Population Growth
How do Ecologists study populations?
There are four areas that ecologists look at when studying populations. They are geographic range, density and distribution, growth rate, and age structure.
- Geographic range - this is the space that a population inhabits. It varies in sized depending on the size and species of the population. For example, a population of bacteria could inhabit an area the size of a deck of cards, while a population of polar bears can inhabit an area that is hundreds of square miles
- Density and Distribution - population density is the number of individuals that live in a particular unit area. In the map below, the population density of the United States is measured by the number of people that live in a square mile. Distribution is how the population is grouped together in that square mile area.
- Growth Rate - this determines whether the size of a population will grow or decline. A population with high growth rate will increase in numbers over time because new individuals are being produced faster than other individuals die off. A negative growth rate means that individuals in a population are dying off faster than new individuals are being produced.
- Age Structure - it is important to know the number of individuals of a population that make up certain age ranges. This means that we need to know how many individuals of a certain age exist in a population. This is important because age often determines if an organism can reproduce. If a population of animals is made up of mostly very young or very old individuals, it will not be able to grow at the same rate as a population of animals that is made up of individuals that are at a mature reproductive age.
What factors affect population growth?
- Birthrate and Deathrate - the relationship of the number of births in a population to the number of deaths is one of the factors that affect population growth. If there are more births than deaths, the population will grow, and if there are more deaths than births, the population will decrease.
- Immigration - this is the process of new individuals arriving into an area, ultimately increasing the size of the population.
- Emigration - this is the process where individuals leave a population causing the size of the population to decrease.
What happens during exponential growth?
Imagine that a small population of two cockroaches were to be placed in an environment where they had an unlimited supply of all the resources they could ever need. What do you think would happen to the size of that population? The population would begin to grow. To start off, the growth would be relatively slow as the two cockroaches are limited to the number of offspring they can have; however, as more offspring join the population, the new cockroaches would have their own offspring. With unlimited resources, this process would continue, with the growth rate of the population rapidly increasing with each successive generation. This is called Exponential Growth. The graph below models this type of growth.
Only populations of organisms that can reproduce rapidly, such as bacteria, can grow exponentially.
What is logistic growth?
Logistic growth is the most common growth model found in nature. Because there is rarely a situation where unlimited resources exist, eventually a population size will max out at a particular size because all the resources in the area are being utilized. Logistic growth has three phases.
- Phase I - The lag phase. This is the time where a small population begins to reproduce. Growth is slow until there are enough individuals of reproductive age to begin increasing the growth rate.
- Phase II - The S-phase. This phase occurs when the population size gets large enough that exponential growth starts taking place.
- Phase III - Carrying Capacity. This is the point where all of the resources of an area are being utilized so that there are not enough resources left to support new individuals. The population size is said to be in equilibrium, where the birthrate is roughly equal to the deathrate.