The recovery of ant communities in regenerating tropical forests
Tropische Regenwälder sind die artenreichsten aller terrestrischen Ökosysteme. Die rasch fortschreitende Zerstörung dieser Wälder stellt eine grosse Bedrohung für die biologische Vielfalt der Erde dar. Jedoch ist die Zerstörung des Waldes selten von Dauer — die landwirtschaftlichen Erträge der umgew...
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Table of Contents: Tropical forests are the most diverse of all terrestrial ecosystems. Though occupying only 7% of the Earth‘s land surface, they probably sustain over two-thirds of all species. Tropical forests are disappearing at fast rates and this has been identified as a major threat to global biodiversity. Deforestation, however, is rarely permanent — converted land is exhausted, abandoned and allowed to regrow. Critical for the future of biodiversity is the degree to which these secondary forests can replace old-growth forests as habitat for forest dwelling species and fulfill similar ecosystem functions. Yet, relatively little is known about the value of secondary forests for the conservation of biodiversity and ecosystem functions. The main objective of my dissertation was to evaluate to what degree tropical secondary forests contribute to the conservation of biodiversity and ecosystem functions. To do so, I used soil and leaf-litter ants (Hymenoptera: Formicidae) — a dominant, species rich, and functionally important group of invertebrates — as model organisms. My approach was twofold: On the one hand, I analyzed the effects of secondary forest succession on various facets of the diversity and community structure of ants. On the other hand, I investigated the successional changes in the ecosystem and how these influence the community assembly of ants. All studies were carried out in the Rio Cachoeira Nature Reserve in the Atlantic Forest of Brazil. I sampled the ant community in a total of 27 study sites that encompassed a gradient of naturally regenerating forests in three different successional age stages. Additionally, I also sampled the ant community in pastures and old-growth forests for comparison. Richness and composition of ant assemblages in secondary forests recovered slowly and did not approach conditions typical of old-growth forests. The distribution of ant taxa along the successional gradient was arranged in a nested pattern — ant taxa of younger successional stages were a subset of taxa present in older stages. Edaphic conditions had only minor influence on the recovery process. Overall, richness of ants was lower at study sites with water-logged soils than at sites where soils did not exhibit hydromorphic properties. The hypogeic ant assemblage recovered more slowly than the epigeic assemblage. Moreover, estimated recovery times of 50 to several hundred years suggest, it would take much longer than previously presumed for complete recolonization. The functional diversity of ant communities was assessed using a set of important functional traits. It became evident that species diversity is closely linked to the functional diversity of ant communities. Since functional complementarity among coexisting species was high, reduced species diversity in secondary forests resulted in proportionally reduced functional diversity of local ant communities. Rare, but functionally unique species, were largely missing in secondary forests. These results suggest that secondary forests may only provide reduced levels of ecosystem functions in comparison to old-growth forests. Knowledge about the rules that govern the assembly of local communities from the regional species pool is useful in providing a scientific foundation for the conservation of biodiversity. Favorable resource availability and habitat conditions appeared to be more important for the assembly of ant communties from the regional species pool than interactive processes such as competition among species. Despite marked differences in species richness and composition of ant communities, the pattern of community assembly varied little in forests of different successional stages. In a field experiment, I studied the resource availability for leaf-litter ants in more detail. I examined bait preferences of ants (protein baits vs. carbohydrate baits) in secondary forests and old-growth forests. The ants preferred protein baits in secondary forests, yet preferred carbohydrate baits in old-growth forests. This suggests a shift in stoichiometric balances between secondary and old-growth forests, which might impact the functional interactions in these ecosystems. In conclusion, secondary forests do not act as refuges for many species which currently occur in discontinuous patches of old-growth forest in the highly endangered Atlantic Forest of Brazil. The structure of ant communities seems to be primarily a product of local environmental conditions, such as the stoichiometric balances of the ecosystem. The observed loss of both species and functional diversity in secondary forests offers no reason to believe that the ecological functions provided by secondary forests are buffered against species loss through functional redundancy. These results highlight the crucial importance of old-growth forests for the conservation of ecosystem functions and biodiversity in the tropical wooded landscape.