Estimating Population Sizes and Density

How might biologists track the population size of a species, such as a group of elephants? To accurately track the population over time, they would need to account for four factors: immigration, emigration, births, and deaths.

Immigration and emigration have to do with individuals entering and leaving a population. For example, if a disturbance occurred in a nearby habitat, some elephants might immigrate, or move into, a new population. Then, competition for resources could increase, causing some elephants to move out of the population, or emigrate, to a new area.

Births and deaths also change a population size over time. Individuals have offspring, which adds more members to the population. Some individuals die each year, which reduces the population.

The growth rate of a population can be measured with an equation that takes into account these four factors that affect population size:

r = (b + i) – (d + e)

In this equation, r = population growth rate, b = birth rate, i = immigration rate, d = death rate, and e = emigration rate.

We can apply the four factors to our locker ecosystem example. A small population of fruit flies immigrated into the locker in search of food. The population increased due to the birth of a new group of fruit flies. Those flies that did not die when you swatted them in surprise emigrated away from the locker when you threw the apple away.

The growth rate of a population can be measured with an equation that takes into account these four factors that affect population size:

r = (b + i) – (d + e)

In this equation, r = population growth rate, b = birth rate, i = immigration rate, d = death rate, and e = emigration rate.

We can apply the four factors to our locker ecosystem example. A small population of fruit flies immigrated into the locker in search of food. The population increased due to the birth of a new group of fruit flies. Those flies that did not die when you swatted them in surprise emigrated away from the locker when you threw the apple away.

The growth rate equation: r = (b + i) – (d + e)

In this equation, r = population growth rate, b = birth rate, i = immigration rate, d = death rate, and e = emigration rate.

100 Flies were born, none emigrated but 15 did immigrate into the locker through a vent. 10 Flies died during this time.

Thus r= (100+ 15)-(10 + 0)

r = 105

A population may grow very rapidly, or it may grow slowly over time. Population growth may be positive, negative, or show no changes. Population growth depends on environmental conditions. The rate of growth for a population is directly determined by the amount of available resources. These resources include any of an organism’s basic needs such as food, living space, water, and oxygen. If the availability of these resources changes significantly, a change in population growth may result. There are two distinct patterns of population growth: exponential growth and logistic growth.

Almost any species that lives in ideal conditions of available resources, space, and other factors will rapidly increase in population size. This type of growth, called exponential growth, occurs when a population size increases dramatically over a relatively short amount of time.

A graph of exponential growth looks like a J-shaped curve. Exponential growth may occur when a species moves into a previously uninhabited area.

A real-world example of exponential growth in a population occurred in 1859, when an Australian landowner brought 24 rabbits into the country for sport hunting and released them into the wild. With no predators, enough space, and plentiful resources, the rabbit population grew exponentially and spread across the country. After many unsuccessful tries to control the population, Australian officials estimate today’s population to be between 100 and 200 million rabbits.