Artenreichtum von Herbivoren-Parasitoiden-Gesellschaften an Leguminosen

Abstract

This dissertation is focused on tropical plant-herbivore-parasitoid systems. More than 8000 pods of 15 species of legumes were collected in Madagascar and their endophagous insect community (herbivores and parasitoids) was analyzed. Patterns found were compared to those of communities in temperate regions (Germany) and other tropical regions (Ivory Coast). The endophagous community in Madagascar comprised 55 species of herbivores and 109 species of parasitoids. Species density was determined by the number of herbivore species per plant species and the number of parasitoid species per host species, respectively. Herbivore load was characterized by the number of herbivore individuals per pod, percent parasitism was characterized by the ratio of parasitoid indivuals to total insect individuals per pod. Specificity of both herbivores and parasitoids was assessed by the number of host plant species used by them. These parameters were related to host plant characterictics. Consequences of resource fragmentation on the community were tested experimentally. Distance between legume individuals was assessed and distances between populations of the same species of leguminous plant were experimentally varied. Malagasy communities showed important differences in comparison to communities in Germany. There were more spcies of herbivores per plant species in Madagascar, supporting the assumption that tropical communities have smaller niches. This coincided with a higher degree of specificity in malagasy herbivores, pointing at the possibly good capacity to cope with natural fragmentation in the tropics. Consequently, no effect of resource fragmentation (distance between host plants) on herbivores could be found. Madagascar had less parasitoid species per host species compared to Germany. Malagasy parasitoids were more specific than those in Germany. As a consequence they were strongly affected by the fragmentation of resources. Percent parasitism also decreased with increasing distance between legumes of the same species, i. e. decrease of species richness was related to a loss of ecosystem functioning. This supports the hypothesis that organisms of higher trophic levels are more susceptible to fragmentation. Percent parasitism did not differ between Madagascar and Germany, despite parasitoid complexes being smaller in Madagascar. This emphasizes the importance of parasitoids in tropical communities. Comparisons of the malagasy communities with those of the Ivory Coast show that tropical communities do not correspond in many characteritics. One can not necessarily derive general patterns from a single tropical locality. Conclusions from Madagascar can therefore not readily be transferred to other tropical regions. Due to the high variation within tropical communities, the consideration of as many different tropical regions as possible is most likely to permit the detection of general patterns. For Madagascar the results permit conclusions that are pointed at the consequences of ongoing fragmentation processes. Their negative outcome does not only threaten species richness but especially ecological functioning in higher trophic levels, likely leading to a decrease of the efficiency of biological control of phytophagous insects.