9.1: Community Structure

Community Structure

Community structure is the ecologist's term for indicating what organisms are present in a given environment, in what numbers, and how they relate to each other. Another way to look at a community is as a collection of niches or slots that organisms can fit into in order to “make a living.” The term community is rather arbitrary. Its boundaries can be as wide or as narrow as one chooses to make them. For example, a rocky-shore community could refer to a thousand miles of coastline or to a kelp community in a band tens to hundreds of feet wide on that coast, or the kelp epiphyte community could refer to those organisms growing on single kelp plants (Figure \(\PageIndex{1}\)). Reasonably sharp community boundaries are required for such a designation, but even that criterion is sometimes rather fuzzy. Traditionally, a biological community is named for a dominant element or elements. An Acropora palmata reef community is conspicuously dominated by that single coral species as a major element of biomass or structure, controlling many other organisms by its presence. A Batis–Distichlis salt marsh community is a salt marsh in which the saltwort Batis and the salt grass Distichlis more or less equally provide the primary vegetation and therefore cover in the marsh.

Figure \(\PageIndex{1}\): A kelp forest community provides habitat and food for a wide range of invertebrates, fish (a) and ephiphytes (algae growing on kelp stipe). (Image a via wikimedia; licence CC 2.0, Image b via wikimedia; licence CC-SA 3.0)

How do we measure community structure?

Two important measures ecologists use to describe the composition of a community are species richness and relative abundance (or species evenness). Species richness is the term used to describe the number of species living in a habitat or other unit. Relative species abundance is the number individuals in a species relative to the total number of individuals in all species within a system. Larger numbers of species and more even abundances of species lead to higher species diversity. For example, both communities in figure \(\PageIndex{2}\) have three different trees species and thus a species richness of three. However, there is a dominant species (represented by six individuals) in community #1. In community #2, there are three of individuals of each species. Therefore, community #2 has a greater species evenness and greater species diversity overall.

Figure\(\PageIndex{2}\): Two hypothetical tree communities have the same species richness, but community #2 (bottom) has a greater species evenness. Both communities have nine trees and three tree species. In community #1, one species is dominant, represented by six individuals. There are two individuals of a conifer species, and only one individual of the final species. In community #2, there are three individuals from each species. Images compiled by Melissa Ha from Alone tree George Hodan, Old Tree Silhouette, and Tree (all public domain).

Definition: Biodiversity

One measure of biodiversity used by ecologists is the number of different species in a particular area and their relative abundance.

Ecologists have struggled to understand the determinants of biodiversity. One well known pattern is that species richness varies with latitude. Communities with the highest species richness tend to be found in areas near the equator, which have lots of solar energy (supporting high primary productivity), warm temperatures, large amounts of rainfall, and little seasonal change. Communities with the lowest species richness lie near the poles, which get less solar energy and are colder, drier, and less amenable to life. This pattern is illustrated below for mammalian species richness (Figure \(\PageIndex{3}\)).

Figure \(\PageIndex{3}\): The greatest species richness for mammals in North America is associated in the equatorial latitudes. (credit: modification of work by NASA, CIESIN, Columbia University). (CC-BY; via OpenStax)

Video: Why Do More Species Live Near the Equator

One of the most striking patterns in ecology is that more species exist in the tropics and biodiversity tends to decline as you move towards the poles. Why does this pattern exist? Watch the video below to find out.

Hypotheses include:

1. There is simply more land and ocean area in the tropics.
2. Tropical locations are more stable and less affected by changes in Earth's climate such as ice ages.
3. The tropics receive more sunlight and thus more energy.
4. Stable environments allow species to carve out narrow niches, allowing multiple similar species to coexist.
5. The process of speciation occurs faster near the equator and/or extinction is less likely.

Other factors influence species richness as well. Island biogeography attempts to explain the great species richness found in isolated islands, and has found relationships between species richness, island size, and distance from the mainland. In general, ecologists think that more diverse ecological communities are more stable (that is, more able to recover after a disturbance) than less diverse communities. However, the diversity-stability relationship isn't a universal rule, and there are some cases where other factors (besides species diversity) are more important in determining community and ecosystem stability.

What factors shape community structure?

The structure of a community is the result of many interacting factors, both abiotic (non-living) and biotic (living organism-related). Here are some important factors that influence community structure:

• The climate patterns of the community's location.
• The geography of the community's location.
• The heterogeneity (patchiness) of the environment.
• The frequency of disturbances, or disruptive events.
• Interactions between organisms.

A community's structure can also be shaped by the chance events that happened during its history. For instance, suppose that a single seed blows into the dirt of a particular area. If it happens to take root, the species may establish itself and, after some period of time, become dominant (excluding similar species). If the seed fails to sprout, another similar species may instead be the lucky one to establish itself and become dominant.

Attribution

This page is a modified derivative of:

• 19.4 Community Ecology Via Open Stax; license CC-BY 4.0
• 9.2 Species Diversity by Melissa Ha and Rachel Schleiger via Environmental Science; license CC BY-NC 4.0