Populations and Carrying Capacity

Objective: Distinguish population, density, and distribution, and evaluate sampling.

A population comprises members of a defined species in a defined area and time. Population size counts individuals; density is individuals per unit area or volume; spatial distribution describes arrangement such as clumped, regular, or dispersed. Ecologists estimate abundance using quadrats, transects, or mark–recapture under method-specific assumptions. Sampling must account for unequal detectability and habitat differences; a count from one patch does not automatically represent an entire region. Immigration and emigration can change a local population without births or deaths. A model must specify boundaries and time interval before interpreting a change.

Objective: Interpret exponential and logistic models and calculate simple net change.

Population change over an interval reflects births, deaths, immigration, and emigration: ΔN = B − D + I − E. Under constant positive per-capita growth and effectively unlimited resources, an exponential model yields a J-shaped trajectory. Logistic growth introduces a carrying-capacity parameter K and slows growth as N approaches K in its simplest form. Real populations may overshoot, fluctuate, and face time-varying resources, so K is an estimate in context rather than a permanent universal ceiling. Growth curves alone do not identify the cause of a change; measurements of resources and demographic rates are needed.

Objective: Separate density-dependent effects from external disturbances.

Density-dependent factors often intensify as a population becomes crowded, such as competition for food or space and transmission of some pathogens. Density-independent disturbances, such as a severe storm or fire, can affect a population irrespective of its density at a given time, although their impacts still depend on context. A single factor can interact with others, and the labels refer to the relationship between impact and population density, not whether a factor is alive. Carrying capacity depends on resources and interactions rather than a fixed property of the species alone. Evidence for density dependence requires comparing demographic rates across densities while considering confounders.