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Biodiversity ensures ecosystem functioning and provisioning of ecosystem services, but it remains unclear how biodiversity–ecosystem multifunctionality relationships depend on the identity and number of functions considered. Here, we demonstrate that ecosystem multifunctionality, based on 82 indicator variables of ecosystem functions in a grassland biodiversity experiment, increases strongly with increasing biodiversity. Analysing subsets of functions showed that the effects of biodiversity on multifunctionality were stronger when more functions were included and that the strength of the biodiversity effects depended on the identity of the functions included. Limits to multifunctionality arose from negative correlations among functions and functions that were not correlated with biodiversity. Our findings underline that the management of ecosystems for the protection of biodiversity cannot be replaced by managing for particular ecosystem functions or services and emphasize the need for specific management to protect biodiversity. More plant species from the experimental pool of 60 species contributed to functioning when more functions were considered. An individual contribution to multifunctionality could be demonstrated for only a fraction of the species.

Most of Earth's biodiversity is found in 36 biodiversity hotspots, yet less than 10% natural intact vegetation remains. We calculated models projecting the future state of most of these hotspots for the year 2050, based on future climatic and agroeconomic pressure. Our models project an increasing demand for agricultural land resulting in the conversion of >50% of remaining natural intact vegetation in about one third of all hotspots, and in 26 hotspots resulting from climatic pressure. This confirms that, in the short term, habitat loss is of greater concern than climate change for hotspots and their biodiversity. Hotspots are most severely threatened in tropical Africa and parts of Asia, where demographic pressure and the demand for agricultural land is highest. The speed and magnitude of pristine habitat loss is, according to our models, much greater than previously shown when combining both scenarios on future climatic and agroeconomic pressure. (VLID)4347668

Rapid ecosystem function assessment (REFA) methods have been proposed as a fast, easy, and cost-efficient solution to approximate ecosystem functions in a standardized way. While REFA has already been applied, no systematic and detailed description of the field protocols and implementation of the method is currently available. This handbook summarizes the details of applying REFA methods for measurements of ecosystem functions in the field.

Human-caused declines in biodiversity have stimulated intensive research on the consequences of biodiversity loss for ecosystem services and policy initiatives to preserve the functioning of ecosystems. Short-term biodiversity experiments have documented positive effects of plant species richness on many ecosystem functions, and longer-term studies indicate, for some ecosystem functions, that biodiversity effects can become stronger over time. Theoretically, a biodiversity effect can strengthen over time by an increasing performance of high-diversity communities, by a decreasing performance of low-diversity communities, or a combination of both processes. Which of these two mechanisms prevail, and whether the increase in the biodiversity effect over time is a general property of many functions remains currently unclear. These questions are an important knowledge gap as a continuing decline in the performance of low-diversity communities would indicate an ecosystem-service debt resulting from delayed effects of species loss on ecosystem functioning. Conversely, an increased performance of high-diversity communities over time would indicate that the benefits of biodiversity are generally underestimated in short-term studies.Analyzing 50 ecosystem variables over 11 years in the world’s largest grassland biodiversity experiment, we show that overall plant diversity effects strengthened over time. Strengthening biodiversity effects were independent of the considered compartment (above- or belowground), organizational level (ecosystem variables associated with the abiotic habitat, primary producers, or higher trophic levels such as herbivores and pollinators), and variable type (measurements of pools or rates). We found evidence that biodiversity effects strengthened because of both a progressive decrease in functioning in species-poor and a progressive increase in functioning in species-rich communities. Our findings provide evidence that negative feedback effects at low biodiversity are as important for biodiversity effects as complementarityamong species at high biodiversity. Finally, our results indicate that a current loss of species will result in a future impairment of ecosystem functioning, potentially decades beyond the moment of species extinction. Human-caused declines in biodiversity have stimulated intensive research on the consequences of biodiversity loss for ecosystem services and policy initiatives to preserve the functioning of ecosystems. Short-term biodiversity experiments have documented positive effects of plant species richness on many ecosystem functions, and longer-term studies indicate, for some ecosystem functions, that biodiversity effects can become stronger over time. Theoretically, a biodiversity effect can strengthen over time by an increasing performance of high-diversity communities, by a decreasing performance of low-diversity communities, or a combination of both processes. Which of these two mechanisms prevail, and whether the increase in the biodiversity effect over time is a general property of many functions remains currently unclear. These questions are an important knowledge gap as a continuing decline in the performance of low-diversity communities would indicate an ecosystem-service debt resulting from delayed effects of species loss on ecosystem functioning. Conversely, an increased performance of high-diversity communities over time would indicate that the benefits of biodiversity are generally underestimated in short-term studies. Analyzing 50 ecosystem variables over 11 years in the world's largest grassland biodiversity experiment, we show that overall plant diversity effects strengthened over time. Strengthening biodiversity effects were independent of the considered compartment (above- or belowground), organizational level (ecosystem variables associated with the abiotic habitat, primary producers, or higher trophic levels such as herbivores and pollinators), and variable type (measurements of pools or rates). We found evidence that biodiversity effects strengthened because of both a progressive decrease in functioning in species-poor and a progressive increase in functioning in species-rich communities. Our findings provide evidence that negative feedback effects at low biodiversity are as important for biodiversity effects as complementarity among species at high biodiversity. Finally, our results indicate that a current loss of species will result in a future impairment of ecosystem functioning, potentially decades beyond the moment of species extinction.

Including ecosystem functions into restoration ecology has been repeatedly suggested, yet there is limited evidence that this is taking place without bias to certain habitats, species, or functions. We reviewed the inclusion of ecosystem functions in restoration and potential relations to habitats and species by extracting 224 publications from the literature (2004-2013). Most studies investigated forests, fewer grasslands or freshwaters, and fewest wetlands or marine habitats. Of all studies, 14% analyzed only ecosystem functions, 44% considered both biotic composition and functions, 42% exclusively studied the biotic component, mostly vascular plants, more rarely invertebrates or vertebrates, and least often microbes. Most studies investigating ecosystem functions focused on nutrient cycling (26%), whereas productivity (18%), water relations (16%), and geomorphological processes (14%) were less covered; carbon sequestration (10%), decomposition (6%), and trophic interactions (6%) were rarely studied. Monitoring of ecosystem functions was common in forests and grasslands, but the functions considered depended on the study organisms. These associations indicate research opportunities for certain habitats, species, and functions. Overall, the call to include ecosystem functions in restoration has been heard; however, a lack of clarity about the ecosystem functions to be included and deficits of feasible field methods are major obstacles for a functional approach. Restoration ecology should learn from recent advances in rapid assessment of ecosystem functions, and by a closer integration with biodiversity-ecosystem functioning research. Not all functions need to be measured in all ecosystems, but more functions than the few commonly addressed would improve the understanding of restored ecosystems.

Fragmentation of habitats, especially of tropical rainforests, ranks globally among the most pervasive man-made disturbances of ecosystems. There is growing evidence for long-term effects of forest frag-mentation and the accompanying creation of artificial edges on ecosystem functioning and forest structure, which are altered in a way that generally transforms these forests into early successional systems. Edge-induced disruption of species interactions can be among the driving mechanisms governing this transformation. These species interactions can be direct (trophic interactions, competition, etc.) or indirect (modification of the resource availability for other organisms). Such indirect interactions are called ecosystem engineering. Leaf-cutting ants of the genus Atta are dominant herbivores and keystone-species in the Neotropics and have been called ecosystem engineers. In contrast to other prominent ecosystem engineers that have been substantially decimated by human activities some species of leaf-cutting ants profit from anthropogenic landscape alterations. Thus, leaf-cutting ants are a highly suitable model to investigate the potentially cascading effects caused by herbivores and ecosystem engineers in modern anthropogenic landscapes following fragmentation. The present thesis aims to describe this interplay between consequences of forest fragmentation for leaf-cutting ants and resulting impacts of leaf-cutting ants in fragmented forests. The cumulative thesis starts out with a review of 55 published articles demonstrating that herbivores, especially generalists, profoundly benefit from forest edges, often due to (1) favourable microenviron-mental conditions, (2) an edge-induced increase in food quantity/quality, and (3; less well documented) disrupted top-down regulation of herbivores (Wirth, Meyer et al. 2008; Progress in Botany 69:423-448). Field investigations in the heavily fragmented Atlantic Forest of Northeast Brazil (Coimbra forest) were subsequently carried out to evaluate patterns and hypotheses emerging from this review using leaf-cutting ants of the genus Atta as a model system. Colony densities of both Atta species occuring in the area changed similarly with distance to the edge but the magnitude of the effect was species-specific. Colony density of A. cephalotes was low in the forest interior (0.33 ± 1.11 /ha, pooling all zones >50 m into the forest) and sharply increased by a factor of about 8.5 towards the first 50 m (2.79 ± 3.3 /ha), while A. sexdens was more uniformly distributed (Wirth, Meyer et al. 2007; Journal of Tropical Ecology 23:501-505). The accumulation of Atta colonies persisted at physically stable forest edges over a four-year interval with no significant difference in densities between years despite high rates of colony turn-over (little less than 50% in 4 years). Stable hyper-abundant populations of leaf-cutting ants accord with the constantly high availability of pioneer plants (their preferred food source) as previously demonstrated at old stabilised forest edges in the region (Meyer et al. submitted; Biotropica). In addition, plants at the forest edge might be more attractive to leaf-cutting ants because of their physiological responses to the edge environment. In bioassays with laboratory colonies I demonstrated that drought-stressed plants are more attractive to leaf-cutting ants because of an increase in leaf nutrient content induced by osmoregulation (Meyer et al. 2006; Functional Ecology 20:973-981). Since plants along forest edges are more prone to experience drought stress, this mechanism might contribute to the high resource availabil-ity for leaf-cutting ants at forest edges. In light of the hyper-abundance of leaf-cutting ants within the forest edge zone (first 50 m), their po-tentially far-reaching ecological importance in anthropogenic landscapes is apparent. Based on previous colony-level estimates, we extrapolated that herbivory by A. cephalotes removes 36% of the available foliage at forest edges (compared to 6% in the forest interior). In addition, A. cephalotes acted as ecosys-tem engineers constructing large nests (on average 55 m2: 95%-CI: 22-136) that drastically altered forest structure. The ants opened gaps in the canopy and forest understory at nest sites, which allowed three times as much light to reach the nest surface as compared to the forest understory. This was accompa-nied by an increase in soil temperatures and a reduction in water availability. Modifications of microcli-mate and forest structure greatly surpassed previously published estimates. Since higher light levels were detectable up to about 4 m away from the nest edge, an area roughly four times as big as the actual nest (about 200 and 50 m2, respectively) was impacted by every colony, amounting to roughly 6% of the total area at the forest edge (Meyer et al. in preparation; Ecology). The hypothesized impacts of high cutting pressure and microclimatic alterations at nest sites on forest regeneration were directly tested using transplanted seedlings of six species of forest trees. Nests of A. cephalotes differentially impacted survival and growth of seedlings. Survival differed highly significantly between habitats and species and was generally high in the forest, yet low on nests where it correlated strongly with seed size of the species. These results indicate that the disturbance regime created by leaf-cutting ants differs from other distur-bances, since nest conditions select for plant species that profit from additional light, yet are large-seeded and have resprouting abilities, which are best suited to tolerate repeated defoliation on a nest (Meyer et al. in preparation; Journal of Tropical Ecology). On an ecosystem scale leaf-cutting ants might amplify edge-driven microclimatic alterations by very high rates of herbivory and the maintenance of canopy gaps above frequent nests. By allowing for an increased light penetration Atta may, ultimately, contribute to a dominating, self-replacing pioneer communities at forest edges, possibly creating a positive feed-back loop. Based on the persisting hyper-abundance of leaf-cutting ants at old edges of Coimbra forest and the multifarious impacts documented, we conclude that the ecological importance of leaf-cutting ants in pristine forests, where they are commonly believed to be keystone species despite very low colony densities, is greatly surpassed in anthropogenic landscapes In fragmented forests, Atta has been identified as an essential component of a disturbance regime that causes a post-fragmentation retrogressive succession. Apparently, these forests have reached a new self-replacing secondary state. I suggest additional human interference in form of thoughtful management in order to break this cycle of self-enhancing disturbance and to enable forest regeneration along the edges of threatened forest remnants. Thereby the situation of the forest as a whole can be ameliorated and the chances for a long-term retention of biodiversity in these landscapes increased. Die Fragmentierung von Habitaten und insbesondere tropischer Regenwälder gehört zu den weitreichendsten Folgen men-schlicher Störung von Ökosystemen. Eine Reihe von Langzeiteffekten verursacht durch Waldfragmentierung und die damit verbundene Schaffung von Waldrändern verändert Ökosystem-Funktion und Struktur von Wäldern nachhaltig. Dadurch werden diese Wälder generell in frühsukessionäre Stadien überführt. Die waldrandbedingte Störung von Artinteraktionen kann dabei maßgeblich zur Transformation dieser Wälder beitragen, wobei die Artinteraktionen direkter (trophische Interaktionen, Konkurrenz, etc.) oder indirekter Natur (Veränderung der Ressourcenverfügbarkeit für andere Organsimen) sein können. Solche indirekte Interaktionen nennt man „Ecosystem engineering“. Blattschneiderameisen können Ökosystemingenieure sein und sind darüber hinaus dominante Herbivoren in den Neotropen. Im Gegensatz zu vielen Ökosystemingenieuren, die durch menschlichen Einfluss stark dezimiert wurden, können Blattschneiderameisen von anthropogenen Landschaftsveränderungen profitieren. Daher sind sie gut geeignete Modellorganismen, um die möglicherweise weitreichenden Auswirkungen von Herbivoren und Ökosystemingenieuren in heutigen, menschgemachten Landschaften zu studieren. Diese kumulative Promotionsarbeit dokumentiert wechselseitige Effekte zwischen Waldfragementierung und den Aktivi-täten von Blattschneiderameisen. Sie beginnt mit einem Literaturreview, der zeigt, dass Herbivoren (besonders Generalisten) von Waldrändern profitieren. Dem zugrunde liegen (1) vorteilhafte Umweltbedingungen, (2) eine höhere Quantität/Qualität von Futter am Waldrand und (3, weniger gut dokumentiert) eine Störung von top-down Kontrolle (Wirth, Meyer et al. 2008; Progress in Botany 69:423-448). Nachfolgend wurden Felduntersuchungen an Blattschneiderameisen der Gattung Atta im stark fragmentierten Atlantischen Regenwald Nordostbrasiliens (Coimbra) durchgeführt, um im Review gefundene Muster und Hypothesen zu überprüfen. Kolonien beider in der Region vorkommender Arten waren am Waldrand konzentriert. Atta cephalotes Kolonien waren selten im Waldesinneren (0,33 ± 1,11 /ha) und etwa 8,5-mal so häufig in den ersten 50 m des Waldes (2,79 ± 3,3 /ha); A. sexdens war etwas homogener verbreitet (Wirth, Meyer et al. 2007; Journal of Tropical Ecology 23:501-505). Die Akkumulierung von Atta-Kolonien entlang alter Waldränder blieb über einen Beobachtungszeitrum von vier Jahren stabil; ohne Änderung der Koloniedichten und trotz eines hohen Kolonieturnovers (etwa 50% in vier Jahren). Solch stabile hyperabundante Populationen von Blattschneiderameisen (Meyer et al. submitted; Biotropica) passen zur bereits zuvor an Waldrändern dokumentierten dauerhaft hohen Verfügbarkeit von Pionierpflanzen (der bevorzugten Futterquelle von Blattschneiderameisen). Darüber hinaus könnten Pflanzen an Waldrändern aufgrund ihrer physiologischen Reaktionen auf die Randbedingungen für Blattschneiderameisen attraktiver sein. In Bioassays mit Laborkolonien habe ich gezeigt, dass trocken-gestresste Pflanzen aufgrund eines höheren Nährstoffgehalts für Blattschneiderameisen attraktiv sind (Meyer et al. 2006; Functional Ecology 20:973-981). Da Pflanzen an Waldrändern anfälliger für Trockenstress sind, kann dieser Mechanismus zur hohen Ressourcenverfügbarkeit für Blattschneiderameisen an Waldrändern beitragen. Welche ökologische Bedeutung haben Blattschneiderameisen in randdominierten anthropogenen Landschaften in Anbetracht ihrer hohen Dichten in den ersten 50 m des Waldes? Extrapolationen basierend auf zuvor bestimmten Herbivorieraten auf Kolonielevel zeigen, dass A. cephalotes an Waldrändern 36% der verfügbaren Blattfläche entfernt (im Vergleich zu nur 6% im Waldesinneren). Außerdem agierte A. cephalotes als Ökosystemingenieur da die Art große Nester anlegte (im Durchschnitt 55 m2: 95%-CI: 22-136), die drastisch die Waldstruktur veränderten. Die Ameisen öffneten Lücken in Kronendach und Unterwuchs des Waldes. Dadurch konnte im Vergleich zum ungestörten Wald eine dreimal höhere Lichtmenge die Nester erreichen, was mit erhöhten Bodentemperaturen und einer verringerten Wasserverfügbarkeit einherging. Ein höherer Lichtgenuss wurde bis in einen Abstand von etwa 4 m vom Nestrand detektiert. Dadurch vervierfacht sich die von jedem Nest beeinflusste Fläche auf ca. 200 m2; diese Flächen addieren sich zu etwa 6% der Gesamtwaldfläche am Rand auf. Diese hier dokumentierte Veränderungen in Mikroklima und Waldstruktur gingen weit über bis dahin publizierte Effekte hinaus (Meyer et al. in preparation; Ecology). Die hypothetisierten Folgen von hohem Schneidedruck und mikroklimatischer Veränderungen an Nestern auf Waldregenration wurden direkt anhand transplantierter Keimlinge von sechs Waldbaumarten untersucht. Die Nester von A. cephalotes veränderten das Wachstum der Pflanzen und ihr Überleben unterschied sich hoch signifikant zwischen Habitaten und Pflanzenarten. Es war generell hoch im Waldesinneren jedoch niedrig auf Nestern wo es stark mit der Samengröße der Art korrelierte. Diese Ergebnisse legen nahe, dass Nestbedingungen Pflanzen selektieren, die von erhöhtem Lichtgenuss profitieren, jedoch großsamig sind und die Fähigkeit zum Wiederaustreiben haben und so am besten befähigt sind wiederholtes Entblättern auf dem Nest zu tolerieren (Meyer et al. in preparation; Journal of Tropical Ecology). Auf Ökosystemlevel kann Atta aufgrund der extrem hohen Herbivorieraten und dem Aufrechterhalten von Nestlichtungen eine höhere Lichtpenetration verursachen und das randtypische Mikroklima verstärken. In letzter Konsequenz würden Blattschneiderameisen so die dominierende, sich selbst verjüngende Pionierpflanzengemeinschaft an Waldrändern begünstigen von der sie selbst profitiert haben, was einen selbstverstärkenden Kreislauf bildet. Aufgrund der anhaltenden Hyperabundanz von Blattschneiderameisen entlang alter Waldränder in Coimbra und den vielfältigen dokumentierten Effekten schlussfolgern wir, dass die ökologische Bedeutung von Blattschneiderameisen, die bereits in ungestörten Wäldern trotz sehr niedriger Koloniedichten gezeigt wurde, in anthropogenen Landschaften drastisch erhöht ist. In fragmentierten Wäldern haben wir Atta als eine maßgebliche Komponente des Störungsregimes identifiziert, das eine rückwärtsgerichtete Sukzession als Folge von Fragmentierung bedingt. Augenscheinlich haben diese Wälder einen neuen, sich reproduzierenden Sekundärzustand erreicht. Um diesen selbstverstärkenden Kreislauf von Störungen zu durchbrechen erscheint weiteres menschliches Eingreifen basierend auf durchdachten Managementstrategien notwendig, um Waldregeneration entlang der Ränder der bedrohten Waldreste zu ermöglichen. Dadurch würde sich die Situation des Waldes insgesamt verbessern und sich die Chancen für ein langfristiges Erhalten der Biodiversität in diesen Landschaften erhöhen.

Fragmentation of habitats, especially of tropical rainforests, ranks globally among the most pervasive man-made disturbances of ecosystems. There is growing evidence for long-term effects of forest frag-mentation and the accompanying creation of artificial edges on ecosystem functioning and forest structure, which are altered in a way that generally transforms these forests into early successional systems. Edge-induced disruption of species interactions can be among the driving mechanisms governing this transformation. These species interactions can be direct (trophic interactions, competition, etc.) or indirect (modification of the resource availability for other organisms). Such indirect interactions are called ecosystem engineering. Leaf-cutting ants of the genus Atta are dominant herbivores and keystone-species in the Neotropics and have been called ecosystem engineers. In contrast to other prominent ecosystem engineers that have been substantially decimated by human activities some species of leaf-cutting ants profit from anthropogenic landscape alterations. Thus, leaf-cutting ants are a highly suitable model to investigate the potentially cascading effects caused by herbivores and ecosystem engineers in modern anthropogenic landscapes following fragmentation. The present thesis aims to describe this interplay between consequences of forest fragmentation for leaf-cutting ants and resulting impacts of leaf-cutting ants in fragmented forests. The cumulative thesis starts out with a review of 55 published articles demonstrating that herbivores, especially generalists, profoundly benefit from forest edges, often due to (1) favourable microenviron-mental conditions, (2) an edge-induced increase in food quantity/quality, and (3; less well documented) disrupted top-down regulation of herbivores (Wirth, Meyer et al. 2008; Progress in Botany 69:423-448). Field investigations in the heavily fragmented Atlantic Forest of Northeast Brazil (Coimbra forest) were subsequently carried out to evaluate patterns and hypotheses emerging from this review using leaf-cutting ants of the genus Atta as a model system. Colony densities of both Atta species occuring in the area changed similarly with distance to the edge but the magnitude of the effect was species-specific. Colony density of A. cephalotes was low in the forest interior (0.33 ± 1.11 /ha, pooling all zones >50 m into the forest) and sharply increased by a factor of about 8.5 towards the first 50 m (2.79 ± 3.3 /ha), while A. sexdens was more uniformly distributed (Wirth, Meyer et al. 2007; Journal of Tropical Ecology 23:501-505). The accumulation of Atta colonies persisted at physically stable forest edges over a four-year interval with no significant difference in densities between years despite high rates of colony turn-over (little less than 50% in 4 years). Stable hyper-abundant populations of leaf-cutting ants accord with the constantly high availability of pioneer plants (their preferred food source) as previously demonstrated at old stabilised forest edges in the region (Meyer et al. submitted; Biotropica). In addition, plants at the forest edge might be more attractive to leaf-cutting ants because of their physiological responses to the edge environment. In bioassays with laboratory colonies I demonstrated that drought-stressed plants are more attractive to leaf-cutting ants because of an increase in leaf nutrient content induced by osmoregulation (Meyer et al. 2006; Functional Ecology 20:973-981). Since plants along forest edges are more prone to experience drought stress, this mechanism might contribute to the high resource availabil-ity for leaf-cutting ants at forest edges. In light of the hyper-abundance of leaf-cutting ants within the forest edge zone (first 50 m), their po-tentially far-reaching ecological importance in anthropogenic landscapes is apparent. Based on previous colony-level estimates, we extrapolated that herbivory by A. cephalotes removes 36% of the available foliage at forest edges (compared to 6% in the forest interior). In addition, A. cephalotes acted as ecosys-tem engineers constructing large nests (on average 55 m2: 95%-CI: 22-136) that drastically altered forest structure. The ants opened gaps in the canopy and forest understory at nest sites, which allowed three times as much light to reach the nest surface as compared to the forest understory. This was accompa-nied by an increase in soil temperatures and a reduction in water availability. Modifications of microcli-mate and forest structure greatly surpassed previously published estimates. Since higher light levels were detectable up to about 4 m away from the nest edge, an area roughly four times as big as the actual nest (about 200 and 50 m2, respectively) was impacted by every colony, amounting to roughly 6% of the total area at the forest edge (Meyer et al. in preparation; Ecology). The hypothesized impacts of high cutting pressure and microclimatic alterations at nest sites on forest regeneration were directly tested using transplanted seedlings of six species of forest trees. Nests of A. cephalotes differentially impacted survival and growth of seedlings. Survival differed highly significantly between habitats and species and was generally high in the forest, yet low on nests where it correlated strongly with seed size of the species. These results indicate that the disturbance regime created by leaf-cutting ants differs from other distur-bances, since nest conditions select for plant species that profit from additional light, yet are large-seeded and have resprouting abilities, which are best suited to tolerate repeated defoliation on a nest (Meyer et al. in preparation; Journal of Tropical Ecology). On an ecosystem scale leaf-cutting ants might amplify edge-driven microclimatic alterations by very high rates of herbivory and the maintenance of canopy gaps above frequent nests. By allowing for an increased light penetration Atta may, ultimately, contribute to a dominating, self-replacing pioneer communities at forest edges, possibly creating a positive feed-back loop. Based on the persisting hyper-abundance of leaf-cutting ants at old edges of Coimbra forest and the multifarious impacts documented, we conclude that the ecological importance of leaf-cutting ants in pristine forests, where they are commonly believed to be keystone species despite very low colony densities, is greatly surpassed in anthropogenic landscapes In fragmented forests, Atta has been identified as an essential component of a disturbance regime that causes a post-fragmentation retrogressive succession. Apparently, these forests have reached a new self-replacing secondary state. I suggest additional human interference in form of thoughtful management in order to break this cycle of self-enhancing disturbance and to enable forest regeneration along the edges of threatened forest remnants. Thereby the situation of the forest as a whole can be ameliorated and the chances for a long-term retention of biodiversity in these landscapes increased. Die Fragmentierung von Habitaten und insbesondere tropischer Regenwälder gehört zu den weitreichendsten Folgen men-schlicher Störung von Ökosystemen. Eine Reihe von Langzeiteffekten verursacht durch Waldfragmentierung und die damit verbundene Schaffung von Waldrändern verändert Ökosystem-Funktion und Struktur von Wäldern nachhaltig. Dadurch werden diese Wälder generell in frühsukessionäre Stadien überführt. Die waldrandbedingte Störung von Artinteraktionen kann dabei maßgeblich zur Transformation dieser Wälder beitragen, wobei die Artinteraktionen direkter (trophische Interaktionen, Konkurrenz, etc.) oder indirekter Natur (Veränderung der Ressourcenverfügbarkeit für andere Organsimen) sein können. Solche indirekte Interaktionen nennt man „Ecosystem engineering“. Blattschneiderameisen können Ökosystemingenieure sein und sind darüber hinaus dominante Herbivoren in den Neotropen. Im Gegensatz zu vielen Ökosystemingenieuren, die durch menschlichen Einfluss stark dezimiert wurden, können Blattschneiderameisen von anthropogenen Landschaftsveränderungen profitieren. Daher sind sie gut geeignete Modellorganismen, um die möglicherweise weitreichenden Auswirkungen von Herbivoren und Ökosystemingenieuren in heutigen, menschgemachten Landschaften zu studieren. Diese kumulative Promotionsarbeit dokumentiert wechselseitige Effekte zwischen Waldfragementierung und den Aktivi-täten von Blattschneiderameisen. Sie beginnt mit einem Literaturreview, der zeigt, dass Herbivoren (besonders Generalisten) von Waldrändern profitieren. Dem zugrunde liegen (1) vorteilhafte Umweltbedingungen, (2) eine höhere Quantität/Qualität von Futter am Waldrand und (3, weniger gut dokumentiert) eine Störung von top-down Kontrolle (Wirth, Meyer et al. 2008; Progress in Botany 69:423-448). Nachfolgend wurden Felduntersuchungen an Blattschneiderameisen der Gattung Atta im stark fragmentierten Atlantischen Regenwald Nordostbrasiliens (Coimbra) durchgeführt, um im Review gefundene Muster und Hypothesen zu überprüfen. Kolonien beider in der Region vorkommender Arten waren am Waldrand konzentriert. Atta cephalotes Kolonien waren selten im Waldesinneren (0,33 ± 1,11 /ha) und etwa 8,5-mal so häufig in den ersten 50 m des Waldes (2,79 ± 3,3 /ha); A. sexdens war etwas homogener verbreitet (Wirth, Meyer et al. 2007; Journal of Tropical Ecology 23:501-505). Die Akkumulierung von Atta-Kolonien entlang alter Waldränder blieb über einen Beobachtungszeitrum von vier Jahren stabil; ohne Änderung der Koloniedichten und trotz eines hohen Kolonieturnovers (etwa 50% in vier Jahren). Solch stabile hyperabundante Populationen von Blattschneiderameisen (Meyer et al. submitted; Biotropica) passen zur bereits zuvor an Waldrändern dokumentierten dauerhaft hohen Verfügbarkeit von Pionierpflanzen (der bevorzugten Futterquelle von Blattschneiderameisen). Darüber hinaus könnten Pflanzen an Waldrändern aufgrund ihrer physiologischen Reaktionen auf die Randbedingungen für Blattschneiderameisen attraktiver sein. In Bioassays mit Laborkolonien habe ich gezeigt, dass trocken-gestresste Pflanzen aufgrund eines höheren Nährstoffgehalts für Blattschneiderameisen attraktiv sind (Meyer et al. 2006; Functional Ecology 20:973-981). Da Pflanzen an Waldrändern anfälliger für Trockenstress sind, kann dieser Mechanismus zur hohen Ressourcenverfügbarkeit für Blattschneiderameisen an Waldrändern beitragen. Welche ökologische Bedeutung haben Blattschneiderameisen in randdominierten anthropogenen Landschaften in Anbetracht ihrer hohen Dichten in den ersten 50 m des Waldes? Extrapolationen basierend auf zuvor bestimmten Herbivorieraten auf Kolonielevel zeigen, dass A. cephalotes an Waldrändern 36% der verfügbaren Blattfläche entfernt (im Vergleich zu nur 6% im Waldesinneren). Außerdem agierte A. cephalotes als Ökosystemingenieur da die Art große Nester anlegte (im Durchschnitt 55 m2: 95%-CI: 22-136), die drastisch die Waldstruktur veränderten. Die Ameisen öffneten Lücken in Kronendach und Unterwuchs des Waldes. Dadurch konnte im Vergleich zum ungestörten Wald eine dreimal höhere Lichtmenge die Nester erreichen, was mit erhöhten Bodentemperaturen und einer verringerten Wasserverfügbarkeit einherging. Ein höherer Lichtgenuss wurde bis in einen Abstand von etwa 4 m vom Nestrand detektiert. Dadurch vervierfacht sich die von jedem Nest beeinflusste Fläche auf ca. 200 m2; diese Flächen addieren sich zu etwa 6% der Gesamtwaldfläche am Rand auf. Diese hier dokumentierte Veränderungen in Mikroklima und Waldstruktur gingen weit über bis dahin publizierte Effekte hinaus (Meyer et al. in preparation; Ecology). Die hypothetisierten Folgen von hohem Schneidedruck und mikroklimatischer Veränderungen an Nestern auf Waldregenration wurden direkt anhand transplantierter Keimlinge von sechs Waldbaumarten untersucht. Die Nester von A. cephalotes veränderten das Wachstum der Pflanzen und ihr Überleben unterschied sich hoch signifikant zwischen Habitaten und Pflanzenarten. Es war generell hoch im Waldesinneren jedoch niedrig auf Nestern wo es stark mit der Samengröße der Art korrelierte. Diese Ergebnisse legen nahe, dass Nestbedingungen Pflanzen selektieren, die von erhöhtem Lichtgenuss profitieren, jedoch großsamig sind und die Fähigkeit zum Wiederaustreiben haben und so am besten befähigt sind wiederholtes Entblättern auf dem Nest zu tolerieren (Meyer et al. in preparation; Journal of Tropical Ecology). Auf Ökosystemlevel kann Atta aufgrund der extrem hohen Herbivorieraten und dem Aufrechterhalten von Nestlichtungen eine höhere Lichtpenetration verursachen und das randtypische Mikroklima verstärken. In letzter Konsequenz würden Blattschneiderameisen so die dominierende, sich selbst verjüngende Pionierpflanzengemeinschaft an Waldrändern begünstigen von der sie selbst profitiert haben, was einen selbstverstärkenden Kreislauf bildet. Aufgrund der anhaltenden Hyperabundanz von Blattschneiderameisen entlang alter Waldränder in Coimbra und den vielfältigen dokumentierten Effekten schlussfolgern wir, dass die ökologische Bedeutung von Blattschneiderameisen, die bereits in ungestörten Wäldern trotz sehr niedriger Koloniedichten gezeigt wurde, in anthropogenen Landschaften drastisch erhöht ist. In fragmentierten Wäldern haben wir Atta als eine maßgebliche Komponente des Störungsregimes identifiziert, das eine rückwärtsgerichtete Sukzession als Folge von Fragmentierung bedingt. Augenscheinlich haben diese Wälder einen neuen, sich reproduzierenden Sekundärzustand erreicht. Um diesen selbstverstärkenden Kreislauf von Störungen zu durchbrechen erscheint weiteres menschliches Eingreifen basierend auf durchdachten Managementstrategien notwendig, um Waldregeneration entlang der Ränder der bedrohten Waldreste zu ermöglichen. Dadurch würde sich die Situation des Waldes insgesamt verbessern und sich die Chancen für ein langfristiges Erhalten der Biodiversität in diesen Landschaften erhöhen.