Contrasting effects of vineyard type, soil and landscape factors on ground- versus above-ground-nesting bees.
Abstract
Agricultural intensification and abandonment of traditional agricultural practices are main drivers of current insect declines. The resulting loss of feeding and nesting opportunities has led to a decrease in pollinator populations like wild bees. While the restoration of floral resources has been widely implemented in wild bee conservation, nesting resources, particularly for ground-nesting species, are barely considered. We assessed wild bee diversity in a wine-growing area in Germany in 15 study sites along a soil gradient and evaluated whether wild bees were distinctly affected by different vineyard types (vertically oriented, terraced, abandoned), local conditions (e.g. shrub and flower cover) and landscape factors in response to divergent nesting needs (above-ground vs. ground-nesting). We found that wild bees responded more strongly to the availability of nesting sites than to flower resources. While ground-nesting bees were determined by the suitability of soil aspects for nesting irrespective of vineyard management types, above-ground-nesting bees profited from vineyard abandonment and shrub encroachment in vineyard fallows and were enhanced by the availability of semi-natural habitats (SNHs) in the surrounding landscape. In contrast, floral resource availability in managed vineyards had only marginal effects on above-ground-nesting bees. Synthesis and applications: Life-history traits like nesting strategies have long been neglected in wild bee conservation approaches, but proved to be highly relevant, especially for ground-nesting bees. For this, agri-environmental schemes can no longer solely focus on the restoration of floral resources, but should equally address nesting resources. Therefore, management efforts for enhancing wild bees in vineyard landscapes should aim at complementing nesting resources for ground-nesting bees (e.g. exposed bare ground patches) and above-ground-nesting bees (e.g. woody elements, hedges) in addition to floral resources. At the landscape level, conserving heterogeneous landscapes at a mixture of actively managed vineyards and semi-natural and woody elements is significant to maintain diverse bee communities.