Effects of Livestock Grazing on Invertebrate-Provided Ecosystem Services in the Aegean

Abstract

Over one-quarter of the world’s terrestrial surface is used for livestock grazing, more land than for any other single human land use. Improperly managed grazing can lead to widespread biodiversity loss and desertification, which can undermine the livelihoods of entire communities and have devastating consequences for local ecosystems. Overgrazing has been shown to impact the diversity and structure of vegetation communities, promote erosion, and have negative consequences for arthropod populations, and worldwide is increasing in intensity and expanding in scope. Yet it is unclear whether or not overgrazing negatively impacts communities of invertebrate predators and pollinators in mixed use agroecological-pastoral systems, and whether such impacts can lead to a reduction in invertebrate-provided ecosystem services like biological pest control and pollination. Preliminary data from previous researchers have suggested that overgrazing may negatively affect native insect populations, which in turn may undermine their ability to provide these essential ecosystem services. This research project attempts to quantify the impact of livestock grazing on biological control and pollination services and answer whether or not increasing grazing intensity results in a decline in invertebrate biodiversity and if such an outcome leads to a corresponding reduction in ecosystem services provided. In a typical Mediterranean ecosystem (Naxos Isl, Aegean Sea, Greece), fourteen study sites were established along a gradient of grazing intensity. Grazing intensity was determined by measuring the stocking rate (livestock/1000 m2) and dung mass (g/m2) at each location. Pitfall traps were deployed at each site to sample the ground-dwelling invertebrate community. To evaluate the differences in pest suppression services between sites, a field experiment was run comparing the differences in growth rates of aphid populations that were either protected from predation by a fine mesh, or open to the local invertebrate predator community; this was repeated at sites experiencing different levels of grazing intensity. To evaluate and quantify pollination services across the gradient of grazing intensity, pollinator observations and pollen grain counts were obtained from conehead thyme flowers (Coridothymus capitatus), the most important nectar-producing plant in the region. Linear mixed-effects models and Pearson’s correlations were run using R software to determine the effect of grazing intensity on pest suppression and pollen counts. Pitfall trap contents revealed that the total available predators per site remained unchanged across the gradient of grazing intensity. However, the data suggest that grazing is associated with declining populations of certain types of invertebrate predators including spiders. Combined with a partial data available from the pest suppression experiment, few clear conclusions can be drawn from the biological control portion of this project. However, a strong positive relationship exists between grazing intensity and pollen count. Additionally, pollen count declines with thyme flower coverage. This effect can be explained by the influence of thyme, rather than the direct effect of grazing itself. Due to its unpalatability, thyme coverage peaks at moderate levels of grazing. However, at even higher livestock densities, nearly all living vegetation is consumed or destroyed. The pattern of decreasing pollen counts with increasing thyme flower coverage can be attributed to the fact that amount of pollen on thyme is negatively correlated with the amount of thyme flowers in an area. The influence of livestock grazing on thyme populations outweighs any potential concomitant deterioration of the pollinator community and results in augmented pollination services in the form of more conspecific pollen grains per stigma on the flowers that do persevere through higher levels of grazing.