Plant - Invertebrate Interactions in Agriculturally Managed Grasslands Under Functional Group Manipulation

Abstract

Within the last few centuries the rapid growth of the human population, along with increasing economic development, has caused increasing demands for agricultural products such as food and fuel. This has led to an extension of agricultural areas, and the intensification of existing agricultural areas to maximize crop production, therewith increasing pressures on ecosystems around the world. Agricultural intensification, with high inputs of fertilizer and increasing cutting regimes, leads to a decline in plant species richness, lower stability to climatic extremes, such as drought or high rainfalls, resulting in a threat to biodiversity on higher trophic levels and important ecosystem processes. In this thesis we analyse the direct and indirect effects of different levels of agricultural management intensity and functional group composition within the Grassland Management Experiment (GrassMan) near the towns of Neuhaus (Solling) and Silberborn in the Solling Mountains, which are situated in Northern Germany. We created twelve treatment combinations by independently combining two cutting frequencies, two levels of fertilizer application and three different sward types via manipulation of functional groups. We further investigate the effects of plant diversity, cutting, and fertilizer application on grassland productivity, herbivorous invertebrates and nutrient cycling, as well as the impact of herbivory on the overall system. The main focus of this thesis lies in exploring the impact of these agricultural management practices and declines in species diversity as well as shifts in functional group composition on plant-insect interactions, such as invertebrate herbivore pressure on plant-performance and its feedbacks on nutrient cycling. In chapter two, we show that overall species-richness had no significant effect on the above-ground biomass production across the five study years. In addition, sward- type diversity had significant effects on the above-ground biomass production only in the experimental year with extremely dry weather conditions for the summer months before the peak standing crop, when control plots had higher yields than the forb- and graminoid-reduced treatments. While management intensification caused an increase in above-ground biomass production, changes in species numbers were not dramatic over the whole investigation period, with increasing cutting frequency promoting the establishment of more species. In our study, existing species composition of the semi-natural grassland was highly resilient. Four years after the manipulation of functional groups, we recorded a composition of grasses, herbs and legumes showing almost no differences between the sward types, with larger shares of grasses being typical of fertilized plots and larger shares of herbs typical of frequent cutting. Shares of legumes were suppressed by fertilization, but were found in almost all plots by the end of the experimental period. In chapter three we show the strong effects of grassland management, plant diversity and composition on slug abundance, which was highest in plots with a low cutting frequency and high food resource availability (increased forb cover and taller vegetation). Higher cutting frequency decreased slug abundance, but with lower impact in control plots, with their naturally developed graminoid-forb communities, giving evidence that more-natural plant species compositions can reduce the impact of disturbances (e.g. through cutting or grazing) on invertebrates. In chapter four, our results show that plant diversity, functional group composition and management regime affect leafhopper species richness and abundance. Higher cutting frequencies directly led to decreased leafhopper diversity, whereas fertilizer application only had a small indirect negative effect via its opposing trends on above- ground plant biomass, diversity and composition. Leafhopper diversity profited from graminoid-rich swards as well as from higher plant diversity, which itself was mediated by functional group manipulation and agricultural management. In chapter five we show that additional herbivore pressure interacts with our initial experimental treatments (cutting frequency, NPK-fertilizer application and functional group manipulation) and affects several vegetation parameters as well as the nitrogen cycle. We found that management and herbivores affect productivity and nitrogen fluxes and show stronger effects than the manipulation of plant functional group composition. In general, we show that grassland management, plant functional group composition and invertebrate herbivory directly and indirectly drive plant productivity, diversity, composition and nutrient cycling. The responses of plant and invertebrate communities, as well as nitrogen cycling, become more complex with increasing number of interacting drivers. Our results suggest that with proper management strategies, providing sufficient resources for the least-productive species, it is possible to maintain high productivity while maintaining high diversity. We revealed positive effects of forb richness on plant diversity and slug density, indicating that populations of invertebrates will increase in abundance not only where there is greater plant species richness but also in more-natural plant communities. Leafhopper diversity profited indirectly in nutrient-poor, forb-dominated plots, since these harbour a greater diversity of plants and therefore greater food-resource heterogeneity. On the other hand, leafhopper diversity profited directly from higher graminoid cover in highly productive plots with lower plant diversity, due to a greater availability of graminoids as a preferred food source for many leafhopper species. We show that management and herbivory had much stronger effects than the manipulation of plant functional group composition. Fertilizer application was the strongest driver of the nitrogen flux rates in our system. Herbivory, however, can short-cut the nitrogen cycling by increasing amounts of litter, excrement and root exudates, consequently positively affecting soil nitrogen contents (NH4 , NO3 ) and flux rates (mineralization, nitrification, N2O). In addition to the strong effect of fertilizer application on the nitrogen cycle, emissions of N2O, a very important greenhouse gas, can also increase under intense herbivory in fertilized grasslands, at least on a short timescale. This leads us to concerns that these N2O emissions can further increase, and therefore contribute to global warming, due to an additive effect of fertilizer application and invertebrate herbivory.