Population, community and ecosystem organisation

Living organisms can be considered at connected levels of organisation, from one species in an area to all the organisms and their surroundings.

A population is a group of organisms of the same species living in the same area at the same time. For example, all the frogs of one species living in a pond at the same time form a population.

A community consists of all the populations of different species in an ecosystem. In the pond, the frog population is part of a community that also includes populations of plants, other animals and microorganisms.

An ecosystem includes the community and its physical environment as an interacting unit. In the pond, organisms interact with one another and with their surroundings, including the water, light and temperature.

Populations therefore make up a community, while the community and the environment together form an ecosystem.

Factors affecting population growth

A population is a group of organisms of the same species living in an area. Its size increases when births exceed deaths and decreases when deaths exceed births. Factors that affect survival or reproduction therefore affect how quickly the population changes.

Food supply and competition

When food is plentiful, more organisms can survive and reproduce, supporting population growth. If food becomes scarce, fewer organisms may survive or reproduce, so growth slows or the population decreases.

Organisms compete when they need the same limited resources. As a population grows, more individuals may compete for the available food and other resources. Some may not obtain enough to survive or reproduce, reducing population growth.

Predation and disease

Predation occurs when one organism kills and eats another. If more members of a population are killed by predators, deaths increase. This can slow population growth or cause the population to decline.

Disease can also increase deaths in a population. If disease causes deaths to exceed births, the population decreases.

The combined effects of food supply, competition, predation and disease help determine the number of births and deaths, and therefore the rate and direction of change in population size.

Population growth curves

A population growth graph shows how the number of individuals in a population changes over time. Time is usually shown on the horizontal axis and population size on the vertical axis. The shape of the curve shows both the direction and the rate of change.

Phases of a population growth curve

In a resource-limited environment, a growth curve may pass through these phases:

1. Lag phase: Population size increases slowly. The curve rises only gently because the number of individuals is changing slowly.
2. Exponential or log phase: Population size increases rapidly. The curve becomes steeper as the population grows more quickly.
3. Stationary phase: Population size levels off and remains approximately stable. Resources are limited, so the population can no longer continue growing rapidly. A stable population size does not mean that individual organisms stop being born or dying; rather, additions to the population approximately balance losses.
4. Death phase: Population size decreases, so the curve slopes downwards. More individuals are lost from the population than are added.

A death phase is not inevitable: a population may remain approximately stable while conditions support it.

Reading the curve

A curve that slopes upwards shows an increasing population; a horizontal curve shows an approximately stable population; and a curve that slopes downwards shows a decreasing population. The steeper the slope, the faster the population size is changing. A shallow slope indicates a slower change, whether the population is increasing or decreasing.

Causes of population growth phases

A population grows when more organisms are born than die, and decreases when more die than are born. Its growth phases reflect changes in births and deaths as limiting factors, such as food shortage and disease, affect survival and reproduction.

Lag phase. The population is small, so relatively few organisms are reproducing. The number of births is low, and the population grows slowly.

Exponential or log phase. Resources are readily available and limiting factors have relatively little effect. Births exceed deaths, so the population increases rapidly.

Stationary phase. As the population grows, food becomes more limited and competition for it intensifies. Food shortage and competition can reduce survival or reproduction; predation and disease can also limit the population. Births and deaths become approximately balanced, so population size remains roughly constant even though individuals are still being born and dying.

Death phase. If limiting factors reduce survival or reproduction enough that deaths exceed births, the population decreases. Food shortage, competition, predation and disease can all contribute to this decline.