Ups and downs in pollinator populations: When is there a decline?

Plant-pollinator systems inherently possess wide variation that limits the applicability of surveys on population dynamics or diversity. Stable habita ts are scarcely studied, whereas dynamics in unprotected habitats are less predictable or more compromised by exotic organisms (Apis, in the case of b ee surveys). An extensively replicated, long-term study of orchid-bees (Eug lossini) was made in protected tropical moist forest in Panama. Over 47,000 bees were recorded in 124 monthly censuses employing 1952 counts. No aggre gate trend in abundance occurred (from 1979 to 2000), although four individ ual species declined, nine increased, 23 showed no change, and species rich ness was stable. No rare or parasitic species showed decreasing trends, whi le the most common of the set of bee species studied gradually declined. Bi odiversity therefore increased. Recorded variability included 300% (fourfol d) differences in bee abundance among years, and changes in species abundan ce up to 14-fold. Surveys in dry and wet seasons (N = 17 and 18 years, 29 a nd 31 species, respectively) indicated no numerical changes in the bee asse mblage over 21 years. El-Ni (n) over tildeo climatic events led to brief in creases in bee abundance. This detailed survey is deconstructed to assess s ampling rigor and strategies, particularly considering the recorded local d ifferences within a single forest. Year-to-year shifts in bee abundance for three tropical and five temperate bee censuses were comparable. In short studies (2-4 years) and during longe r studies (17-21 years), 59 species that included solitary, social, and hig hly social bees had mean abundances that varied by factors of 2.06 for temp erate bees and 2.16 for tropical bees. "Normal" bee populations commonly ha lved or doubled in 1-yr intervals. Longer term data are only available for the tropics. Stochastic variation and limitations of monitoring methods sug gest that minimum series of four years (i.e., three intervals) of several c ounts during the active season may demonstrate genuine trends. Longer term, continuous studies are still needed for meaningful insights on pollinator population shifts in nature.