High abundance and complementarity, but not species richness, of rare and declining bees in flowering agricultural crops
Agriculture is a primary cause of biodiversity loss worldwide, and conservation objectives and agricultural production are typically viewed as divergent priorities. Yet agricultural habitats are also used by many native species, including rare ones. Here we compare the number of rare and declining bee species between two land use categories: flowering crop fields, and natural habitats. We then determine whether the different habitat types within each land use category are complementary to each other in terms of the rare and declining bee species found in each. We first defined the rare and declining bees for our region using a large data set based on museum specimens. We then used data we collected on bees hand-netted while foraging on flowers in six habitat types (three crop types and three natural communities). We compared abundance and species richness of rare and declining bees between land use types using a Wilcoxon Mann-Whitney rank sum test. To test complementarity between habitat types, we adapted an equation used for determining overyielding in biodiversity-ecosystem function research (Dmax) to determine whether combinations of habitats “overyielded” with a greater species richness than any individual habitat type.
In total we analyzed 7357 specimens of 172 bee species. Our rank test indicated that rare bees were significantly more abundant in agricultural as compared with natural habitats (p=0.005, Z = 2.8); however, our results were largely driven by two crop-specialist species (Habropoda laboriosa and Melitta americana) that accounted for the majority of rare bees on crops. There was no significant difference between land use categories for abundance of declining bees (Z=0.27, p=0.79) or for species richness of rare (Z=-0.22, p=0.85) or declining (Z=-0.89, p=0.38) bees. We also found evidence of complementarity (Dmax>0 for all combinations) between habitats in the agricultural land use category but not in the natural land use category; combinations of agricultural habitat types (but not natural habitat types) supported greater bee species richness than did any single habitat type.