Insects in forests. Assemblages, effects of tree diversity and population dynamics
Sound management and conservation of insect communities and forests require a thorough understanding of the factors affecting insects and their habitats. Insects play an essential role in forest ecosystems. Many species of forest insects, e.g., the gypsy moth (Lymantria dispar L), impose a great dan...
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|Sound management and conservation of insect communities and forests require a thorough understanding of the factors affecting insects and their habitats. Insects play an essential role in forest ecosystems. Many species of forest insects, e.g., the gypsy moth (Lymantria dispar L), impose a great danger on forests by defoliating trees. Other species, such as butterflies, are considered indicator species for nature conservation. The purpose of this dissertation is to provide some insight into the relationship between insects and forests. I use moths as a model to study the processes affecting insect assemblages, tree-insect interactions and population dynamics of insects.
In the first chapter, I present the topics of my dissertation starting with a general introduction about the roles of insects in forests. The second chapter points out the effects of environmental and neutral processes on species compositions. I particularly disentangle the effects of environmental versus spatial distance on the compositions of moth assemblages. Two statistical approaches were applied for this purpose: the raw and distance approaches. Our results show that both environmental and spatial distance influence the composition of species assemblages. Thus, environmental and neutral processes contribute to the diversity of Lepidoptera insects in Bavarian forests. However, the statistical methods (raw and distance approaches) showed inconsistent results with regard to the relative importance of each process on moth assemblages.
In the third chapter, I investigate the effects of the diversity of tree stands on insect herbivory on oak and whether the effects of tree diversity on herbivory damage are reflected by the performance (leaf consumption, growth) of the generalist herbivore Lymantria dispar. The study shows that the damage on oak caused by herbivores decreases with the increased diversity of tree stands. This decrease is not reflected by the performance of a generalist herbivore (Lymantria dispar), neither in field nor in lab assays. Hence, the changes in the tree quality do not explain the reduction of damage by herbivores. Alternative mechanisms such as natural enemies and resource dilution (associational resistance) are suggested.
In the fourth chapter, I study and review factors affecting the population dynamics of forest insects. I use the foliage feeding insect, the gypsy moth (Lymantria dispar), as a study-
model. I highlight the role of natural enemies in the population dynamics and population cycles of the gypsy moth in its native and invasive ranges. The results show that natural enemies have a large impact on the population dynamics of the gypsy moth because they cause the highest mortality rates. Among parasitoids, the tachinids cause the highest rates of mortality in larvae and pupae populations. These mortality rates increase in the northern latitudes. Furthermore, the effects of the parasitoids seem to be density dependent. Parasitoids are thought to influence the population cycles in the native range. In the invasive range, predators were reported to be responsible for the population cycles. Certain types of forests and host plants were additionally reported to influence population cycles in the invasive range. We speculate that the population dynamics of the gypsy moth is largely influenced by the interaction of several factors, basically weather, host plants and natural enemies. However, this may differ between native and invasive ranges.