Host specificity of insect herbivores in tropical forests

Abstract

Studies of host specificity in tropical insect herbivores are evolving from a focus on insect distribution data obtained by canopy fogging and other mass collecting methods, to a focus on obtaining data on insect rearing and experimentally verified feeding patterns. We review this transition and identify persisting methodological problems. Replicated quantitative surveys of plant-herbivore food webs, based on sampling efforts of an order of magnitude greater than is customary at present, may be cost-effectively achieved by small research teams supported by local assistants. Survey designs that separate historical and ecological determinants of host specificity by studying herbivores feeding on the same plant species exposed to different environmental or experimental conditions are rare. Further, we advocate the use of host-specificity measures based on plant phylogeny. Existing data suggest that a minority of species in herbivore communities feed on a single plant species when alternative congeneric hosts are available. Thus, host plant range limits tend to coincide with those of plant genera, rather than species or suprageneric taxa. Host specificity among tropical herbivore guilds decreases in the sequence: granivoresO leaf-minersOfructivoresOleaf-chewersZsap-suckersOxylophagesOroot-feeders, thus paralleling patterns observed in temperate forests. Differences in host specificity between temperate and tropical forests are difficult to assess since data on tropical herbivores originate from recent field studies, whereas their temperate counterparts derive from regional host species lists, assembled over many years. No major increase in host specificity from temperate to tropical communities is evident. This conclusion, together with the recent downward revisions of extremely high estimates of tropical species richness, suggest that tropical ecosystems may not be as biodiverse as previously thought.

Influence of plant resistance traits in selectiveness and species strength in a tropical plant‐herbivore network

“…Th e most abundant plant species (i.e., those with most biomass or cover) tend to be related to a greater number of herbivore species ( Basset, 1991 ;Marques et al, 2000 ;Whitfeld et al, 2012 ) simply by having a greater probability of being found, that is, a greater apparency ( Feeny, 1976 ). However, in the particular case of tropical systems, where herbivorous insects are highly specialized to their host plants ( Novotny et al, 2002 ;Novotny and Basset, 2005 ;Ødegaard et al, 2005 ;Weiblen et al, 2006 ;Forister et al, 2012Forister et al, , 2015, it is likely that species strength is independent from the host plants biomass or abundance. In this case, plant resistance traits play an important role infl uencing herbivore preferences ( Coley and Barone, 1996 ;Stamp, 2003 ;Agrawal, 2005 ;Poelman et al, 2008 ) and thus their species strength, or their relative importance as food for herbivores.…”

“…Th is discrepancy could be the result of diff erences in the degree of generalization between these types of networks; whereas networks of mutualistic interactions tend toward generalization ( Bascompte et al, 2006 ), plant-herbivore networks are highly specialized ( Prado and Lewinsohn, 2004 ;Cagnolo et al, 2011 ;López-Carretero et al, 2014 ). Th is specialization has developed from a complex set of ecological, biochemical and historical factors ( Ehrlich and Raven, 1964 ;Novotny et al, 2002 ;Novotny and Basset, 2005 ;Ødegaard et al, 2005 ;Weiblen et al, 2006 ;Gómez-Zurita et al, 2007 ) that, in general, have promoted interactions within subgroups of available plant species regardless of their coverage or density.…”

Vertical canopy gradient shaping the stratification of leaf‐chewer–parasitoid interactions in a temperate forest

“…Therefore, parasitoids are a highly threatened group that is extremely vulnerable to extinction (Shaw & Hochberg, 2001; Smith, Wood, Janzen, Hallwachs, & Hebert, 2007). In temperate and tropical forests, the major source of insect diversity is the forest canopy (Novotny & Basset, 2005; Stork, 1988). Many arthropod species in temperate deciduous forests depend on canopy habitats, and approximately half of them are predators or parasitoids (Moran & Southwood, 1982; Ulyshen, 2011).…”

Are insect pollinators more generalist than insect herbivores?

“…Although historically studies in terrestrial plant-animal interactions have focused on direct pairwise interactions, it is now acknowledged that generalization in interactions among species is more widespread than previously thought. This relatively high prevalence of generalist species has been highlighted in both mutualistic (Waser et al 1996) and trophic (Novotny & Basset 2005) interaction networks. Species generalism has some important consequences on the functioning and evolution of ecological systems, and the way we study them.…”

“…The explanatory variables were the interaction type, the total number of plants per web and the interaction term. We incorporated this covariate in the analyses because estimates of species generalism should indeed be considered with regard to the number of potentially available alternative plants (Novotny & Basset 2005). To test which part of the distribution differed, we then analysed each octave separately using linear models (gls function of nlme package) with the same model structure.…”