Search

Abstract: Studies of grassland communities have demonstrated that increasing vertebrate grazing decreases the diversity of specialised herbivorous insects, while plant diversity is maintained or increased. However, we still have a limited understanding of the causal mechanisms underlying these contrasting observations of two tightly linked groups of organisms. We used spatially linked plant and moth observations from salt marshes, sampled for 3 years along an experimental sheep‐grazing gradient (0, 1–2, 3–4 and 10 sheep ha−1), to test whether the disruption of plant–insect interactions by large herbivores accounts for these contrasting grazing effects. Moths were caught using emergence traps, which were moved and repositioned every 3 weeks. Firstly, we quantified species turnover between the grazing regimes for both taxa (measured as Sørensen dissimilarity) using a null‐model approach. Secondly, we analysed the number of observed insect ̶ host associations under the different regimes. Species turnover between grazing regimes was significant (after correcting for rarefaction effects) for moth species, but not for plants, indicating very few and random effects of grazing on plant species composition. The percentage of realised plant–moth associations decreased from 37% in the absence of grazing to 6.5% under high stocking densities. We thus conclude that vertebrate grazing caused a disruption of plant–moth associations, probably by rendering the host‐plants unsuitable for most of the moth species. Our findings provide further mechanistic understanding on how large herbivores shape arthropod communities and illustrate the importance of host‐plant associations in explaining effects of natural or anthropogenic habitat modification. [ABSTRACT FROM AUTHOR]

• Abundance is an important property to assess change in ecological communities. • It's unclear if individual number or biomass is the best measure of community size. • We compared change in these measures in plant- and leafhopper communities. • We found strong changes in species and trait composition for both measures. • Neither measure of community size was clearly superior. Declines in insect biomass and individual numbers have been demonstrated in many parts of the world, including central European dry grasslands. It is, howver, unclear if biomass data or individual numbers are superior as measures of change in species composition, and in trait composition of insect communities. We revisited a former study of ours demonstrating severe losses in abundance of planthoppers and leafhoppers in Central European grasslands since the 1960s. We performed a series of univariate, multivariate and trait-based tests to investigate the relationship between biodiversity change and environment, and compared the results for number-weighted and biomass-weighted community data. While both individual numbers and biomass declined strongly over time, no changes in species diversity (Jost's D) were observed irrespective of the measure used for weighting. Surprisingly, metrics for trait diversity tended to increase over time in both the number-weighted and biomass-weighted data. For both measures, community composition changed strongly over time for species and trait data, and was also associated with landscape structure and mean annual precipitation. Of the tested traits, we only found associations between an early phenology and time, and a temporal increase of species over-wintering in a nymphal stage, but again there was no major difference between the measure used for weighting. Our study shows that declining assemblage size (regardless of the measure used) does not necessarily translate in concomitant changes in functional diversity or trait diversity. From a methodological perspective, no clear evidence emerged for either biomass or individual number as measure of assemblage size being more sensitive to environmental change. This implies that conversion of widely used data of individual numbers to biomass values is unlikely to bring much additional insights. With respect to the general scarcity of quantitative data on insect communities, the focus should be on using whatever data is available. [ABSTRACT FROM AUTHOR]

Climate change and land-use intensification pose increasing threats to biodiversity, with climate change expected to eventually surpass other global environmental change drivers and become the greatest threat to biodiversity in the future. Understanding the combined ecological impacts of multiple global change drivers is crucial to predict future scenarios of biodiversity change. However, experimental evidence for the impacts of land-use intensification under current and future climate scenarios is lacking, even though this is imperative for understanding future trajectories of biodiversity in agricultural landscapes. We experimentally tested for the simultaneous effects of land-use intensification and climate change on arthropod biodiversity in a field-scale grassland experiment known as the Global Change Experimental Facility (GCEF). Specifically, we tested whether future scenarios of climate change are likely to exacerbate impacts of land-use intensification on arthropod diversity and abundance across different trophic levels by sampling aboveground arthropod communities in low and high land-use intensity grasslands under current and future climatic conditions. We found that climate change reduced total abundances of arthropods and increased evenness of the whole community, while only having trophic level-specific effects on detritivore abundance and evenness. Land-use intensification reduced abundance of the whole community, predators and detritivores, but only eroded species richness of the whole community and herbivores, with the magnitude of declines in predator and detritivore abundance depending on the climate scenario. Additionally, both land-use intensification and climate altered species composition of the whole community and within the predator, herbivore, and detritivore trophic levels. We show that climate change and land use intensification cause simultaneous shifts in arthropod abundance, species richness, and species composition across trophic levels. Changes in arthropod communities as a result of climate change and land-use intensification will likely have profound consequences for ecosystem functioning under future environmental conditions. • We experimentally test for combined land-use and climate change impacts on arthropod communities. • Land-use and climate change reduced total abundance and diversity, but increased arthropod evenness. • Climate change interactively reduced land-use impacts on predator and detritivore abundance. • Land-use intensity had more widespread impacts on arthropods across trophic groups than climate change. • Both drivers caused shifts in species composition, within and across trophic groups. [ABSTRACT FROM AUTHOR]