Complex long-term dynamics of pollinator abundance in undisturbed Mediterranean montane habitats over two decades

Current notions of "pollinator decline" and "pollination crisis" mainly arose from studies on pollinators of economic value in anthropogenic ecosystems of mid-latitude temperate regions. Comprehensive long-term pollinator data from biologically diverse, undisturbed communities are needed to evaluate the actual extent of the so-called "global pollination crisis." This paper analyzes the long-term dynamics of pollinator abundance in undisturbed Mediterranean montane habitats using pollinator visitation data for 65 plant species collected over two decades. Objectives are (1) to elucidate patterns of long-term changes in pollinator abundance from the perspectives of individual plant species, major pollinator groups, and the whole plant community and (2) to propose a novel methodological implementation based on combining a planned missing data design with the analytical strength of mixed effects models, which allows one to draw community-wide inferences on long-term pollinator trends in species-rich natural habitats. Probabilistic measurements ("patch visitation probability" and "flower visitation probability" per time unit) were used to assess pollinator functional abundance for each plant species on two separate, randomly chosen years. A total of 13,054 pollinator censuses accounting for a total watching effort of 2,877,039 flower-min were carried out on 299 different dates. Supra-annual unstability in pollinator functional abundance was the rule, with visitation probability to flowering patches and/or individual flowers exhibiting significant heterogeneity between years in the majority of plant species (83%). At the plant-community level, there was a significant linear increase in pollinator functional abundance over the study period. Probability of pollinator visitation to flowering patches and individual flowers increased due to increasing visitation by small solitary bees and, to a lesser extent, small beetles. Visitation to different plant species exhibited contrasting changes, and insect orders and genera differed widely in sign and magnitude of linear abundance trends, thus exemplifying the complex dynamics of community-wide changes in pollinator functional abundance. Results of this investigation indicate that pollinator declines are not universal beyond anthropogenic ecosystems; stress the need for considering broader ecological scenarios and comprehensive samples of plants and pollinators; and illustrate the crucial importance of combining ambitious sampling designs with powerful analytical schemes to draw reliable inferences on pollinator trends at the plant community level.