6.1: Defining Characteristics of Communities

In general, populations of one species never live in isolation from populations of other species. The interacting populations occupying a given habitat form an ecological community. The number of species occupying the same habitat and their relative abundance is known as the diversity of the community. Areas with low species diversity, such as the glaciers of Antarctica, still contain a wide variety of living organisms, whereas the diversity of tropical rainforests is so great that it cannot be accurately assessed. Scientists study ecology at the community level to understand how species interact with each other and compete for the same resources.

Niche

Each species within a community exploits the environment and interacts with other species in a particular manner. Ecologists use the word niche to describe the role of a species in its community, which can also be viewed as its “occupation” or livelihood. Some niches are relatively narrow and specialized, as is the niche of bats that feed only on flying insects of a certain size, or wasps that pollinate only one or a few species of plants. Other niches, however, are much broader, such as those of bears and humans, both of which forage over an extremely broad range and affect their ecosystem in diverse ways.

The fundamental niche is determined by the range of a species’ tolerance of environmental conditions. Many different physical, abiotic (non- living) factors influence where species live, including temperature, humidity, soil chemistry, pH, salinity and oxygen levels. For each species, there is a set of environmental conditions within which it can best survive and reproduce. It is under these conditions that the species is best adapted. 

 

Foundation Species

Foundation species are considered the “base” or “bedrock” of a community, having the greatest influence on its overall structure. They are often primary producers, and they are typically an abundant organism. For example, kelp, a species of brown algae, is a foundation species that forms the basis of the kelp forests off the coast of California.

Foundation species may physically modify the environment to produce and maintain habitats that benefit the other organisms that use them. Examples include the kelp described above or tree species found in a forest. The photosynthetic corals of the coral reef also provide structure by physically modifying the environment (Figure 19.24). The exoskeletons of living and dead coral make up most of the reef structure, which protects many other species from waves and ocean currents.

Figure 19.24 Coral is the foundation species of coral reef ecosystems. (credit: Jim E. Maragos, USFWS)

 

Keystone Species

A keystone species is one whose presence has a large influence in maintaining the prevalence of various species in an ecosystem, the ecological community’s structure, and sometimes its biodiversity. Pisaster ochraceus, the intertidal sea star, is a keystone species in the northwestern portion of the United States (Figure 19.25). Studies have shown that when this organism is removed from communities, mussel populations (their natural prey) increase, which completely alters the species composition and reduces biodiversity. Another keystone species is the banded tetra, a fish in tropical streams, which supplies nearly all of the phosphorus, a necessary inorganic nutrient, to the rest of the community. The banded tetra feeds largely on insects from the terrestrial ecosystem and then excretes phosphorus into the aquatic ecosystem. The relationships between populations in the community, and possibly the biodiversity, would change dramatically if these fish were to become extinct.

Figure 19.25 The Pisaster ochraceus sea star is a keystone species. (credit: Jerry Kirkhart)

Attributions:

This page is a modified derivative of: 

• Ch 9 - Ecology: From Individuals to the Biosphere by Open Library, license CC-BY-NC 4.0

• The Ecological Niche by Laci M. Gerhart-Barley, Libratext license CC BY-NC-SA 3.0 

• Ch 19.4 Community Ecology by OpenStax Concepts in Biology CC-BY-4.0