Population Attributes

Besides size and density, a population has several further characteristic attributes: natality, mortality, immigration, emigration, its age structure (often shown as an age pyramid), the shape of its expanding-population growth curve, and its overall biotic potential.

Population size / density (N): although the total number of individuals is usually the most workable measure of population density, it is not always the best one. If, in a given area, there are millions of termites or ants but only a handful of animals that feed on them, saying that the predators have a 'low population density' would be misleading -- in such cases, biomass is often a more meaningful measure of population size than a raw head count. Where a population is too large or too dispersed to count directly, ecologists commonly fall back on relative measures (for instance, the number of birds or insects caught per trap is a perfectly good proxy for their overall density) or on indirect estimation methods (the tiger census in India's national parks, for example, is largely based on counting pug marks and faecal pellets rather than the animals themselves). In nature, population size can be as small as fewer than ten individuals (as with Siberian cranes in a bird sanctuary) or run into the millions (as with Chlamydomonas in a pond).

Natality: the birth rate of a population, and generally the single biggest influence on how fast a population grows. The crude birth rate is used when working out overall population size (expressed as births per 1000 individuals per year), while a specific birth rate is calculated relative to some particular criterion, such as age. For example, if a pond held 200 carp last year, and 800 new carp were added over the year through reproduction (bringing the current population to 1000), the birth rate works out as 800 / 200 = 4 offspring per carp per year. Absolute natality is the number of births that would occur under ideal conditions -- no competition, abundant food and water -- while realized natality is the number of births that actually occur once real environmental pressures are factored in; absolute natality is therefore always greater than or equal to realized natality.

Mortality: the death rate of a population -- a measure of the number of deaths (overall, or due to some specific cause) in proportion to population size, per unit of time, typically expressed as deaths per 1000 individuals per year. A mortality rate of 9.5 (out of 1000) in a population of 1000 would mean 9.5 deaths a year in that whole population, or 0.95% of it. Absolute mortality is the number of deaths under ideal conditions (no competition, abundant resources), while realized mortality is the number of deaths once environmental pressures come into play; absolute mortality is therefore always less than or equal to realized mortality.

Sex ratio: the ratio of individuals of one sex to individuals of the other sex in a population. It both affects, and is affected by, the population's birth, death, immigration and emigration rates. A ratio of 1:1 males to females is generally the most common evolutionarily stable strategy (ESS) found in nature. …