Problems of boundaries in community ecology

There may be communities that are separated by clear, sharp boundaries, where groups of species lie adjacent to, but do not intergrade into, each other. If they exist, they are exceptional. The meeting of terrestrial and aquatic environments might appear to be a sharp boundary but its ecological unreality is emphasized by the otters or frogs that regularly cross it and the many aquatic insects that spend their larval lives in the water but their adult lives as winged stages on land or in the air. On land, quite sharp boundaries occur between the vegetation types on acidic and basic rocks where outcrops meet, or where serpentine (a term applied to a mineral rich in magnesium silicate) and nonserpentine rocks are juxtaposed. However, even in such situations, minerals are leached across the boundaries, which become increasingly blurred. The safest statement we can make about community boundaries is probably that they do not exist, but that some communities are much more sharply defined than others. The ecologist is usually better employed looking at the ways in which communities grade into each other, than in searching for sharp cartographic boundaries.

In the first quarter of the 20th century there was considerable debate about the nature of the community. Clements (1916) conceived of the community as a sort of superorganism whose member species were tightly bound together both now and in their common evolutionary history. Thus, individuals, populations and communities bore a relationship to each other resembling that between cells, tissues and organisms.

In contrast, the individualistic concept devised by Gleason (1926) and others saw the relationship of coexisting species as simply the results of similarities in their requirements and tolerances (and partly the result of chance). Taking this view, community boundaries need not be sharp, and associations of species would be much less predictable than one would expect from the superorganism concept.

The current view is close to the individualistic concept. Results of direct gradient analysis, ordination and classification all indicate

Fine-scale effects of roots, organic particles and soil structure ^

Plot-scale to field-scale effects of burrowing _ animals, individual plants and plant communities

Large-scale gradients of texture, soil carbon, topography and ^ vegetation systems

Figure 16.8 Determinants of spatial heterogeneity of communities of soil organisms including bacteria, fungi, nematodes, mites and collembolans. (After Ettema & Wardle, 2002.)

that a given location, by virtue mainly of its physical characteristics, possesses a reasonably predictable association of species. However, a given species that occurs in one predictable association is also quite likely to occur with another group of species under different conditions elsewhere.

A further point needs to be born in mind when considering the question of environmental patchiness and boundaries. Spatial heterogeneity in the distribution of communities can be viewed within a series of nested scales. Figure 16.8, for example, shows patterns in spatial heterogeneity in communities of soil organisms operating at scales from hectares to square millimeters (Ettema & Wardle, 2002). At the largest scale, these reflect patterns in environmental factors related to topography and the distribution of different plant communities. But at the other extreme, fine-scale patterns may be present as a result of the location of individual plant roots or local soil structure. The boundaries of patterns at these various scale are also likely to be blurred.

Whether or not communities have more or less clear boundaries is an important question, but it is not the fundamental consideration. Community ecology is the study of the community level of organization rather than of a spatially and temporally definable unit. It is concerned with the structure and activities of the multispecies assemblage, usually at one point in space and time. It is not necessary to have discrete boundaries between communities to study community ecology.

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