Pattern: Background Information

Note: hot linked bold faced terms are in the Glossary

Abundance and Spatial Patterns of Distribution

The most readily available references to biogeographical patterns are vegetation maps at various scales. Maps, however, especially at coarser scales, usually hide some of the variability within the mapping unit (a polygon within the larger biogeographical unit) because they are generalizations of reality and thus do not show the entire geographic range of individual taxa (a class or category, e.g., a species). There are, however, other ways to show geographic distributions of organisms that illustrate the reported range of occurrence better. One may, for example, portray the differential abundances of taxa as rank order of abundance. To show differential abundance of species, one might use dots to represent the species density over a region. Such a dot map would tell you how many species can be found in an area, but not which ones. Other types of abundance maps may tell you how frequently one particular species or land use can be found in different areas (e.g., the hog distribution or alfalfa production [e.g., as yield per acre] across the United States).

Abundance maps of "naturally occurring" organisms are much less common because of the lack of data. Pollen analysts use fossil pollen to infer the distribution of trees and other species in the past to show the dynamics of species. Abundances of some grass species have also been mapped. These have all been small-scale maps of large regions or continents, so not very much detail is portrayed in these maps.

The abundance of many grass and tree species commonly exhibits high spatial variability. Differences in abundances at sites within a landscape (local variation) are frequently as large as those over greater distances (regional variation). These varied local patterns indicate that the regional distributions are not spatially continuous, as is implied by the use of isarithmic lines (lines of equal value like contour lines that connects points of equal elevation) on maps to portray such patterns. Cartographic theory holds that point symbols are the only appropriate way to show spatially discontinuous phenomena (point data). That is, we cannot presume to show the values that lie between control points unless we have reason to believe that the surface is spatially continuous (like air pressure where we can reliably interpolate between observation points).

Pattern and Disturbance

When we are able to look at regional patterns of plant distributions at the level of individual organisms, we see that species are not uniformly distributed throughout a landscape. At this scale of observation, we can see that the assumed relationship between climate and plant abundance does not really hold. Disturbances create opportunities for invaders, thus diminishing the relative role of climate in determining the abundance of certain species. Climate is of course involved in the creation of disturbances, particularly erosion, flooding, wind-fall, fire, and deposition, but there is a different link between plants and climate than has traditionally been gleaned from the study of regional to continental scale patterns.

At the landscape scale, local patterns reveal that the presence of any organism is the result of invasion at some point in time. Patterns are thus products of biotic interactions between colonists (those organisms already present) and invaders (those newly arriving) whose presence is often facilitated by corridors of disturbance. An aquatic example demonstrating this is the change in fish population in the North American Great Lakes (see Lodge 1993: 378-380). The native fish assemblage of the Upper Great Lake once was dominated by the lake trout and coregonids (the colonists), but then changed significantly with the construction of the Welland Canal in 1829 (the corridor and hydrologic disturbance) which allowed two marine species, the lamprey and alewife (the invaders), to reach the Upper Lake. Together with a number of other factors, this eventually led to a dramatic decline of lake trout and coregonid. An important point to make here is that the terms colonist and invader don't inherently carry any value judgement with them. It really depends on which species' viewpoint you take, whether you see this change in lake population as negative or positive. Invasion is generally viewed as negative when the invading species displaces another one that had special value for use (e.g., an endangered plant, a tree we harvest for some human purpose, or a very beautiful songbird). On the other hand, if this endangered or useful or treasured species successfully colonizes a new space, we welcome it (see the examples described in Root and Weckstein 1995; Oglesby and Smith 1995; Morse, Kutner, and Kartesz 1995).

Differences in pattern among the co-occurring organisms are thus dispersal patterns for the invaders and patterns of disintegration for earlier invaders. Pattern analysis shows that some distributions are random and have been for a long time. The occurrence of some species may still be controlled by disturbance patterns that are not spatially systematic (e.g., buffalo wallows), but which are ideally suited to establishment of the species (e.g., buffalo grass seed burrs carried on the hides of bison).

By modeling the processes of disturbance and dispersal that exist at the landscape scale over wider areas, we can begin to see how these regional patterns develop. Some conceptual models of pattern development through organism dispersal are presented in the next unit as a basis for understanding the contribution of these patterns to the diversity of organisms in a space.