Insect Responses And Adaptations To Plant Diversity Subjectd To Large Herbivore Grazing In Meadow Steppe

In nature, insects represent a large proportion of invertebrates in many ecosystems, and play important ecological functions. Individual behaviors of insects are important part of Entomology, and affect the dynamics of insect populations and communities. Insects coexist with plants and large herbivores in natural grassland ecosystems, and the study on their relationships is an important issue in grazing ecology and grassland ecology. Habitat and food that insects confront are highly variable, and this variation may be caused by not only large herbivore grazing but also by the complexity of vegetation, such as plant species diversity. In this thesis, we analyzed the behavioral activity of a generalist grasshopper(Euchorthippus unicolor) on a meadow steppe, especially the time devoted to different behavioral activities, its foraging strategies and habitat selection, responding to grazing by large herbivores and altering plant diversity. This study is of great importance for the development of animal behavioral ecology, sustaining the stability of grazing grassland systems, and effectively managing the grasslands.A manipulated experiment with two plant diversity (low and high plant species richness) and four grazing treatments by large herbivores (no grazing, cattle, sheep, and mixed by cattle and sheep) was used to examine insect diversity, abundance at the community level, and behavioral activity at the individual level. We obtained the important results and conclusions as follows.(1) Insect diversity was higher in ungrazed plots compared to grazed plots independently of plant diversity. Differences in vegetation characteristics and aboveground biomass between grazed and ungrazed plots may be the probable explanations of these results. Insect abundances were lowest in low plant diversity treatment and highest at high plant diversity level. This difference in insect abundances may be related to higher structural complexity in high plant diversity treatment and increased productivity due to increased plant diversity.(2) This study mainly investigated how a generalist grasshopper herbivore, Euchorthippus unicolor, adapted to changes resulting from large herbivore grazing. Three behavioral activities including quiescence, locomotion and feeding were observed and the time allocated to each behavioral activity was recorded. We found that E. unicolor displayed contrasting behaviors in low and high plant diversity treatments:E. unicolor increased the feeding and locomotion time in high plant diversity treatment whereas quiescence activity decreased, and we found opposite time pattern at low plant diversity level. The changes in plant attributes (cover, height) and food supply, which varied with plant diversity treatments, explained these contrasting behavioral activities.(3) Owing to the reduced food resources resulting from large herbivore grazing, this generalist grasshopper fed upon many plant species to meet its nutritional requirements and this may increase its feeding time under grazing conditions. Large herbivore grazing influenced the behavioral activities for E. unicolor by mediating food supply (quality or quantity) and plant attributes which influenced the physical habitat conditions. The feeding heights of the grasshopper varied among grazing treatments and within grazing treatment. In ungrazed plots, E. unicolor fed mostly in upper vegetation heights. Under grazing conditions, the grasshopper used mostly low and middle vegetation heights to select its food items. These differences are related to the effect of large herbivore grazing which removes the upper leaves and stems located in the upper vegetation heights which contain higher tissue quality. Under grazing conditions, the grasshopper may concentrate on remaining food resources located in lower vegetation heights which also provided better protection against predators.(4) In this study, plant species diversity coupled with different livestock grazing treatments influenced E. unicolor microhabitat selection. Overall, the grasshopper increased the number of plant species used as microhabitats under grazing conditions compared with ungrazed plots and this was explained by the fact that increasing the range of plant species used as microhabitats may increase the probabilities of getting suitable habitats and /or refuges from probable predators. This experiment also found that the grasshopper used microhabitat located in high vegetation height in ungrazed plots while it used microhabitats located in low and middle vegetation heights under grazing conditions. Possible explanation for these results was that grazing reduced the availability/accessibility of microhabitats located in high vegetation height and forced the grasshopper to select lower microhabitats than grasshopper would occupy in absence of large herbivore grazing.(5) The aspect of plant parts revealed that E. unicolor spent more time on leave surface in ungrazed plots than on stem while we found opposite trend under grazing conditions. These results showed that E. unicolor preferred mostly leaf surface in absence of large herbivore grazing. The preference of leaf surface is presumably related to its role as shelter and food resources as well. These results were related to the fact that grazing removed upper shoots which encompass mainly leaves. Therefore, the grasshopper presumably used stems as surrogate microhabitats under grazing conditions.Based on the above obtained outcomes from our experiments, we acquired further knowledge and insights into the relationship among insects, plants, and large herbivores. Large herbivore grazing strongly affected behavior activity of a generalist grasshopper herbivore, especially its foraging selection or strategies. These alterations are an adaptive and evolutionary consequence for grazing by large herbivores and plant diversity. The adaptive behavior is an important determinant factor affecting dynamics of grasshopper population and community, which are of paramount ecological significance. Additionally, the effects of grazing on grasshopper behavioral activity markedly depend on plant diversity, implying that plant diversity affects the behavioral activity of large herbivores and especially small herbivore as well. We therefore suggested that diversity of grassland plant community was one of the most important conditions to ensure stable coexistence of large generalist herbivore-plant-grasshopper. This study further emphasized the importance for protecting grassland plant diversity.