Seasonal variation of a plant-pollinator network in the Brazilian Cerrado: Implications for community structure and robustness.

Seasonal variation in the availability of floral hosts or pollinators is a key factor influencing diversity in plant-pollinator communities. In seasonally dry Neotropical habitats, where month-long periods of extreme drought are followed by a long rainy season, flowering is often synchronized with the beginning of precipitation, when environmental conditions are most beneficial for plant reproduction. In the Brazilian Cerrado, a seasonally dry ecosystem considered one of the world's biodiversity hotspots for angiosperms, plants with shallow root systems flower predominantly during the rainy season. Foraging activity in social bees however, the major pollinators in this biome, is not restricted to any particular season because a constant supply of resources is necessary to sustain their perennial colonies. Despite the Cerrado's importance as a center of plant diversity, the influence of its extreme cycles of drought and precipitation on the dynamics and stability of plant-pollinator communities is not well understood. We sampled plant-pollinator interactions of a Cerrado community weekly for one year and used network analyses to characterize intra-annual seasonal variation in community structure. We also compared seasonal differences in community robustness to species loss by simulating extinctions of plants and pollinators. We find that the community shrinks significantly in size during the dry season, becoming more vulnerable to disturbance due to the smaller pool of floral hosts available to pollinators during this period. Major changes in plant species composition but not in pollinators has led to high levels of turnover in plant-pollinator associations across seasons, indicated by in interaction dissimilarity (<3% of shared interactions). Aseasonal pollinators, which mainly include social bees and some solitary specialized bees, functioned as keystone species, maintaining robustness during periods of drastic changes in climatic conditions.

Taxonomic revision of Perdita subgenus Heteroperdita Timberlake (Hymenoptera: Andrenidae), with descriptions of two ant-like males.

Perdita subgenus Heteroperdita Timberlake, a distinctive subgenus of 22 species from the southwestern United States and adjacent Mexico, all specialists on Tiquilia (Boraginaceae), is revised. Nine new species are described: Perdita (Heteroperdita) desdemona Portman, sp. n., P. (H.) exusta Portman & Griswold, sp. n., P. (H.) hippolyta Portman & Griswold, sp. n. (male previously incorrectly described as P. pilonotata Timberlake), P. (H.) hooki Portman & Neff, sp. n., P. (H.) nuttalliae Portman, sp. n., P. (H.) prodigiosa Portman & Griswold, sp. n., P. (H.) sycorax Portman, sp. n., P. (H.) titania Portman & Griswold, sp. n., and P. (H.) yanegai Portman, sp. n. The following sexes are associated and described for the first time: the male of P. (H.) frontalis Timberlake, 1968, the female of P. (H.) optiva Timberlake, 1954, and the true male of P. (H.) pilonotata Timberlake, 1980. Perdita (H.) fasciatella Timberlake, 1980 is proposed as a junior synonym of P. (H.) sexfasciata Timberlake, 1954. A neotype is designated for P. (H.) pilonotata Timberlake, 1980. Two species in particular, P. prodigiosa and P. pilonotata, are sexually dimorphic with distinctive ant-like males. Information is presented on floral relationships, phenology, and geographic distribution. Identification keys for males and females are provided.