Only a small proportion (27%) of species in herbivore communities feed on a single plant species when alternative congeneric hosts are available (). By contrast, the proportions of species feeding on a single plant genus (48%) and family (58%) are higher and not statistically different from one another. Thus, limits of host plant range tend to coincide with those of plant genera, rather than those of families or higher taxonomic ranks. However, this analysis employed taxonomic ranks as a substitute for plant phylogeny and also ignored numerous methodological discrepancies among individual datasets and is therefore preliminary.
In theory, host specificity comparisons between different herbivore taxa and guilds are straightforward since all herbivores may be sampled simultaneously from identical study plants and sites. However, while some studies include herbivores from several insect orders (), studies of more than one guild on the same hosts are lacking from tropical forests, probably because each guild requires a specific sampling method.
Concealed herbivore guilds are more specialized than those that feed externally on temperate vegetation (Mattson et al. 1988
). In tropical forests, the average percentage of family specialists decreases among guilds in the sequence: granivores (99%)>leaf-miners (96%)>fructivores (83%)>leaf-chewers (56%)=sap-suckers (56%)>xylophages (24%)>root-feeders (10%) (see ). The proportion of genus specialists (but not species specialists) follows the same pattern. With the exception of xylophages, which feed on dead plant material, all endophytic guilds are thus more specialized than the external feeders. These conclusions remain tentative since replicated studies produced inconsistent host specificity estimates in several guilds: xylophagous beetles, leaf-chewing beetles, granivorous beetles and leaf-chewing Orthoptera (). Some of these differences may have been caused by different methods of study, but they emphasize the need for replicated studies.
Although it is believed that larval host plant ranges are more restricted than those of adults, rigorous larval–adult comparisons are rare. This is a notable problem in herbivorous beetles where larvae and adults often belong to different guilds. The species richness (Basset 2001a
) and host specificity (Ødegaard 2000b
) of leaf-chewing beetles were studied extensively in tropical forests. However, adults predominate in this guild, while their root-feeding, xylophagous and granivorous larvae are mostly ignored.
Trends in the diversity of species along environmental gradients (e.g. latitudinal or altitudinal) are popular themes for study, whereas analogous trends in the diversity of interactions, including plant–herbivore trophic links, are not (Ollerton & Cranmer 2002
). Although there is extensive literature comparing species richness in herbivorous communities at different altitudes (Brehm et al. 2003
), host specificity comparisons between lowland and montane tropical forests are few () since lowland forests remain the most popular topic for study (Basset 2001a
). Steep altitudinal gradients in temperature, rainfall and the abundance of ants (important as predators and mutualists of herbivores) across small geographical distances provide yet unexplored opportunities for the study of host specificity determinants in the tropics.
Whether there is a latitudinal gradient in host specificity between temperate and tropical forests is surprisingly hard to assess given a methodological dichotomy between tropical and temperate research. Almost all information on the host plants of tropical herbivores comes from local community studies on selected plant species (), while even incomplete regional host plant lists are available for only a few taxa (e.g. Fiedler 1995
; Flowers & Janzen 1997
). By contrast, regional host plant lists for temperate herbivores, detailed particularly for Great Britain and central Europe, have been compiled for over two centuries, while quantitative community studies that include feeding experiments and rearing of larvae (e.g. Futuyma & Gould 1979
) are rare. Incongruent methodology represents a serious problem as both host specificity estimates based on community and regional data have their own sets of biases (Ward 1988
Given that reliable regional host species lists are unlikely to be forthcoming for tropical habitats, there is a need for renewed effort in collecting quantitative host specificity data for herbivorous insects from temperate vegetation. Ideally, such studies should produce quantitative food webs (Godfray et al. 1999
) that measure the abundance or biomass of both plant and herbivore species as well as the intensity of their trophic interactions. It is important that food web studies are replicated as any tropical/temperate trends are only likely to emerge by analysing what may appear as unnecessary ‘superabundance’ of datasets.
There is no marked difference in host specificity between tropical and temperate butterflies (Fiedler 1995
), although particular lineages may be less (Lycaenidae: Polyommatini; Fiedler 1995
) or more specialized (Papilionidae: Scriber 1988
) in the tropics than in temperate regions. Bark beetles (Coleoptera: Curculionidae: Scolytinae; Beaver 1979
) and treehoppers (Hemiptera: Membracidae; Wood 1984
) appear to be less specialized in the tropics than in temperate regions, while a community study of temperate caterpillars (Futuyma & Gould 1979
) shows lower host specificity than analogous data from the tropics (Barone 1998
; Novotny et al. 2002a
). The limited evidence available indicates that there may be no major increase in host specificity from temperate to tropical herbivore communities. This conclusion, and the recent downward revisions of extremely high estimates of tropical species richness (Basset et al. 1996
; Ødegaard 2000b
; Novotny et al. 2002b
), suggests that tropical ecosystems, although different from those temperate in many respects, may not be as species rich as thought previously.
Empirical study of host specificity patterns is only the first step towards the ultimate goal of explaining host plant use by evolutionary and ecological factors. The evolutionary dynamics of host plant selection range from cospeciation between plants and herbivores, through sequential speciation of herbivores following the evolution of their hosts, to multiple colonizations of unrelated plants with similar chemical or other traits (e.g. Becerra 1997
; Weiblen 2004
). These evolutionary processes generate regional species pools of herbivores that serve as sources for local communities. The composition of communities may be tested against an estimate derived from a random selection from the regional species pool. This approach, already applied to plants (Webb 2000
), could be useful in the study of herbivorous communities.