In this topic you will learn about how ecosystems can change over time.

The gradual replacement of one community by another is called ecological succession. Ecological succession can begin in two different kinds of places. It can begin where a community already exists, such as an abandoned farm field. Ecological succession in a place where a community already exists is called secondary succession.

Ecological succession can also happen where there are few, if any, living things. This is called primary succession. Primary succession can begin where communities were wiped out. Such places would include land swept clean by a volcanic eruption or a forest fire. The first species to be living in an otherwise lifeless area is a pioneer species. Fireweed is an example of a pioneer species. It is often the first plant to grow after a forest fire. Plants like a fireweed in bloom attract animals such as insects and an occasional insect-eating bird. A new community, called a pioneer community—the first community living in an area—begins to thrive.

The first organisms in a pioneer community are not always plants. This is usually the case in newly formed volcanic islands that rise from the sea. Here the pioneer community is made up of bacteria, fungi, and algae.

Over the years pioneer species slowly break down rock into soil. Many years will pass, perhaps hundreds of them. The climate will remain pretty much unchanged. The community will continue to grow. The populations will become stable and balanced. Few new animals and plants will arrive. Few will leave. Eventually ecological succession will slow down or stop altogether. A scientist visiting the area will see a climax community. A climax community is the final stage of succession in an area, unless a major change happens.

Certain kinds of events can change an entire ecosystem. A hurricane may sweep over the area, ripping out trees. A volcano might erupt. People might build hotels or introduce new plants or animals. The climate might change. Then the process of ecological succession would begin all over again. A new climax community would eventually develop. This community might or might not be the same as the previous climax community.