Grazing and the effects on insect communities

Grazing is an important method in managing natural communities. The interaction between grazers and the plant community is relatively well known. The interaction between grazers and insect community is less well known. The vegetation affects the insect community by means of three factors, plant diversity, vegetation height and spatial heterogeneity. Plant diversity positively influences insect diversity, for there are many plants to specialize on. Plant diversity is often positively influenced by grazing. The intermediate disturbance theory can the negative effect. Disturbance itself can be an additional influence on insect communities. An increasing vegetation height can increase the abundance and thereby the numbers of species of insects. Although the height may be a measure of more area for insects to specialize on, the structural complexity of a plant can also increase the insect diversity or species richness. However, both height and complexity are decreased by grazing and thus the overall effect of grazing on insects can differ much. Spatial heterogeneity, or the spatial structure, also positively affects the insect community. The effect of grazing on spatial heterogeneity is most of the times positive, but the result of grazing on heterogeneity may be of similar causes as the plant diversity, namely part of intermediate disturbance. All factors positively effect insects. However, not all factors are positively influenced by grazing. Another cause for different results of grazing is the fact that trophic levels of insects respond differently to changes in vegetation. Mobile/predatory insects respond more to structural changes and immobile/phytophagous insects respond at a smaller scale and mainly at the plant diversity. Disturbance itself can give yet other results of grazing on insects. The best level of disturbance for insects differs with that of plants. The optimal regime for management is therefore hard to find.