Superhost Plants Alter the Structure of Plant-Galling Insect Networks in Neotropical Savannas

Habitat area and edges affect the length of trophic chains in a fragmented forest

The food chain length represents how much energy reaches different trophic levels in food webs. Environmental changes derived from human activities have the potential to affect chain length. We explore how habitat area and edges affect chain length through: (1) a bottom-up effect of abundance ('pyramid hypothesis'); (2) the truncation of the highest trophic level ('trophic-rank hypothesis'); and (3) changes in species connectivity patterns ('connectivity hypothesis'). We built plant-leaf miner-parasitoid food webs in 19 remnants of a fragmented Chaco forest from central Argentina. On each remnant, we constructed food webs from different locations at the forest interior and edges. For each food web, we registered the abundance of species, the species richness of each trophic level, estimated the connectivity of their networks, and the average food chain length. We used structural equation models to evaluate the direct and indirect effects of habitat area and edge/interior location on food chain length mediated by species richness, abundance and connectivity. We found no direct effects of habitat area on chain length but chains were longer at forest edges than at their interior. The three mechanisms were supported by our results, although they showed different strengths. First, we found that the interior favours a bottom-up abundance effect from herbivores to parasitoids that positively affected chain length; second, we found that the forest area positively affects plant richness, which has a strong effect on the number of resources used by consumers, with a positive effect on chain length. Third, the remnant area and interior position favoured plant richness with a negative effect on the abundance of parasitoids, which had a positive effect on chain length. In general, the strongest effects on chain length were detected through changes in abundance rather than species richness although abundance was less affected by habitat fragmentation. We evaluated for the first time the effects of human-driven habitat fragmentation on the length of trophic chains in highly diverse plant-herbivore-parasitoid networks. Despite the loss of species, small habitat fragments and edges embedded in the agricultural matrix can support interaction networks, making them conservation targets in managed landscapes.

Diversity of Gall-Inducing Insects Associated With a Widely Distributed Tropical Tree Species: Testing the Environmental Stress Hypothesis

Abiotic factors can affect plant performance and cause stress, which in turn affects plant-herbivore interactions. The Environmental Stress Hypothesis (ESH) predicts that gall-inducing insect diversity will be greater on host plants that grow in stressful habitats. We tested this hypothesis, considering both historical and ecological scales, using the plant Copaifera langsdorffii Desf. (Fabaceae) as a model because it has a wide geographic distribution and is a super-host of gall-inducing insects. According to the ESH, we predicted that 1) on a historical scale, the diversity of gall-inducing insects will be higher in habitats with greater environmental stress and 2) on an ecological scale, gall-inducing insect diversity will be greater on plants that possess greater levels of foliar sclerophylly. We sampled gall-inducing insects on plants of C. langsdorffii in five sites with different levels of water and soil nutrient availability and separated from each other by a distance of up to 470 km. The composition, richness, and abundance of gall-inducing insects varied among study sites. Plants located in more stressful habitats had higher levels of foliar sclerophylly; but richness and abundance of gall-inducing insects were not affected by host plant sclerophylly. Habitat stress was a good predictor of gall-inducing insect diversity on a regional scale, thus corroborating the first prediction of the ESH. No relationship was found between plant sclerophylly and gall-inducing insect diversity within habitats. Therefore, on a local scale, we did not find support for our second prediction related to the ESH.