Physiological and behavioral ecologists are concerned primarily with individual organisms. Coexisting individuals of a single species possess characteristics - such as density, sex ratio, age-class structure, rates of natality and immigration, mortality and emigration - that are unique to populations. We explain the behavior of a population in terms of the behavior of the individuals that comprise it. In their turn, activities at the population level have consequences for the next level up - that of the community. The community is an assemblage of species populations that occur together in space and time. Community ecology seeks to understand the manner in which groupings of species are distributed in nature, and the ways these groupings can be influenced by their abiotic environment (Part 1 of this textbook) and by interactions among species populations (Part 2). One challenge for community ecologists is to discern and explain patterns arising from this multitude of influences.
In very general terms, the species the search for rules of that assemble to make up a com-community assembly munity are determined by: (i) dispersal constraints; (ii) environmental constraints; and (iii) internal dynamics (Figure 16.1) (Belyea & Lancaster, 1999). Ecologists search for rules of community assembly, and we discuss these in this chapter and a number of others (particularly Chapters 19-21).
A community is composed of individuals and populations, and we can identify and study straightforward collective properties, such as species diversity and community biomass. However, we have already seen that organisms of the same and different species interact with each other in communities have collective properties... ... and emergent properties not possessed by the individual populations that comprise them processes of mutualism, parasitism, predation and competition. The nature of the community is obviously more than just the sum of its constituent species. There are emergent properties that appear when the community is the focus of attention, as there are in other cases where we are concerned with the behavior of complex mixtures. A cake has emergent properties of texture and flavor that are not apparent simply from a survey of the ingredients. In the case of ecological communities, the limits to similarity of competing species (see Chapter 19) and the stability of the food web in the face of disturbance (see Chapter 20) are examples of emergent properties.
Total species pool
Habitat species pool
Ecological species pool Internal dynamics
Figure 16.1 The relationships among five types of species pools: the total pool of species in a region, the geographic pool (species able to arrive at a site), the habitat pool (species able to persist under the abiotic conditions of the site), the ecological pool (the overlapping set of species that can both arrive and persist) and the community (the pool that remains in the face of biotic interactions). (Adapted from Belyea & Lancaster, 1999; Booth & Swanton, 2002.)
Science at the community level poses daunting problems because the database may be enormous and complex. A first step is usually to search for patterns in the community's collective and emergent properties. Patterns are repeated consistencies, such as the repeated grouping of similar growth forms in different places, or repeated trends in species richness along different environmental gradients. Recognition of patterns leads, in turn, to the forming of hypotheses about the causes of these patterns. The hypotheses may then be tested by making further observations or by doing experiments.
A community can be defined at any scale within a hierarchy of habitats. At one extreme, broad patterns in the distribution of community types can be recognized on a global scale. The temperate forest biome is one example; its range in North America is shown in Figure 16.2. At this scale, ecologists usually recognize climate as the overwhelming factor that determines the limits of vegetation types. At a finer scale, the temperate forest biome in parts of New Jersey is represented by communities of two species of tree in particular, beech and maple, together with a very large number of other, less conspicuous species of plants, animals and microorganisms. Study of the community may be focused at this scale. On an even finer habitat scale, the characteristic invertebrate community that inhabits water-filled holes in beech trees may be studied, or the flora and fauna in the gut of a deer in the forest. Amongst these various scales of community study, no one is more legitimate than another. The scale appropriate for investigation depends on the sorts of questions that are being asked.
Community ecologists sometimes consider all of the organisms existing together in one area, although it is rarely possible to do this without a large team of taxonomists. Others restrict their attention within the community to a single taxonomic group (e.g. birds, insects or trees), or a group with a particular activity (e.g. herbivores or detritivores).
The rest of this chapter is in six sections. We start by explaining how the structure of communities can be measured and described (Section 16.2). Then we focus on patterns in community structure: in space (Section 16.3), in time (Sections 16.4-16.6) and finally in a combined spatiotemporal setting (Section 16.7).
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