That is, there are many rare species and only a few common species. In nearly every community in which species have been identified and counted, distributions are highly skewed. As a result, the pattern in species abundance may be more uniform for the many species that co-occur in fen wetlands, differences in their abundance are much more gradual, sometimes also referred to as being more evenly distributed (Figure 2D). The rich structural complexity in fen wetlands (Figure 2B) may lead to a fine partitioning of available habitat space. As a result, there is a large disparity in the abundance of species from the species-poor communities found in salt marshes (Figure 2C). For example, the periodic intrusion of seawater into salt marshes (Figure 2A) may constitute a disturbance that prevents most species from becoming abundant other than a few species that are able to cope with the periodic exposure to salt water. Contrasting patterns in species abundance may similarly indicate differences in the way that communities are organized. By condensing the information on species abundances, they allow for comparisons of how various communities differ in the way they are organized. Species abundance curves, in which the number of species is plotted against their abundance, are used to deal with this complexity. Comparing species abundance among communities can be difficult because communities often comprise many different species whose abundance profiles differ widely among the communities. For example, observations that two species occur together in many places, yet never co-occur at high densities (i.e., when one species is numerous, the other is scarce), suggests that these species compete with each other. Data on species abundances are relatively easy to obtain, and may give insight into less visible aspects of a community, such as competition and predation. Knowing the abundance of different species can provide insight into how a community functions. This relationship is termed the distribution-abundance relationship. This pattern in distribution of species abundances is accompanied on a larger scale level, by the tendency of widespread species to also occur in higher densities compared to species restricted in their geographic distribution. Within most natural assemblages a few species comprise the majority of the individuals (Figure 1 Preston 1948). Although there are few rules that are universally true in community ecology (Lawton 1999), there are regular, predictable patterns. Such general rules would be very useful as they increase our understanding of how communities function and allow predictions as to how they would respond to environmental changes, for example global warming or restoration efforts. Such a pattern may be explained in various ways: one species may exclude the other through competition or, alternatively, they simply prefer different habitats.Īlthough it may be relatively easy to explain patterns when only a handful of species are involved, it has proven difficult to provide general rules about entire communities where many species co-occur and interact. An example of such a pattern is that some species never occur together in the same place. A central aim of community ecology is to understand how communities are organized by identifying, describing, and explaining general patterns that underlie the structure of communities. Community ecology is the study of a set of species co-occurring at a given time and place.

Pure point measure distribution