The Trophic Cascade Theory has been used to explain the organization of herbivorous insect communities in tropical ecosystems. In addition, the insect community associated with a species of plant can also be determined by the geographical distribution and taxonomic isolation of the plant. In this study, the following predictions about the number of herbivores associated with particular host plants were tested: (i) plant species belonging to large taxonomic groups with broad geographical distributions have a higher number, (ii) the abundance of ants negatively affects herbivore insect diversity, (iii) local plant diversity positively affects chewing herbivore diversity and (iv) local abundance of a specific host plant positively affects the diversity of sucking herbivores. The samples of insect herbivores were collected from 32 plants (16 plants of Erythroxylum suberosum and 16 of Qualea parviflora) by beating. A total of 71 ants (13 species) and 158 herbivorous insects (90 species) were collected from these two species of plants. The richness and abundance of the insect herbivores collected from E. suberosum differed from those collected from Q. parviflora. The abundance of ants negatively affected the diversity of sucking insects associated with E. suberosum. In addition, the interaction between the variables total plant richness per plot and ant abundance affected the diversity of chewing insects associated with E. suberosum. The density of Q. parviflora per plot affected the diversity of associated sucking insects. In addition, the interaction of the variables abundance of ants and abundance of Q. parviflora influenced the diversity of chewing insects. Our results indicate that there is no predominance of bottom-up or top-down forces in the organization of herbivorous insect communities in this area of tropical savanna, but the roles of these forces on insect communities are guild-dependent., Juliana Kuchenbecker, Marcílio Fagundes., and Obsahuje bibliografii
Physical disturbance by large herbivores can affect species diversity at the community level and concurrently genetic diversity at the species level. As seedling establishment is rarely observed in clonal plants, short-term experiments and demographic studies are unlikely to reveal the response of clonal plants to disturbances. A long-term (30-year) field experiment and the availability of molecular markers allowed us to investigate the clonal structure of populations of Elytrigia atherica subjected to different management regimes. The long-term field study provided us with five replicated blocks that had been subjected to three different management regimes, grazing by cattle, mowing and abandonment. In this study we examined the effects of herbivore grazing and mowing on clonal richness and genetic diversity of populations in salt marshes using multilocus microsatellite genotypes. In addition, phenotypic traits and spatial positions of E. atherica ramets were determined for 20 samples in a 5 × 10m plot in each of the blocks. Abundance and phenotypic traits were affected by the management regimes, resulting in a higher abundance in abandoned fields and plants having shorter and narrower leaves in managed fields. Biomass removal did affect the clonal structure of populations and increased the genetic diversity compared to that in abandoned fields. However, no distinct difference was found between the two management regimes, mowing and grazing. Although seedling recruitment has rarely been observed, the present study shows that such rare events have occurred within the populations studied. Thus, molecular tools can greatly increase our understanding of vegetation dynamics and processes within populations growing under different conditions.
While the key role of termites in the decomposition of litter in the tropics has been acknowledged for a long time, much less information exists on their importance in the recycling of dung of primary consumers, especially herbivores. A review of published studies shows that a diverse group of termites (at least 126 species) has been reported to feed on a wide range of mammalian dung (18 species). Predominantly, wood-feeding and polyphagous wood-litter feeding species were found to feed also frequently on dung. Moreover, we found that termites can quickly remove large amounts of mammalian dung, especially in the dry season, when on average about 1/3 of the dung deposited in a given habitat is removed by termites within one month (with the highest rates observed in savannas). No distinctive preference for mammalian dung over other organic food sources was observed for fungus-growing termites (Macrotermitinae), whereas the majority of the non-fungus growing taxa studied prefer dung over other food. As termites bring large quantities of dung below the soil surface, disturb and enrich soils with nutrients, dung feeding by termites appears to be a previously underestimated process important in the functioning of tropical ecosystems.