Ecosystem resilience and change

Ecosystems are dynamic, containing many interacting living and nonliving components. The variety of these components and interactions contribute to an ecosystem’s complexity. Ecosystems can differ in their make-up and, as a result, some ecosystems are more complex than others.

Scientists can look at a variety of factors to help determine the complexity of an ecosystem. One of these factors is species richness, or the total number of species in a community.

Another factor is species diversity, which takes into account species richness as well as the relative number of individuals within each species. An ecosystem that is dominated by a single species is typically considered less diverse than an ecosystem with a more even distribution of species. Species diversity is usually calculated using a mathematical model called an index.

To help illustrate the difference between species richness and species diversity, imagine two communities, each with four different species. In Community 1, the majority of individuals come from just one of the species. In Community 2, the individuals are spread more evenly across the species.

In this example, both Community 1 and Community 2 have four species, so they have the same species richness. However, the relative abundance of each species is more evenly distributed in Community 2, so Community 2 has greater species diversity.

Disturbance, or disruption, is a normal phenomenon in ecosystems, and it can range in severity from mild to extreme. Moderate disturbances, such as localized weather events, can actually promote species diversity by freeing up resources and increasing habitat complexity. In contrast, extreme disturbances, such as large-scale natural disasters, can greatly change an ecosystem, altering community structures and reducing species diversity.

Ecological succession describes the sequence of changes that occur in an ecosystem after a disturbance. Primary succession is the initial colonization of an area by living organisms. This occurs in relatively rare cases, such as when volcanic activity creates a new landscape. Most succession is a form of secondary succession, which is the recolonization of an area after a disturbance. This can occur after a natural disaster, such as a fire or a hurricane.

Ecosystems that are more complex are more resilient, or better able to tolerate and recover from disturbances, than ecosystems that are less complex. To help illustrate why this is, imagine a complex ecosystem with many components and many interactions between those components. If a disturbance decreases the availability of a food source for a certain species, that species will not be greatly affected because it is likely able to get food from a variety of sources.