Diversity and distribution of leaf litter beetle assemblages in secondary and old-growth forests of the Mata Atlântica

Abstract

Although insects make up the major component of tropical forest biodiversity, they are still poorly studied. Therefore, I explored the value of secondary forests and agroforests for the conservation of leaf litter beetle assemblages. The study was conducted in the coastal submontane Atlantic forest of Paraná, Southern Brazil. Secondary forests ranging in age from 5 to 50 years, old-growth forest as well as small-scale banana monocultures and shade banana-palm plantations were studied at different points in time using a chronosequence approach. Beetles were sampled by sieving from leaf litter and extracting using the Winkler method as well as using pitfall traps. Relevant environmental variables, such as litter volume, temperature, C/N-ratio and humidity, tree richness, pH-value, canopy openness, patch size and distance to old-growth forest, were measured in each site. The results revealed much lower species densities and significantly different and heterogeneous species compositions in young secondary forests compared to old-growth forests. During forest regeneration, species density greatly increased, and the species composition of older secondary forests was similar to that of old-growth forests. Most measured environmental variables were significantly correlated with beetle density and richness; but, litter volume and litter temperature showed the strongest effect on the beetle recovery pattern. Litter temperature mediated by canopy openness and soil pH, interpreted as mediated by land-use history, seemed to be important factors for the shift in species composition during forest regrowth. Chronosequences in adjacent reserves showed comparable recovery patterns and environmental relationships. Sampling at different points in time (August: dry season and April: after rainy season) showed that although species density and abundance was lower in April compared to August, assemblage composition of the corresponding forest stages was very similar at both months and resulted in similar recovery pattern during forest regrowth. Thus, it indicates that sampling of litter inhabiting beetles at different points in time using the Winkler method not only reveals identical ecological pattern (in this case along forest regeneration) but also that these pattern are identified by very similar data sets, whose differences are more likely influenced by sample incompleteness than by strong differences in their assemblage composition. A strong interconnection between litter quantity and beetle occurrences seems to mediate abundance and species density not only along a successional gradient, but also among temporal variations of this physical parameter. The agroforestry systems showed comparable low species densities than young secondary forests. Only very few mature forest species occurred in agroforests and younger forest stages, which thus may be able to use these forest types as habitat. The accuracy of these results gained using the Winkler method were studied by comparing beetle data sets sampled by litter sifting and using pitfall traps. Pitfall traps sampled few individuals and species. Over 60 % of the species was also sampled using the Winkler method. Although surely more beetles would be found by expanding the sampling effort, it indicates that the local leaf litter beetle fauna is mainly constituted of small to very small beetles, featuring moderate to low surface mobility, which were adequately sampled with the Winkler method. Given these results, I suggest that young secondary forests of up to 15 years and agroforests, although embedded in a forest dominated matrix, are of limited value for the conservation of most litter inhabiting old-growth forests beetle species. However, older secondary forests of about 35-50 years after abandonment provide a suitable habitat for many old-growth forest species and may play an important role for the conservation of tropical insect communities. Important modulating factors for the recovery of litter beetle communities are a closing canopy, which buffer extreme micro-climatic conditions and an increase of the quantity and quality of habitable substrate during secondary succession. Hence, I consider these relationships as main reason why old secondary forests, which differ in plant composition to old-growth forests, but are similar in physical and biotic conditions, already feature old-growth forest like beetle assemblages. Thus, litter temperature and litter quantity are factors to consider by looking for high priority sites for biodiversity conservation and could be of interest in restoration activities, which try to combine reforestation and insect conservation