Thermal Biology and Foraging Responses of Insect Pollinators to the Forest Floor Irradiance Mosaic

This paper examines the responses of the insect pollinators of the summer-flowering, understorey shrub, Lavandula latjfolia (Labiatae) to the irradiance mosaic of the forest floor in a southeastern Spanish locality, and assesses the relationship between these responses and interspecific variation in thermal biology. Pollinators differed significantly in the mean irradiance at capture points (MICP), and a large percentage of species (59%) preferentially selected plants in either shaded or sunlit conditions. Taxonomic affiliation (at the order level) and body size explained 78% of interspecific variance in MICP. Dipterans tended to be restricted to situations of relatively low irradiance, whereas hymenopterans foraged over the entire irradiance gradient. MICP was inversely related to body size. Pollinators differed significantly in mean thoracic temperature (Tth), thoracic temperature excess (Texc = Tth — Ta [air temperature]), and slope of the Tth/Ta regression (which may be taken as a rough index of thermoregulatory ability). Hymenopterans had higher Tth and Texc, and smaller Tth/Ta slopes, than dipterans. After accounting for this taxonomic effect, 7h and Texc, but not Tth/Ta slope, increased with body size. MICP depended significantly on Texc and, to a lesser degree, on Tth, and species characterised by high Texc tended to select low-irradiance conditions. The relationship between MICP and body size was thus mediated by the correlations between the latter variable and thermal biology parameters. In the hot Mediterranean summer, as in other thermally stressing environments, thermal constraints of the kind reported in this study may generate within- and between-habitat variation in the composition and size structure of pollinator assemblages, which may be consequential for plant reproduction.