Population attributes

A population is a group of individuals of the same species living in a defined area at the same time, sharing or competing for the same resources, and able to interbreed. Lotus plants in a pond, the deer in a forest, or the people of a village are populations.

A single animal is born and dies; it cannot have a 'birth rate'. But a population has features that individuals do not, its attributes:

• Birth rate and death rate, written per individual (per capita) over a period of time. If a pond with 50 frogs gains 10 young in a year, the birth rate is 10 ÷ 50 = 0.2 per frog per year. If 3 of them die in that year, the death rate is 3 ÷ 50 = 0.06.
• Sex ratio: the proportion of females to males, for example 60% females and 40% males.
• Age distribution: the share of pre-reproductive, reproductive and post-reproductive individuals. Drawn as bars, this makes an age pyramid. A wide base means many young: a growing population. A base about as wide as the middle means a stable one. A narrow base means a declining one.

Population size, or density (N), is the most important attribute, but it is not always counted as a simple number:

• A single huge banyan tree shading many small plants is better measured by per cent cover or biomass than by counting '1'.
• Sometimes a relative count is enough: the number of fish caught per trap shows whether a lake's fish are increasing.
• Some animals are hard to see, so they are counted indirectly. Tigers in national parks are counted from their pug marks (footprints) and fecal pellets.

Four things change a population's size:

• Natality (births) and immigration (individuals arriving) increase it.
• Mortality (deaths) and emigration (individuals leaving) decrease it.

So, if the population is Nₜ at time t, then one time unit later:

Nₜ₊₁ = Nₜ + [(B + I) − (D + E)]

For example, 50 frogs with 20 births, 5 deaths, 3 arrivals and 2 departures become 50 + 23 − 7 = 66 frogs.