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Communities and Ecosystems

Ecological Communities

A group of species living together is an ecological community in which they grow, reproduce to create offspring, and survive through periods of succession in their own habitat.

When groups of organisms naturally occur together and interact, an ecological community is formed. An ecological community is a group of species in a specific area. Within a community, a network forms in which species are connected because of direct interactions they have with each other, such as those connected through a food web or a parasite and its host. These interactions can indirectly affect the entire ecological community. Types of ecological communities include grasslands, forests, and wetlands. For example, consider the diversity of species that inhabit a tropical rainforest. The species that live here are an ecological community. From the birds that live in the canopy created by trees to the snakes that feed on the frogs, several interactions take place within this community. Because of these interactions, every organism must face the challenge of survival, but in different ways. Each organism thus plays a specific role within a community.

The role of a species within an ecosystem is called a niche. A niche is all interactions by a species with the living and nonliving things in an environment. It describes an organism's role with the flow of energy through a community. Energy flow refers to how organisms obtain energy—often through what they eat—and how this energy is passed through the ecosystem. A niche also includes an organism's role in interacting with other species in an ecosystem. However, a niche is not the same as a habitat. Habitat describes the physical area where an organism lives, while niche focuses on the organism's role or pattern of living within the habitat. For example, a dung beetle of South Africa specifically lives in and feeds on the dung of wild elephants and buffalo. Dung beetles are entirely dependent on the elephants and buffalo and serve the purpose of aiding in the breakdown of dung to more rapidly add nutrients back into the soil.

As the number of different species increases in a community, biodiversity also increases. However, when biodiversity declines within a community, ecological succession can occur, making an entire community less stable or unable to withstand disturbances that affect its overall structure and function. Ecological succession is a series of progressive changes in species composition that happen after a disturbance in an ecosystem. It happens, for example, when a community undergoes a series of changes that causes organisms to change their environment or causes new species to move into the community. It describes how the structure of species in a community evolves over time. Ecological succession is not sudden; rather, it is a gradual process. One example of an ecological succession is the migration of birds during a natural disaster, such as a hurricane. When a hurricane destroys an entire habitat, it can cause species structure to change as old species leave and new species move into the habitat.

Different Types of Ecological Succession

In primary succession, living organisms colonize an area for the first time (e.g., newly formed rock from a lava flow), whereas in secondary succession, an existing community responds to a disturbance in the environment (e.g., a forest fire).
In any community, certain species may become more or less abundant. Sometimes they may even face extinction—the situation in which a species vanishes altogether from the community. Whether extinction occurs depends on the level of biodiversity within the community and the magnitude of the ecological disturbance. In the event that a community becomes unstable, two types of ecological succession can occur: primary succession or secondary succession. Primary succession is when all the organisms have been removed from the environment, as occurs after a volcanic eruption. Secondary succession is when some organisms have been removed from the environment, such as trees being removed from a forest by logging.

Primary Succession

Primary succession results in the formation of new ecological communities after a disturbance has completely destroyed the ecosystem and left no soil present. The process begins with a successive stage in which an ecological disturbance, such as a volcanic eruption or a landslide, causes new land surfaces to form and creates a barren area with no soil present. These surfaces often consist of materials such as rock, sand, clay, or minerals. Typically the first organisms, or pioneer species, that can colonize when no soil is present include fungi, bacteria, or lichens. They are pioneers because they are capable of living in an environment that is not conducive to most life forms and they give rise to other new colonies of organisms. Gradually, over many years, more successive stages will occur, increasing biodiversity within a community.

Primary Succession

Primary succession occurs when new communities of organisms form after a large ecological disturbance, such as a volcano eruption. The establishment and growth of these organisms happens in succession or through a series of progressive changes. The first organisms to start these successive stages are those that can withstand an environment without soil, such as lichen. They are the pioneers that pave the way for other organisms to colonize over time.

Secondary Succession

Secondary succession occurs when new ecological communities are formed in an area after a disturbance removed most of the organisms but there is soil present afterward. Ecological disturbances that cause secondary succession often include the destruction or partial destruction of vegetation. Disturbances such as a fire, windstorm, or flood can cause this type of destruction but still leave some or all of the soil intact. Thus, there is a regrowth of a community of organisms after the disturbance. Typically, organisms that were present before the disturbance remain after the disturbance ends. Pioneer species pave the way for the colonization of new organisms or recolonization of existing organisms. Regrowth can occur because soil is present. After eventual successive stages, biodiversity increases within this community.

Secondary Succession

When a disturbance such as a flood, fire, or windstorm damages a community, this can trigger secondary succession. During this type of succession, soil is present, which allows organisms present before the disturbance as well as new organisms to colonize the area. This process of community regrowth happens in successive stages, which are a series of progressive changes. These stages happen over time.
Typically each species will grow under a certain set of favorable environmental conditions, such as a specific temperature range or the correct amount of sunlight. When these conditions are optimal, species growth is maximized. As long as these conditions remain constant, the species will continue to flourish. However, if something changes these conditions, it will cause a succession to occur. When a community is stable (the number of births and deaths are balanced) at the end of a succession sequence, it is called a climax community. A climax community is the point in ecological succession when species turnover has reached a steady state. At this point, ecological succession has essentially stopped. However, the possibility does exist for the succession process to start again if future disturbances occur in the community. For example, a population of antelope living in a grassland would be considered a climax community when the population size remains constant. The plant life in the grassland is sufficient to support the number of animals (antelopes and others) living there, and the birthrate of the antelopes matches the death rate from all causes (such as age, disease, and predators). If, however, a wildfire were to destroy a significant portion of the grassland, there would be a shift in the balance and succession would occur.