Diets maintained in a changing world: Does land-use intensification alter wild bee communities by selecting for flexible generalists?

Abstract

Biodiversity loss, as often found in intensively managed agricultural landscapes, correlates with reduced ecosystem functioning, for example, pollination by insects, and with altered plant composition, diversity, and abundance. But how does this change in floral resource diversity and composition relate to occurrence and resource use patterns of trap-nesting solitary bees? To better understand the impact of land-use intensification on communities of trap-nesting solitary bees in managed grasslands, we investigated their pollen foraging, reproductive fitness, and the nutritional quality of larval food along a land-use intensity gradient in Germany. We found bee species diversity to decrease with increasing land-use intensity irrespective of region-specific community compositions and interaction networks. Land use also strongly affected the diversity and composition of pollen collected by bees. Lack of suitable pollen sources likely explains the absence of several bee species at sites of high land-use intensity. The only species present throughout, Osmia bicornis (red mason bee), foraged on largely different pollen sources across sites. In doing so, it maintained a relatively stable, albeit variable nutritional quality of larval diets (i.e., protein to lipid (P:L) ratio). The observed changes in bee-plant pollen interaction patterns indicate that only the flexible generalists, such as O. bicornis, may be able to compensate the strong alterations in floral resource landscapes and to obtain food of sufficient quality through readily shifting to alternative plant sources. In contrast, other, less flexible, bee species disappear.

Keywords: Bee decline; biodiversity exploratories; foraging; metabarcoding; pollen nutrients; solitary bees.

FIGURE 1

Bipartite network showing interactions between trap nesting solitary bee species and plant species based on larval pollen provisions sampled from nests installed at plots differing in land‐use intensity (LUI: represented by categories: low, intermediate, and high) in three biogeographical regions in Germany (Exploratories: Swabian Alb, Hainich‐Dün and Schorfheide‐Chorin) (assignment of ASVs up to species level). Plant species were included if they occurred in relative abundances of ≥1% in the respective dataset. Osmia cornuta is represented with 7 nest chambers, O. bicornis with 90, Chelostoma florisomne with 7, Megachile rotundata with 11, Heriades truncorum with 7, O. caerulescens with 15 and O. leaiana with 3, M. versicolor with 7 and Hylaeus spp. with 3. Colored bars below bee species show occurrence of each bee species at plots differing in land‐use intensity and in geographical regions; top bars indicate geographical distributions: red: Schorfheide‐Chorin, blue: Swabian Alb, darkgreen: Hainich‐Dün; bottom bars indicate land‐use intensity (LUI): lightgreen: low, yellow: intermediate, brown: high; white bars indicate that a species was absent from a specific bioregion or land‐use intensity category

FIGURE 2

Effect of land‐use intensity (LUI) on (a) plant taxonomic Shannon diversity, (b) total fatty acid (FA) concentration (c) total amino (AA) acid concentration, and (d) total essential AA concentration and (e) the ratio of total FA to total AA in Osmia bicornis larval pollen provisions sampled from nests installed at plots differing in land‐use intensity (LUI) in three biogeographical regions in Germany (Exploratories: Swabian Alb, Hainich‐Dün and Schorfheide‐Chorin). Plant diversity is based on revealed ASVs (Amplicon sequent variants) per bee nest