The challenge of accurately documenting bee species richness in agroecosystems: bee diversity in eastern apple orchards

Abstract

Bees are important pollinators of agricultural crops, and bee diversity has been shown to be closely associated with pollination, a valuable ecosystem service. Higher functional diversity and species richness of bees have been shown to lead to higher crop yield. Bees simultaneously represent a mega-diverse taxon that is extremely challenging to sample thoroughly and an important group to understand because of pollination services. We sampled bees visiting apple blossoms in 28 orchards over 6 years. We used species rarefaction analyses to test for the completeness of sampling and the relationship between species richness and sampling effort, orchard size, and percent agriculture in the surrounding landscape. We performed more than 190 h of sampling, collecting 11,219 specimens representing 104 species. Despite the sampling intensity, we captured <75% of expected species richness at more than half of the sites. For most of these, the variation in bee community composition between years was greater than among sites. Species richness was influenced by percent agriculture, orchard size, and sampling effort, but we found no factors explaining the difference between observed and expected species richness. Competition between honeybees and wild bees did not appear to be a factor, as we found no correlation between honeybee and wild bee abundance. Our study shows that the pollinator fauna of agroecosystems can be diverse and challenging to thoroughly sample. We demonstrate that there is high temporal variation in community composition and that sites vary widely in the sampling effort required to fully describe their diversity. In order to maximize pollination services provided by wild bee species, we must first accurately estimate species richness. For researchers interested in providing this estimate, we recommend multiyear studies and rarefaction analyses to quantify the gap between observed and expected species richness.

Keywords: Agroecosystem; apple orchard; bee diversity; biodiversity; ecosystem function; pollination services; sampling.

Figure 1

A wild mining bee, Andrena mandibularis, visiting an apple blossom in New York, USA.

Figure 2

A map of the study sites (black circles) in the Finger Lakes region of western New York, USA.

Figure 3

Rarefaction curves for the 22 orchards individually analyzed (A-V), as well as all sites pooled together (W). The dark gray area is the 95% confidence interval around the Chao 1 mean (solid line) for expected species richness as a function of sampling. The dashed line is the observed number of species. The letter in the upper right-hand corner refers to the name of the orchard; it refers to Table1 for a list of the orchard names. For 13 sites (marked by an X on the upper left hand corner of the graph), we captured <75% of the expected bee diversity (B, E, F, I, J, K, L, M, N, Q, R, S, and V). The rarefaction curve in eight of these 13 sites does not reach an asymptote (E, F, I, J, K, M, Q, and R).

Figure 4

Scatter plots of the species richness versus the number of samples at each of the individually sampled sites for all collections combined (A), just the standardized collections (B), and just the general collections (C). There is a significant correlation between richness and samples for all collections combined (P < 0.05, R² = 0.47), standardized collections (P < 0.05, R² = 0.51), and general collections (P < 0.05, R² = 0.52), but this correlation is not significant for orchards where we captured 75% or more of the expected species richness (dark circles) (P > 0.05).