Chapter 51 Population Ecology

Ecology Basics Terms to know… Ecology Biotic factors Abiotic factors 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/Weather Other populations 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

Population dynamics… Dispersion – spacing in relation to other members of the population Three basic varieties: 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 Uniform Individuals are fairly evenly spaced Reasons: social behavior of animals (territories), high levels of competition between individuals Advantages: reduced competition 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:

Density-dependent regulatory factors Cause an increase in death rate as the population increases Affect a larger proportion of the population, not just a larger number Can also cause a decrease in death rate as the population decreases Tend to regulate a population at a relatively constant size near the carrying capacity An example of a negative feedback system

Cyclical population fluctuations

Intraspecific competition Interference competition Also called contest competition Dominant individuals obtain resources at the expense of others (they interfere) Can cause small drops in population as the individuals unable to compete die Exploitation competition Also called scramble competition All individuals share the limited resource Can cause wide variation in population size

Life history traits Reproductive strategies differ: Two extremes: Semelparous – a single, large reproductive effort Examples: Iteroparous – repeated reproductive cycles over the life span Two extremes: r-selected – rapid population growth rate Small size, early maturity, short life span, large broods, little or no parental care Found in variable, temporary, or unstable environments K-selected – keeps population at or near carrying capacity Large size, late reproduction, long life span, small broods, parental care of young Found in constant or fairly stable environments

Survivorship curves Measures the probability of survival to a particular age Three basic types: Type I – young have a high chance of survival, probability of survival decreases with age humans Type II – the probability of survival does not change with age squirrels Type III – the probability of death is highest early in life oysters

metapopulations Different populations within an ecosystem Due to variations of habitats: Source habitats – high quality habitats, plenty of resources, lots of reproductive success, greater population density Sink habitats – lower-quality habitats, fewer resources, less reproductive success, lower population density Humans are increasing the number of metapopulations as they fragment ecosystems

Human Populations Our population continues to increase, but this is due to a decrease in the death rate rather than an increase in the birth rate. This decrease is due to: Greater food production Better medical care Improved sanitation practices Zero population growth (r = 0)

Human Demographics Highly developed countries: Developing countries: Low population growth rates Highly industrialized Low infant mortality rates Example: Developing countries: Moderately developed Birth rates and infant mortality rates high but declining Medium industrialization Examples: Less developed Highest birth and infant mortality rates Lowest amounts of industrialization

Age Structure Diagram The number and proportion of people at each age of a population

Age Structure Diagrams… Rapid growth: Pyramid shaped – as children mature, they become parents of the next generation and there are more of them than the previous group Slower growth or decline in population: More tapered bases – the number of prereproductive and reproductive ages are close to the same  slower, but still positive growth Small bases – a higher percentages of postreproductive ages than reproductive and prereproductive ages  negative growth (decline)

Environmental degradation People overpopulation…. Developing countries Examples: Consumption overpopulation… Developed countries