Distribution of insect galls in xeric and mesic habitats of Floresta Nacional de Silvânia, Brazil

<div><p>ABSTRACT We investigated the insect gall distribution along savanna (xeric) and forest (mesic) vegetation in the Floresta Nacional de Silvânia, Goiás, Brazil. We tested if the insect gall diversity is higher in the xeric vegetation than in the mesic vegetation, as predicted by the hygrothermal stress hypothesis. The insect gall fauna was surveyed between December 2009 and June 2010 in two transects established each vegetation type. In total we found 186 insect gall morphotypes, distributed on 35 botanical families and 61 plant species. Cecidomyiidae (Diptera) induced the most insect galls (34.1%), and the plant family Fabaceae had the greatest richness of insect gall morphotypes (18). We recorded 99 insect gall morphotypes in the forest and 87 morphotypes in the savanna vegetation, being that none insect gall morphotype occurred in both habitats. We found that the insect gall richness and abundance did not differ between forest and savanna transects. On the other hand, the estimated insect gall richness was higher in the forest than in the savanna. Our findings contrary the hygrothermal stress hypothesis possibly because forest habitats have higher plant architecture complexity and occurrence of super-host taxa than the savanna habitats.</p></div

Contrasting Effects of Land Use Intensity and Exotic Host Plants on the Specialization of Interactions in Plant-Herbivore Networks

<div><p>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.</p></div

Path analyses of residual connectance in plant-insect networks of (A) all herbivores, (B) endophages, and (C) exophages, explained by land use intensity, proportion of exotic plants, plant taxonomic span, herbivore taxonomic span, and average taxonomic distinctness (AvTD) of plants and of insects.

<p>Numbers on paths between variables are standardized path coefficients (scaled by the standard deviations of the variables). Green arrows represent positive effects and red arrows represent negative effects. The thickness of lines and arrows is proportional to effect size.</p

Path analyses of proportion of monophages from plant-insect networks of (A) all herbivores, (B) endophages, and (C) exophages.

<p>For details and explanations, see legend for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115606#pone.0115606.g003" target="_blank">Fig. 3</a>.</p

Explanatory models for the residual connectance according to path analyses in Fig. 3 with direct and indirect coefficients and their relative contributions to the explained variation (R²), following Lewinsohn [49].

<p>Only variables with significant total effects are presented. The “direct effect” (<i>d</i>) expresses how much a given variable changes in response to changes in another variable while controlling for the effect of all other variables in the model. The ‘‘indirect effect’’ (<i>i</i>) expresses the influence of a given variable on another variable that is mediated by one or more variables through causal relationships presented in the model.</p><p>Explanatory models for the residual connectance according to path analyses in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115606#pone.0115606.g003" target="_blank">Fig. 3</a> with direct and indirect coefficients and their relative contributions to the explained variation (R²), following Lewinsohn [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115606#pone.0115606.ref049" target="_blank">49</a>].</p

Statistical values of fit, power and explanation of path models for the residual connectance and proportion of monophages.

<p>TLI: Tucker-Lewis Fit Index; CFI: Comparative Fit Index; RMSEA: Root Mean Square Error of Approximation. Statistical power was calculated using the approach by MacCallum et al. [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0115606#pone.0115606.ref032" target="_blank">32</a>].</p><p>Statistical values of fit, power and explanation of path models for the residual connectance and proportion of monophages.</p

Generalized linear mixed model for the effects of phylogenetic isolation (PI) and plant origin (PO) on the richness of insect herbivores, for all herbivores, for assemblages of endophages only, or for assemblages of exophages only.

<p>Positive z-values for the plant-origin effects suggest a higher richness in native plants, whereas negative values suggest a higher richness in exotic plants. Marginal (R²_m) and conditional (R²_c) r-squared values are shown. The number of observations for the entire dataset includes 728 plant species in 30 local assemblages. The number of observations for the endophagous assemblages represents 215 plant species in 13 local assemblages, and that for the exophagous assemblages represents 513 plant species in 17 local assemblages.</p

Global distribution of the 30 plant-herbivore assemblages used in this study.

<p>Green dots represent plant-insect assemblages that are composed of endophagous herbivores and red dots represent those assemblages composed of exophagous herbivores.</p

Generalized mixed model for the effects of phylogenetic isolation (PI) and plant origin (PO) on the mean specialization of insect herbivores, for all herbivores, only endophagous assemblages or only exophagous assemblages.

<p>Positive t-values for the plant origin effects suggest a higher mean specialization of the herbivores on native plants, whereas negative values mean a higher suggest specialization on exotic plants. Marginal (R²_m) and conditional (R²_c) r-squared values are shown. The number of observations for the entire dataset represent 728 plant species, in 30 local assemblages; that for the endophagous assemblages represent 215 plant species in 13 local assemblages, and that for the exophagous assemblages represent 513 plant species in 17 local assemblages.</p

Standardized coefficients of the relationship between phylogenetic isolation (PI) and plant origin (PO) on the response variables (richness, Jaccard’s index, Simpson’s index and specialization) of insect herbivores, for all herbivores, only endophagous assemblages, or only exophagous assemblages.

<p>Positive coefficients for the plant origin effect mean higher values of the response variables on natives compared to exotic plants. Error bars represent ±1.96 standard errors.</p