Interações multitróficas no cerrado: resultados condicionais nas relações entre plantas, herbívoros e predadores

Abstract

The Cerrado biome (Brazilian savanna) is the predominant natural vegetation of Central Brazil and the occurrence of organisms in this biome is restrained by biotic and abiotic factors which may influence the inter-specific interactions between insects and plants. The conditions for the persistence of organisms in nature are a main concern in conservation ecology. Nonetheless, only with the knowledge of organisms natural history and ecological interactions we can give support for their preservation in natural areas. In this thesis we present a series of original studies assessing the diverse interactions among extrafloral nectaried plants, herbivores and their possible natural enemies; and the costs and benefits to plants in supporting insect interactions. In the first part of the thesis we show how an abiotic variable (fire) affected the facultative mutualism between ants and plants and how ants and herbivores responded to the resprouting of plants after fire. It was shown that fire promoted a rapid growth of Banisteriopsis campestris and affected the quality of extrafloral nectar, by increasing sugar concentration. As a consequence, more ants were attracted to the plants, thus decreasing herbivory rates. Nonetheless, in other plant species, B. malifolia, tiny herbivorous were immune to ant attack and inflicted serious damage to leaves. The second part of the thesis is dedicated to evaluate the role of predatory wasps and ants on the deterrence of endophytic herbivores. We show that predatory wasps were more important than ants in the deterrence of endophytic beetles. To conclude, in the third part of this thesis we also show how plant-insect relationships are conditional (to biotic and abiotic factors) and the strategies that caterpillars and thrips use to escape from ant attack while feeding on plant reproductive parts. Florivorous myrmecophilous lycaenid caterpillars release sugared substances to antguards and are not attacked. Thrips co-occurred with ants in extrafloral nectaried plants and their herbivory may seem small and negligible in leaves and flower buds, but fruits (samaras) damaged by thrips are distorted and asymmetric, what restrains its dispersive capabilities by wind. Our studies contribute to a better understanding of plant-insect relationships in the cerrado, the largest hotspot in Brazil. The mutualism between extrafloral nectaried plants and ants is one of the most interesting interactions in nature, but it is highly conditional and the costs and benefits involved in these associations depend on the parties involved (ants and herbivores) and abiotic factors.