Ecosystems constantly change as organisms interact with one another and their environment. These changes can occur slowly or be triggered by sudden events, and they shape the structure of biological communities.

A community is made up of different populations that live in the same area and interact with one another. It includes the types and numbers of species present, as well as their feeding relationships and interactions. Changes in ecosystems are often caused by disturbances, such as fires, storms, or droughts, which can alter populations and communities.  The survival of organisms in a community depends on environmental conditions. A limiting factor is any condition, such as water availability, that restricts an organism’s ability to survive or reproduce. For example, a lack of water can cause grass to die, which may lead to a decline in animal populations that depend on it. Tolerance refers to an organism’s ability to withstand changes in its environment; species with greater tolerance are more likely to survive disturbances.

Ecological succession is the gradual, natural process of change and species replacement in a community over time. One type, primary succession, occurs in areas where no life previously existed, such as after a volcanic eruption. In these environments, the land is initially bare.

The first organisms to colonize the area are called pioneer species, such as lichens and mosses. These organisms begin to break down rock and contribute organic material, helping to form soil. As soil develops, it can support grasses, herbs, and small shrubs. Over time, more complex communities form, including insects, small animals, and eventually larger plants and animals.  This process can take hundreds to thousands of years, gradually transforming a lifeless area into a stable, thriving ecosystem.

Secondary succession is the pattern of change that occurs after an existing community is disturbed or destroyed, but the soil and some organisms still remain. Common causes include wildfires, floods, and storms.  For example, after a wildfire, nutrients are returned to the soil from burned plants, and some seeds are activated by heat. Within a few years, grasses, weeds, and small plants begin to grow. These plants support small animals, such as insects and rodents, which find food and shelter in the new vegetation.

Over time, larger plants begin to grow. After about 20 years, shrubs and young trees, such as pines and cedars, begin to form a developing forest. As the habitat becomes more complex, it can support a wider variety of animals, including birds and small mammals.  After many decades, the forest continues to grow and mature. Tall trees, such as pines and oaks, dominate the area, providing shelter and resources for larger animals like deer and bears.  Eventually, the ecosystem reaches a climax community, a stable stage where species composition changes very little over time unless another disturbance occurs.

Review:

1. What is a pioneer species?
2. Compare primary succession and secondary succession.
3. What is a climax community?