Notwithstanding the important interactions between competition and environmental heterogeneity, and the complications of apparent competition, a great deal of attention has been focused on conventional competition itself. We have already noted the difficulties in interpreting merely observational evidence (but see Freckleton & Watkinson, 2001), and it is for this reason that many studies of the ecological effects of interspecific competition have taken an experimental approach. For example, we have seen manipulative field experiments involving barnacles (see Section 8.2.2), birds (see Section 8.2.5), cattails (see Section 8.3.3) and snails (see Section 8.5.4), where the density of one or both species was altered (usually reduced). The fecundity, the survivorship, the abundance or the resource utilization of the remaining species was subsequently monitored. It was then compared either with the situation prior to the manipulation, or, far better, with a comparable control plot in which no manipulation had occurred. Such experiments have consistently provided valuable information, but they are typically easier to perform on some types of organism (e.g. sessile organisms) than they are on others.
The second type of experimental evidence has come from work carried out under artificial, controlled (often laboratory) conditions. Again, the crucial element has usually been a comparison between the responses of species living alone and their responses when in combination. Such experiments have the advantage of being comparatively easy to perform and control, but they have two major disadvantages. The first is that species are examined in environments that are different from those they experience naturally. The second is the simplicity of the environment: it may preclude niche differentiation because niche dimensions are missing that would otherwise be important. Nevertheless, these experiments can provide useful clues to the likely effects of competition in nature.
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