The impact of forest management on saproxylic beetles and other arthropods in a semi-deciduous forest in Southern Benin

Tropical forest ecosystems worldwide are suffering from anthropogenic pressures leading to deforestation and forest degradation, and resulting in a loss of biodiversity. Because tropical forests sustain over half of the planet’s life forms, their destruction might evoke an ecological crisis of global dimension. However, in spite of an increasing awareness of the general public and political authorities, tropical forests continue to deteriorate unabatedly, and numerous species are getting extinct every day. At the same time, large-scale reforestation and afforestation programmes in the tropics have been launched to produce timber and fuelwood. Moreover, it has been proposed that plantation forests have an important role to play in maintaining or enhancing regional biodiversity pools and in alleviating anthropogenic pressures on the natural forests remaining. Consequently, studies about the role of forest plantations for biodiversity conservation are emerging worldwide. However, there is a dearth of information from Africa, and basically none about the arthropod fauna in African forest plantations. The present study steps into this gap and aims to generate baseline data about the diversity of arthropods in natural and plantation forests in tropical West Africa. In doing so, it wishes to encourage and inspire the development and implementation of conservation-oriented forest management strategies.
We studied arthropod assemblages in the Lama forest reserve in Southern Benin, an area harbouring one of the last remnants of natural forest in the Dahomey gap. We compared arthropods from different types of forest, including natural, degraded and plantation forest. The fully protected core of the reserve, the so-called Noyau central, is composed of a small-scale mosaic of natural and degraded semi-deciduous Guineo-Congolian forest. It is surrounded by teak (Tectona grandis) and fuelwood (predominantly Senna siamea) plantations. The particular geographical configuration of the reserve provided an ideal set-up to study the impact of different forestry management systems on arthropod diversity and the role of plantations for biodiversity conservation.
In the first chapter, we give an overview of arthropod diversity in the Lama forest reserve, focusing on species related to biomass decomposition, and highlight its importance for biodiversity conservation. No differences in arthropod species richness were found among different forests types within the Noyau central. However, great differences were observed between forest plantations. Of the environmental variables influencing arthropod assemblages, soil type was the most important natural factor, and species richness was highest in soils with coarse texture (old plantations and isolated forest fragments). The remaining variables naturalness, understorey cover and canopy height are related to silvicultural practices and are therefore amenable to an improved, conservation-oriented forest management. High values for β-diversity suggest that all major forest habitats contribute significantly to regional species pools und should therefore be protected.
Chapters two to four deal with beetles dependent on dead wood (saproxylics). Saproxylic beetles are an exceptionally species-rich ecological group. Moreover, they are known to be highly sensitive to forest management and are therefore particularly endangered. Despite this, invertebrate diversity studies in Africa have largely ignored saproxylic beetles and their dead wood habitat.
In the second chapter, we investigate the attraction of freshly cut wood for pioneer beetles, using a newly designed trap (twin-Malaise trap) baited with native and exotic wood and exposed in both natural and plantation forests. Six species showed primary attraction and two repulsion. Three species were native wood specialists, but none were teak specialists. Among the beetles showing primary attraction, we found no economically important pest species. The twin-Malaise trap proved to be a suitable device to examine the primary attraction of beetles. However, our study also showed that the entire spectrum of wood-associated beetles cannot be sampled with this type of trap.
In the third chapter, the diversity of saproxylic beetles and their interaction with dead wood are examined for the first time in an African tropical forest. Both the quantity and quality of dead wood resources differed greatly among natural forest and plantations. Dead wood of advanced decay stage and larger size was basically absent in plantations. Despite this paucity of dead wood, saproxylic beetle diversity in teak and fuelwood plantations was surprisingly high. Nonetheless, overall species richness of saproxylic beetles was higher in natural forest, and wood beetle assemblages differed clearly between natural forest and plantations. The volumes of recently dead wood, large pieces of dead wood, coarse woody debris and standing dead trees were found to have the most significant influence on the composition of saproxylic beetle assemblages.
The last chapter focuses on standing dead trees (snags). We found similar species richness of saproxylic beetle assemblages from native and exotic snags. However, overall species richness and β-diversity were higher for native snags, reflecting higher habitat heterogeneity in the protected forest. Species assemblage composition, influenced mainly by the forest system, revealed high heterogeneity, especially among snags of native tree species. Our results showed that retaining or even creating snags is crucial to the protection of saproxylic beetle assemblages.
The results presented here provide evidence that the cultivation of exotic trees in the vicinity of natural forest may be an important element in biodiversity conservation. The system practiced in Lama forest, which combines different management intensities from unmanaged natural forest to intensively managed, short-rotation fuelwood plantations, goes well beyond the single-objective forestry approaches practiced in other tropical areas. The functional and spatial diversity of Lama forest is reflected in a high diversity of saproxylic beetles and other arthropods. The forest ecosystem as a whole benefits from the diversity and abundance of these invertebrates and the ecological services they provide (decomposition, nutrient cycling). Last not least, the rich and varied invertebrate fauna promotes species higher up the food chain feeding on them.
Several conservation-oriented management measures can be proposed to foster the role of plantation forests as habitat surrogates for forest species. In the short term, the regeneration and development of species-rich understorey vegetation of the Guineo-Congolean phytogeographical region should be promoted in plantations. Moreover, different types of dead wood should be retained to improve the resource base for saproxylic beetles. When adopting long-term strategies that aim to integrate biodiversity conservation into tropical forestry, native trees should be planted or allowed to regrow spontaneously within plantation schemes so as to provide a stock of trees that mature and die in plantations.