Insect diversity and trophic interactions in shaded cacao agroforestry and natural forests in Indonesia | Insektenvielfalt und trophische Interaktionen in beschatteten Kakao-Agrarforsten und Naturwäldern in Indonesien

In den Tropen bedingen anthropogene Veränderungen natürlicher Habitate gegenwärtig einen nie da gewesenen Verlust der Biodiversität. Agrarforsten tropischer Landschaften, mit traditionellem Schattenbaumbestand, repräsentieren in immer größerem Maße die einzigen Habitate mit einem ausreichenden Baumbestand und unterstützen somit hohe Biodiversitätsraten. Umfassend werden in den Tropen Agrarforsten mit heterogenem Schattenbaumbestand in homogene, unbeschattete Systeme umgewandelt. Diese großräumigen Vereinheitlichungen der Lebensräume im Dienste steigernder Produktivität, bedrohen jedoch sowohl Biodiversität, als auch wichtige ökologische Prozesse, die ihrerseits Bestäubung der Nutzpflanzen, Schädlingsbefall und Nachhaltigkeit der Landnutzung beeinflussen.In unserer Studie untersuchten wir den Umfang, in welchem durch Kakao dominierte Agrarforsten zur Erhaltung der Diversität von Insekten beitragen können. Zur Einzuschätzung der direkten und indirekten Faktoren, welche die Käfer- und Ameisendiversität beeinflussen, dienten uns Agrarforsten mit unterschiedlicher Zusammensetzungen des Schattenbaumbestandes. Gleichzeitig beobachteten wir die Wechselbeziehungen von Kakao-Schädlingen und Kakao-Bestäubern in Bezug zu den unterschiedlich bewirtschafteten Agroforsten.

In the tropics, human modification of pristine habitats is currently causing unprecedented biodiversity losses. In tropical landscapes, traditional shade agroforestry increasingly makes up the only remaining habitat with a considerable tree cover and supports high levels of biodiversity. In this study we investigated the extent to which cacao dominated agroforests can contribute to the conservation of insect diversity. Concomitantly, we investigated the cacao-pest and cacao-pollinator interactions in relation to the differences between shade management. The study took place in and around the Toro village in the border of the large Lore Lindu National Park, Central Sulawesi, Indonesia. We selected cacao dominated agroforests that were shaded by three different stands of shade trees: Trees that remained from the previous forest cover, a diverse stand of planted shade trees and a stand of shade trees dominated by two non-native species of planted leguminous trees. From each type of agroforest we selected four replicate sites. Additionally, four forest sites were selected for biodiversity comparisons. For the first time, we show that cacao trees in shaded agroforests can harbor levels of beetle and ant diversity that resemble that of lower canopy trees in undisturbed forests. However, species turnover was high between forests and agroforests, particularly of beetles. Although total ant species richness on cacao trees was not related to canopy cover, the proportion of species that also occurred in one of the forest sites, decreased clearly with canopy reduction. Moreover, associated changes in microclimate appeared to promote the increase in dominance by the aggressive invasive Crazy Ant Anoplolepis gracilipes. The proportion of ant species that also occurred in the forest sites was not affected by the changing microclimate, but decreased significantly with increasing abundance by A. gracilipes, which illustrates an important, indirect effect of changing management practices in shaded agroforests. Our study on the causes of fruit mortality was the first that underlined the fact that shade removal may not represent the only management measure that needs to be taken in order to increase cacao yields. A heterogeneous stand of (natural) shade trees released the trees from pest pressures. The intercropping of leguminose trees released the trees from pressures that drive early fruit abortion, possibly due to nitrogen fixing qualities of those trees. The study on cacao pollination illustrated that those environmental factors that drive fruit mortality could even blur initial benefits from artificially increased pollen deposition. By comparing these results with those from pollination and yield studies on coffee and passion fruit, we exposed an important aspect that is overlooked in numerous recent ecological studies on pollinator services: that the chain of ecosystem processes that make up the final service (e.g., crop yields) is as strong as its weakest link. This means that, for example, enhanced crop pollination will not be expressed in crop yields as long as factors that drive fruit abortion and herbivore pressures are not optimized. In conclusion, shaded cacao agroforests are important habitats for species rich beetle and ant communities, two major aspects of tropical biodiversity. However, species from natural forests are rare among beetles on cacao and forest ants on cacao may indirectly depend on shade management in that microclimatic changes mediate dominance by invasive ants, which can drive losses of native assemblages. Cacao is productive in wide ranges of habitats, but factors that drive fruit mortality can be of major importance for yields. Although shade removal is predicted to increase cacao yields, we experimentally approached cacao fruit mortality and showed that sustainable shade management has a potential to increase yields. Shaded cacao agroforests are important aspects of tropical landscapes that undergo continuing deforestation, and should be protected in order to serve as a powerful tool in the conservation of tropical biodiversity.