removal of one or more of variables that maintains “Stable” population size.

•Nearly all offspring produced in population survive and reproduce

–capacity for growth of any population is exponential.

•Even when rate remains constant, actual increase in number accelerates as the population size grows.

Population Growth

•Carrying Capacity

–Carrying capacity = maximum number of individuals resources in given area can support.

•Exponential growth beyond carrying capacity often followed by population crashes

•Logistic Growth

–As a population approaches carrying capacity, growth rate slows as resources become depleted.

Population Growth

Influence of Population Density

•Density Dependent Population Controls

–Populations approaching carrying capacity = increased competition for resources = decreased birth rate & increased mortality.

–In some cases, growth rates increase with population size (locusts)

•Easier to find mates

•Often followed by population crash

Influence of Population Density

•Density Independent Population Controls

–Growth limited by factor other than population size.

•External environmental conditions.

•Population Cycles

–Snowshoe Hares

•Food Plants (willows) and Predators (Lynx) both control population.

Predator-Prey Cycle

Population Growth and Life History Models

Survivorship Curves

Population Growth and Life History Models

Exponential Growth in Humans

•In past centuries, human populations regulated by food availability, disease, and predators.

–environmental restraints reduced by agriculture & medicine,

–explosive human population growth

Tracking Exponential Growth

•Population Pyramids

–Bar graph displaying number of people in each age category.

•Stable - Rectangular

•Rapid Growth - Triangular

•Decreasing - Inverted Pyramid

Population Pyramids

•Uncertain Future

–Rapidly growing human population is and will continue to stress global ecosystems

•Raw materials

•Food resources

•Waste disposal

–Uneven Resource Distribution

•Population Distribution

•Resource use gap

Worldwide Population Growth