Pollinator trait diversity: functional implications at different land-use intensities and environmental conditions

It is increasingly noticed that species richness alone is not a sufficient predictor of ecosystem functioning and resilience. The diversity of species responses to environmental variation could be a stabilizing factor to ecosystems, as it can ensure a higher probability that at least some species continue to perform their functions even at unfavorable conditions. Specific functional traits (morphological-, physiological- or life-history traits) could be more beneficial in some habitats and under certain conditions, than others. When species with particular traits and responses to environmental variation are lost, this may cause changes in ecosystem processes and thus have profound consequences for services that humans depend on. It is critical that we understand how these different issues of species diversity influence the role of organisms in ecosystem functioning and resilience. Therefore, we investigated the response diversity and functional traits in pollinator communities experiencing different land-use intensities and varying environmental conditions in three studies. The first study focuses on the variation of thermal niches of pollinator communities and whether the diversity of thermal responses and the projected thermal resilience are affected by land-use intensity. We recorded pollinator communities including flies, bees, beetles and butterflies (511 species) in 40 grasslands at various weather conditions and determined the thermal niche of each species. Temperature generally explained 84% of the variation in pollinator activity. Species in more intensively used grasslands had broader thermal niches and were also more complementary in their thermal optima. Quantified thermal resilience increased with land-use intensification mainly driven by flies that prefer cooler temperatures and compensated for losses of other taxa. We show that the diversity of thermal responses of pollinators contributes to a higher projected resilience of ecosystems under land-use change. The second study addressed to the variability in water loss as physiological trait of pollinators that can influence many aspects of species performance. In the view of ongoing climate change, associated with rising temperatures and longer periods of dryness, water loss can be crucial. We measured water loss of 67 pollinator species gravimetrically at extremely dry air conditions for two hours at 15° and 30°C. To investigate differences in water loss of pollinators, we quantified for the first time surface to volume ratios (SA/V ratios) of insects, by creating 3D surface models as obtained by structured light scanning methods. Quantified SA/V ratios better explained the variation in water loss across species than body mass alone. Small insects with a proportionally larger surface area had the highest water loss rates and, therefore, are most vulnerable to high temperatures and dryness. The four orders did not differ significantly. Directly measured SA/V ratios thus provide a promising method to predict physiological responses of insects, improving the potential of extrapolated relative changes of SA/V ratios based on body mass allometry alone. The third study shows how morphological traits of pollinator communities are filtered by land-use intensity. We recorded pollinator communities on 40 grassland sites along a land-use gradient and measured several morphological characteristics of 476 pollinator species. Community means of body size, hairiness, relative wing size and proboscis length decreased with land-use intensity, although species diversity remain constant. Relative femur length, eye size, antenna length and mandible length were unaffected by land-use. The variation in size of head and relative size of wings and eyes increased with land-use intensity. Shifts in trait means with land-use intensity strongly correspond to shifts in relative abundance of insect orders, whereas consistent land-use changes were rarely found within an order. These findings highlight that functional traits may be more sensible indicators of land-use effects than species diversity alone, and many of these traits can be relevant for ecosystem functionality. Taken together, quantifying the diversity of responses and mean functional traits of pollinator communities is a promising approach to assess the vulnerability of ecosystems to land-use intensification and climate change. Knowledge about the vulnerability of communities to several factors and the potential for predictions enables us to initiate steps for protection, rather than just documenting the negative impact of already occurred disturbances. And unlike a community of specific species, functional traits can be easily generalized across functional groups and extrapolated to different regions.