Influência da cobertura florestal Nativa e da estrutura da vegetação local nas interações tróficas em plantações de cacau sombreadas no sul da Bahia

Abstract

In southern Bahia, a center of endemism, cacao is the most traded agricultural product and about 70% of production is made under the agroforestry system known as cabruca. This system reconciles cacao production with maintaining the original biodiversity. Although we know that arthropods can cause a reduction in annual cacao production, we do not know if changes in the abundance of predatory birds and arthropods, triggered by changes in the amount of forest in the landscape or the structural simplification of cacao crops due to increased intensity. Management through the removal of native vegetation and the use of agrochemicals to increase productivity can influence the local herbivory rate. Therefore, the objective of this study is to investigate the impact of the deforestation process and the simplification of vegetation structure on local herbivory patterns. And test, both experimentally and through effective sampling of birds and arthropods, whether these changes may be due to a relaxation of top-down control at a landscape scale in Atlantic Forest of Southern Bahia State. This study was conducted on 18 cabrucas on a forest cover gradient (3-66%), in which dummy caterpillar were placed and observed for 4 days to verify the predation rate. We sampled understory birds and arthropods on cacao trees, and collected leaves to measure herbivory. Data were also collected from five local descriptors (vertical stratification, basal area, shading, abundance of native trees and cacao abudance). Statistical analysis was based on structural equation modeling and multiple linear regression. We reveal that predation pressure on herbivorous arthropods of cacao leaves, herein assessed by dummy caterpillars, was mainly carried out by invertebrates and not by birds or other vertebrates. Such top-down control was further corroborated by the findings of our path analysis, in which the actual increasing abundance of herbivorous artropods of cacao leaves local was not related with the local abundance of birds but rather, occurs as a response of a decreasing in the local abundance of predatory arthropods. Interesting, although predation rate of dummy catterpillars increased with deforestation while lessened under higher densities of cacao trees, we found no evidence that landscape or local features herein assessed influenced neither the actual invertebrates assemblages (predators or herbivorous) nor cacao leaf damage. Although we detected a trophic cascade involving invertebrates, i.e. predators controlling herbivorous, this interaction did not influence the actual amount of observed leaf damage in cacao trees.