On commonness and rarity of insect pollinators: patterns of species abundance and diversity in a hyperdiverse Mediterranean assemblage

The notion of "pollinator diversity" is central to most research and interpretations in animal pollination ecology. Nevertheless, when the term "diversity" is applied to pollinators its usage is often closer to the vernacular meaning (variety of kinds) than to concepts rooted in the "ecological diversity" tradition of community and statistical ecology. This paper attempts to fill a conspicuous knowledge gap in pollination ecology by presenting a comprehensive analysis of patterns of species abundance and diversity in a hyperdiverse insect pollinator assemblage from well-preserved Mediterranean montane habitats of southeastern Spain. Data on pollinator visitation to flowers of the community of entomophilous plants (288 species) were gathered over a 29-year period, and [~]95% of the pollinator individuals recorded were identified to species, totalling 46,401 individuals in 845 species. The shape of species abundance distributions (SADs) was virtually identical at regional (N = 56 sites) and local (one intensively studied site) scales, and SADs were best predicted by the log-series distribution. Pollinator diversity estimates corresponding to the first three Hill numbers (Species richness, Shannon diversity and Simpson diversity; 0D, 1D and 2D, respectively) were obtained for each plant species x site x year combinations ("sampling occasions", N = 472). Pollinator diversity measures varied widely among plant species; their frequency distributions were continuous, unimodal and strongly right-skewed; and variation was related to plant phylogeny, floral features (open vs. restrictive perianth, single flower vs. flower packet), and pollinator visitation to flowers and flowering patches. Pollinator diversity of individual plant species depended on habitat type, with those from dolomitic outcrops, rock cliffs and forest interior having the least diverse pollinators. 0D, 1D and 2D tended to vary independently of each other among habitats and years, revealing a complex spatio-temporal patterning of pollinator species richness and dominance. Estimated proportions of undetected pollinator diversity ("dark diversity") depended on insect order (highest for Diptera) and diversity measure (highest for 0D). Adoption of community ecology tools (SAD, sampling adequacy estimation, complementary diversity measures) to assess pollinator diversity will improve our ability to elucidate pollinator responses to natural and anthropogenic environmental change and permit hitherto unexplored questions in pollination ecology. "The ecologist sees in any measure of diversity an expression of the possibilities of constructing feedback systems or any sort of links, in a given assemblage of species" Margalef (1968, p. 19).