Community patterns in time

Just as the relative importance of species varies in space, so their patterns of abundance may change with time. In either case, a are communities discrete entities with sharp boundaries?

the community: not so much a superorganism ...

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

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

. . . more a level of organization species will occur only where and when: (i) it is capable of reaching a location; (ii) appropriate conditions and resources exist there; and (iii) competitors, predators and parasites do not preclude it. A temporal sequence in the appearance and disappearance of species therefore seems to require that conditions, resources and/or the influence of enemies themselves vary with time.

For many organisms, and particularly short-lived ones, their relative importance in the community changes with time of year as the individuals act out their life cycles against a background of seasonal change. Sometimes community composition shifts because of externally driven physical change, such as the build up of silt in a coastal salt marsh leading to its replacement by forest. In other cases, temporal patterns are simply a reflection of changes in key resources, as in the sequence of heterotrophic organisms associated with fecal deposits or dead bodies as they decompose (see Figure 11.2). The explanation for such temporal patterns is relatively straightforward and will not concern us here. Nor will we dwell on the variations in abundance of species in a community from year to year as individual populations respond to a multitude of factors that influence their reproduction and survival (dealt with in Chapters 5, 6 and 8-14).

Our focus will be on patterns of community change that follow a disturbance, defined as a relatively discrete event that removes organisms (Townsend & Hildrew, 1994) or otherwise disrupts the community by influencing the availability of space or food resources, or by changing the physical environment (Pickett & White, 1985). Such disturbances are common in all kinds of community. In forests, they may be caused by high winds, lightning, earthquakes, elephants, lumberjacks or simply by the death of a tree through disease or old age. Agents of disturbance in grassland include frost, burrowing animals and the teeth, feet, dung or dead bodies of grazers. On rocky shores or coral reefs, disturbances may result from severe wave action during hurricanes, tidal waves, battering by logs or moored boats or the fins of careless scuba divers.

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