Understanding variation in resource specialization is important for progress on issues that include coevolution, community assembly, ecosystem processes, and the latitudinal gradient of species richness. Herbivorous insects are useful models for studying resource specialization, and the interaction between plants and herbivorous insects is one of the most common and consequential ecological associations on the planet. However, uncertainty persists regarding fundamental features of herbivore diet breadth, including its relationship to latitude and plant species richness. Here, we use a global dataset to investigate host range for over 7,500 insect herbivore species covering a wide taxonomic breadth and interacting with more than 2,000 species of plants in 165 families. We ask whether relatively specialized and generalized herbivores represent a dichotomy rather than a continuum from few to many host families and species attacked and whether diet breadth changes with increasing plant species richness toward the tropics. Across geographic regions and taxonomic subsets of the data, we find that the distribution of diet breadth is fit well by a discrete, truncated Pareto power law characterized by the predominance of specialized herbivores and a long, thin tail of more generalized species. Both the taxonomic and phylogenetic distributions of diet breadth shift globally with latitude, consistent with a higher frequency of specialized insects in tropical regions. We also find that more diverse lineages of plants support assemblages of relatively more specialized herbivores and that the global distribution of plant diversity contributes to but does not fully explain the latitudinal gradient in insect herbivore specialization.

For numerous taxa, species richness is much higher in tropical than in temperate zone habitats 1 . A major challenge in community ecology and evolutionary biogeography is to reveal the mechanisms underlying these differences. For herbivorous insects, one such mechanism leading to an increased number of species in a given locale could be increased ecological specialization, resulting in a greater proportion of insect species occupying narrow niches within a community. We tested this hypothesis by comparing host specialization in larval Lepidoptera (moths and butterflies) at eight different New World forest sites ranging in latitude from 15° S to 55° N. Here we show that larval diets of tropical Lepidoptera are more specialized than those of their temperate forest counterparts: tropical species on average feed on fewer plant species, genera and families than do temperate caterpillars. This result holds true whether calculated per lepidopteran family or for a caterpillar assemblage as a whole. As a result, there is greater turnover in caterpillar species composition (greater fi diversity) between tree species in tropical faunas than in temperate faunas. We suggest that greater specialization in tropical faunas is the result of differences in trophic interactions; for example, there are more distinct plant secondary chemical profiles from one tree species to the next in tropical forests than in temperate forests as well as more diverse and chronic pressures from natural enemy communities.Ecological theory requires that organisms differ in their use of shared, limiting resources if they are to coexist. The role of resource specialization in fostering biodiversity is thus a central issue in ecology and evolutionary biology. A longstanding hypothesis predicts a direct relationship between ecological specialization and species richness in communities 2 . Specialization reduces interspecific competition and facilitates species coexistence by partitioning niche space 3,4 . Character divergence across generations in response to trophic interactions or competition 5 provides an evolutionary mechanism by which species richness and specialization can increase together 6 " 8 . Beginning with observations recounted by Darwin 9 and Wallace 10 , examples of ecological specialization in tropical organisms have fostered a widespread perception that specificity of interactions is a hallmark of the high-diversity tropics.Although biotic inventories often confirm the higher species richness of tropical communities than those at higher latitudes 1 , few studies have quantified increased ecological specialization along a latitudinal gradient 11 . Novotny et al. 12 recently challenged the notion that herbivorous insects are more specialized in the tropics by the use of a quantitative comparison of host specificity of herbivorous insects in tropical forests of Papua New Guinea and those in temperate forests of central Europe. They reported a similar host specificity among temperate and tropical herbivorous insects and concluded that the ...

Abstract. Ecological specialization is a fundamental and well-studied concept, yet its great reach and complexity limit current understanding in important ways. More than 20 years after the publication of D. J. Futuyma and G. Moreno's oft-cited, major review of the topic, we synthesize new developments in the evolution of ecological specialization. Using insectplant interactions as a model, we focus on important developments in four critical areas: genetic architecture, behavior, interaction complexity, and macroevolution. We find that theory based on simple genetic trade-offs in host use is being replaced by more subtle and complex pictures of genetic architecture, and multitrophic interactions have risen as a necessary framework for understanding specialization. A wealth of phylogenetic data has made possible a more detailed consideration of the macroevolutionary dimension of specialization, revealing (among other things) bidirectionality in transitions between generalist and specialist lineages. Technological advances, including genomic sequencing and analytical techniques at the community level, raise the possibility that the next decade will see research on specialization spanning multiple levels of biological organization in non-model organisms, from genes to populations to networks of interactions in natural communities. Finally, we offer a set of research questions that we find to be particularly pressing and fruitful for future research on ecological specialization.

Insect outbreaks are expected to increase in frequency and intensity with projected changes in global climate through direct effects of climate change on insect populations and through disruption of community interactions. Although there is much concern about mean changes in global climate, the impact of climatic variability itself on species interactions has been little explored. Here, we compare caterpillar-parasitoid interactions across a broad gradient of climatic variability and find that the combined data in 15 geographically dispersed databases show a decrease in levels of parasitism as climatic variability increases. The dominant contribution to this pattern by relatively specialized parasitoid wasps suggests that climatic variability impairs the ability of parasitoids to track host populations. Given the important role of parasitoids in regulating insect herbivore populations in natural and managed systems, we predict an increase in the frequency and intensity of herbivore outbreaks through a disruption of enemy-herbivore dynamics as climates become more variable.climate change ͉ herbivore ͉ outbreak ͉ parasitoid ͉ top-down

Tachinidae are one of the most diverse and ecologically important families in the order Diptera. As parasitoids, they are important natural enemies in most terrestrial ecological communities, particularly as natural enemies of larval Lepidoptera. Despite their diversity and ecological impact, relatively little is known about the evolution and ecology of tachinids, and what is known tends to be widely dispersed in specialized reports, journals, or texts. In this review we synthesize information on the evolutionary history, behavior, and ecology of tachinids and discuss promising directions for future research involving tachinids. We provide an overview of the phylogenetic history and geographic diversity of tachinids, examine the evolution of oviposition strategies and host associations, review known mechanisms of host location, and discuss recent studies dealing with the ecological interactions between tachinids and their hosts. In doing so, we highlight ways in which investigation of these parasitoids provides insight into such topics as biogeographic patterns of diversity, the evolution of ecological specialization, the tritrophic context of enemy-herbivore interactions, and the role of host location behavior in shaping host range.

A conceptual divide exists between ecological and evolutionary approaches to understanding adaptive radiation, although the phenomenon is inherently both ecological and evolutionary. This divide is evident in studies of phytophagous insects, a highly diverse group that has been frequently investigated with the implicit or explicit goal of understanding its diversity. Whereas ecological studies of phytophagous insects increasingly recognize the importance of tri-trophic interactions as determinants of niche dimensions such as host-plant associations, evolutionary studies typically neglect the third trophic level. Here we attempt to reconcile ecological and evolutionary approaches through the concept of the ecological niche. We specifically present a tri-trophic niche concept as a foil to the traditional bi-trophic niche concept for phytophagous insects. We argue that these niche concepts have different implications for understanding herbivore community structure, population divergence, and evolutionary diversification. To this end, we offer contrasting empirical predictions of bi-and tri-trophic niche concepts for patterns of community structure, the process of population divergence, and patterns of evolutionary diversification of phytophagous insects.

The extraordinary diversity of phytophagous insects may be attributable to their narrow specialization as parasites of plants, with selective tradeoffs associated with alternate host plants driving genetic divergence of host-associated forms via ecological speciation. Most phytophagous insects in turn are attacked by parasitoid insects, which are similarly specialized and may also undergo host-associated differentiation (HAD). A particularly interesting possibility is that HAD by phytophagous insects might lead to HAD in parasitoids, as parasitoids evolve divergent lineages on the new host plant-specific lineages of their phytophagous hosts. We call this process 'cascading host-associated differentiation' (cascading HAD). We tested for cascading HAD in parasitoids of two phytophagous insects, each of which consists of genetically distinct host-associated lineages on the same pair of goldenrods (Solidago). Each parasitoid exhibited significant host-associated genetic divergence, and the distribution and patterns of divergence are consistent with divergence in sympatry. Although evidence for cascading HAD is currently limited, our results suggest that it could play an important role in the diversification of parasitoids attacking phytophagous insects. The existence of cryptic host-associated lineages also suggests that the diversity of parasitoids may be vastly underestimated.

There is growing awareness of the importance of natural selection in driving genetic divergence and speciation, and several of the most apparent cases of this ecological speciation are provided by the existence of genetically distinct host forms in phytophagous insects. Such examples of host-associated differentiation (HAD) have become increasingly documented, and the implications of this phenomenon for the diversification of insects are becoming widely appreciated. However, instances of HAD remain rare relative to insect diversity and are sparsely distributed both ecologically and taxonomically. We sought to assess the frequency of HAD in a model herbivore community by examining genetic divergence in a variety of herbivores that feed on two closely related and broadly sympatric species of goldenrod (Solidago altissima and S. gigantea). Using mitochondrial DNA and allozyme data, in conjunction with previously published studies, we found that four of nine herbivores exhibited evidence of HAD, including possible host races or cryptic species. Using a range of reasonable substitution rate estimates for cytochrome oxidase I mitochondrial DNA, we found that HAD appears to have proceeded asynchronously across taxa. This pattern, along with the broadly sympatric distribution of host plants and the specialized life histories of the phytophagous insects, is consistent with sympatric divergence in some or all of these taxa. Although further behavioral and ecological study is needed, our survey of HAD in a community of herbivores indicates that ecological (perhaps sympatric) speciation may have been responsible for generating a significant fraction of the extant diversity of phytophagous insects.