The shifting importance of abiotic and biotic factors across the life cycles of wild pollinators

Abstract

Abstract Organisms living in seasonal environments are exposed to different environmental conditions as they transition from one life stage to the next across their life cycle. How different life stages respond to these varying conditions is a fundamental aspect of biology and is critical for understanding how organisms will respond to environmental change. Despite the importance of animal pollinators, we lack a basic understanding of the influence of different environmental factors across their life cycles. We investigated the relative importance of climate conditions, food availability, and previous life stage abundance in a community of wild bumble bee species, asking: how do these three factors influence bee abundance at each life stage? We used a 7-year dataset to examine the importance of environmental conditions on the abundance of life stages in seven wild bumble bee species. We monitored climate conditions, the abundance of floral resources, and abundances of bees in each life stage across the active colony life cycle in a highly seasonal subalpine ecosystem in the Colorado Rocky Mountains, USA. Bee abundance at different life stages responded to environmental conditions in a consistent manner across the seven species. The survival and recruitment stage of the life cycle (assessed as abundance of overwintered queens) responded negatively to longer winters; the growth stage (assessed as abundance of workers) responded positively to floral resource availability; and the reproductive stage (assessed as abundance of males) was positively related to the abundance of the previous life stage (workers). Our long-term examination of annual bumble bees reveals a consistent set of responses in the abundance of each life stage to climate conditions, floral resource availability, and previous life stage abundance. Across species, these three factors each influenced a distinct life stage, illustrating how their relative importance can shift throughout the life cycle. The life-cycle approach we have taken with wild bumble bees highlights that important details about pollinator demography are overlooked without considering life stage-specific responses. Ultimately, it is these life-stage specific responses that shape population outcomes, not only for animal pollinators, but for many organisms living in seasonal environments.

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