Abstract
Human land use tends to decrease the diversity of native plant species and facilitate the invasion and establishment of exotic ones. Such changes in land use and plant community composition usually have negative impacts on the assemblages of native herbivorous insects. Highly specialized herbivores are expected to be especially sensitive to land use intensification and the presence of exotic plant species because they are neither capable of consuming alternative plant species of the native flora nor exotic plant species. Therefore, higher levels of land use intensity might reduce the proportion of highly specialized herbivores, which ultimately would lead to changes in the specialization of interactions in plant-herbivore networks. This study investigates the community-wide effects of land use intensity on the degree of specialization of 72 plant-herbivore networks, including effects mediated by the increase in the proportion of exotic plant species. Contrary to our expectation, the net effect of land use intensity on network specialization was positive. However, this positive effect of land use intensity was partially canceled by an opposite effect of the proportion of exotic plant species on network specialization. When we analyzed networks composed exclusively of endophagous herbivores separately from those composed exclusively of exophagous herbivores, we found that only endophages showed a consistent change in network specialization at higher land use levels. Altogether, these results indicate that land use intensity is an important ecological driver of network specialization, by way of reducing the local host range of herbivore guilds with highly specialized feeding habits. However, because the effect of land use intensity is offset by an opposite effect owing to the proportion of exotic host species, the net effect of land use in a given herbivore assemblage will likely depend on the extent of the replacement of native host species with exotic ones.
Highlights
Consumer-resource interactions between herbivorous insects and their host plants represent a large fraction of terrestrial food webs
We addressed the following questions: 1) Do higher levels of human land use and higher proportions of exotic host species lead to consistent changes in the connectance of plant-herbivore networks? 2) Are monophagous herbivores more sensitive to the effects of land use intensity and proportion of exotic host species? 3) Do plant-herbivore networks composed of internal- versus external-plant feeders respond differently to the increase in land use intensity and in the proportion of exotic host species? We outline the expected effects of land use intensification and proportion of exotic host plant species in a conceptual model (Fig. 1)
We found that the total effect of land use intensity on the residual connectance is partly offset by the proportion of exotic host plant species (Table 2; Fig. 3)
Summary
Consumer-resource interactions between herbivorous insects and their host plants represent a large fraction of terrestrial food webs. Plant-herbivore networks are mostly characterized by highly specialized interactions, since most insect species consume only a small subset of the total set of host plant species [1,2]. Given the accelerated rates of habitat conversion into human-modified environments, it is important to understand whether interaction specialization in plant-herbivore networks increases or decreases with land use intensification. There is a broad consensus that habitat alteration by human activities has negative consequences on native species, usually reducing their overall diversity [3,4,5,6]. The effect of land use intensity on ecological interaction networks, if any, is potentially modified by the concomitant increase in the proportion of exotic species in human-altered environments
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