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- Beta diversity analysis applied to food webs
Important methodological advances in the analysis of species distribution patterns on large spatial scales include null models of species distribution, based on speciation and dispersal in uniform environments (Hubbell, 2001; Condit et al., 2002; Volkov et al., 2003; Leigh et al., 2004), models of metapopulation dynamics (Hanski, 1999), and increasingly sophisticated extrapolation of species distribution using spatially explicit environmental information (Corsi et al., 2000; Segurado & Araujo, 2004), often obtained by remote sensing (Tuomisto et al., 2003a).
This progress has not been matched by advances in collecting empirical data on tropical beta diversity, which remains the least studied of the alpha, beta and gamma aspects of diversity in tropical forests (Novotny & Weiblen, 2005). This bias is largely due to incomplete taxonomic knowledge of tropical plants and particularly insects, making comparisons of multiple communities difficult.
The studies of plants and insects on large geographical scales fall into two broad categories: those studying spatial variability in species composition of particular taxa or guilds, including plants (Condit et al., 2002; Tuomisto et al., 2003b) and their insect herbivores (Ødegaard, 2006; Novotny et al., 2007), while ignoring their interactions with the rest of the food web, and those focusing on the structural parameters of plant–insect food webs, including the number of herbivore species per plant species (Lewinsohn et al., 2005), number of host species per herbivore species (Novotny & Basset, 2005), or web connectance (Kitching, 2000; Tylianakis et al., 2007), while ignoring changes in species composition among these food webs.
We suggest that these two approaches can be combined and the study of beta diversity usefully broadened from plant and insect communities to plant–insect food webs, applying analytical tools used in community studies. In particular, Lewinsohn and Roslin (2008) demonstrate the importance of simultaneous study of plant and herbivore alpha and beta diversity in combination with herbivore host specificity for the analysis of tropical biodiversity. This amounts de facto to the study of beta diversity of plant–herbivore food webs.
Ideally, a contiguous area of rainforest vegetation should be censused for plants, herbivores and their trophic interactions in order to document plant–herbivore food webs. A more feasible alternative includes comprehensive sampling of herbivores from a particular taxon or guild, feeding on plant species from a particular taxon, or the entire vegetation, with sample size on each plant species proportional to its biomass or abundance (Novotny et al., 2004). This sampling protocol is markedly different from the one used by a majority of rainforest studies, which focus mostly on the host specificity of herbivores (Novotny & Basset, 2005). These studies typically examine arbitrarily selected sets of plant species, often including a balance of closely and distantly related species and/or single representatives of numerous plant lineages, sampled with constant sampling effort per plant species (Basset, 1996; Barone, 1998; Ødegaard, 2000). This dichotomy in sampling protocols leads to two largely separate lines of research, one on herbivore host specificity, another on food web structure.
Although the present paper is concerned mostly with plant–herbivore food webs, the concepts and methods discussed here can be equally well applied to insect–insect food webs, for instance food webs between herbivorous hosts and their parasitoids (Morris et al., 2005), as well as to tri-trophic plant–herbivore–predator systems (Dyer & Letourneau, 2003).