Este é um Blog educacional, dedicado a discussões acadêmicas sobre a Ecologia Evolutiva. Contém chamadas específicas relacionadas às disciplinas de Ecologia da Universidade Federal de Ouro Preto, e textos didáticos gerais.
Este texto do Begon vai lhe ajudar a se preparar para uma das questões. Leia atentamente, mas não esqueça que o conteúdo dado nas aulas aborda algo de toda a Ecologia de Comunidade. O Economia da Natureza é suficiente para a maioria das leituras, mas alguns aspectos eu quero que você aprofunde, e estes vão aparecer aqui no blog.
Um detalhe: este capítulo lida com um aspecto muito importante da teoria ecológica, e para melhorar a estima daqueles que acham que só artigo de gringo compõe teoria, os colegas de Viçosa são citados frontalmente aqui, no exemplo do artigo de Soares et al (esta Soares quase foi professora de vocês).
(Soares, S.M., Schoereder, J.H. & DeSouza, O. (2001) Processes
involved in species saturation of ground-dwelling ant communities
(Hymenoptera, Formicidae). Austral Ecology, 26, 187–192.)
Mais tarde hoje ou até amanha eu entro outro texto.
21.2.1 The relationship between local and regional
One way to assess the degree to which communities are saturated with species is to plot the relationship between local species richness (assessed on a spatial scale where all the species could encounter each other in a community) and regional species richness (the number of species in the regional pool that could theoretically colonize the community). Local species richness is sometimes referred to as α richness (or α diversity) and regional species richness as γ richness. If communities are saturated with species (i.e. the niche space is fully utilized), local richness will reach an asymptote in its relationship with regional richness (Figure 21.2a).
This appears to be the case for the Brazilian ground-dwelling ant communities studied by Soares et al. (2001) (Figure 21.2b). Similar patterns have been described for aquatic and terrestrial plant groups, fish, mammals and parasites, but nonsaturating patterns have just as often been described for a variety of taxa, including fish (Figure 21.2c), insects, birds, mammals, reptiles, molluscs and corals (reviewed by Srivastava, 1999). Local regional richness plots provide a useful tool for addressing the question of community saturation, but they must be used with caution. For example, Loreau (2000) points out that the nature of the relationship depends on the way that total richness (γ) is partitioned between within-community (α) and between-community richness (β), and this is a matter of the scale at which different communities are distinguished from one another. In other words, researchers might erroneously include within a single community several habitats that should be considered as different communities, or, alternatively, they may study local communities at an inappropriately small scale (e.g. 1 m2 quadrats may have been too small to be ‘local’ communities in the ground-dwelling ant study of Soares et al., 2001).
21.2.2 Species interactions and the simple model of
We can also consider the relationship between the model in Figure 21.1 and two important kinds of species interactions described in previous chapters – interspecific competition and predation (see especially Chapter 19). If a community is dominated by interspecific competition, the resources are likely to be fully exploited. Species richness will then depend on the range of available resources, the extent to which species are specialists and the permitted extent of niche overlap (see Figure 21.1a– c). Predation, on the other hand, is capable of exerting contrasting effects.
First, we know that predators can exclude certain prey species; in the absence of these species the community may then be less than fully saturated, in the sense that some available resources may be unexploited (see Figure 21.1d). In this way, predation may reduce species richness. Second, though, predation may tend to keep species below their carrying capacities for much of the time, reducing the intensity and importance of direct interspecific competition for resources. This may then permit much more niche overlap and a greater richness of species than in a community dominated by competition (see Figure 21.1c). Finally, predation may generate richness patterns similar to those produced by competition when prey species compete for ‘enemy-free space’ (see Chapter 8). Such ‘apparent competition’ means that invasion and the stable coexistence of prey are favored by prey being sufficiently different from other prey species already present. In other words, there may be a limit to the similarity of prey that can coexist (equivalent to the presumed limits to similarity of coexisting competitors).