For each species, communities have numbers {population, ecology} of individuals. Populations have relative numbers at each age range {age distribution}.
Population-change rate depends on growth rate, population, and available resources {carrying capacity, population}|: dN / dt = r * N * (1 - N/K), where N is total population, r is growth rate, and K is maximum possible population.
Populations {deme}| {genetic population} can be one interbreeding species.
Regions have average distances {dispersal}| {dispersion, distance} among animals. Emigration, immigration, and migration rates depend on mobility and terrain. Populations disperse until reaching barriers.
Populations have numbers of individuals per area {population density, ecology}.
effects
Denser populations have more pathogens, parasites, and competition. Infanticide, cannibalism, competition, disease, genetic change, breeding-behavior changes, and food-supply changes can increase.
regulation
Species have population-density-dependent population controls, to avoid extinction. Controls can reduce population if density increases: increased predation by predators, decreased food supply, increased territoriality, increased emigration, reduced hormones from increased stress, reduced fertility, and more inhibited development. Low-density populations and stable populations respond quicker to population-density-dependent parameters. High-density and unstable populations respond slower to density-dependent parameters and respond at higher thresholds.
In environments with different heights, such as rain forests, populations differ at different heights {vertical stratification}|.
Populations can grow {population growth} exponentially (Malthus). At time t, number n is growth rate r times number n at previous time t - 1: n(t) = r * n(t - 1).
curve
Population growth often has S-shaped curves, with slow increase at first, then exponential growth, and then flattening rate when approaching environmental capacity. Populations typically increase until stopped by environmental shortages.
cycles
Lemming and snowshoe-hare populations have growth and decline cycles. Perhaps, crowding and competition stresses cause cycles.
Undercrowding can be population-limiting factor {Allée's principle} {Allée principle}.
To account for death from food lacks, predators, and diseases, current number n(t) is growth rate r times previous number n(t - 1) times one minus previous number {logistic difference equation}: n(t) = r * n(t - 1) * (1 - n(t - 1)).
If environment has no limiting food, predators, or disease factors, population-increase rate {biotic potential} is maximum.
If no limiting food, predators, or disease factors affect reproduction, population-increase rate {reproductive potential}|is maximum.
Populations have births per 1000 people each year {birth rate}|.
Populations have deaths per 1000 people each year {death rate}|.
Populations have number who survive per 1000 people each year {survival rate}|, which is opposite of death rate.
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Date Modified: 2022.0225