Species-specific shifts in flowering phenology influence community-level patterns: implications for plant-pollinator interactions
With unprecedented rates of temperature increases and altered precipitation regimes across the globe, species are showing strong directional shifts in the timing of their life history events in accordance with these changes in their abiotic environment. Although the overall trend is for earlier phenological events, the phenological shifts of individual species can differ substantially in both magnitude and direction. Given the variation and complexity of phenological responses across species, the emergent community-level response is unlikely to shift in a simple uniform pattern through time. To explore how phenological shifts of individual species scale up to influence community-level patterns of phenology, we analyze data from a long-term, ongoing study of flowering time for 60 species at a site in the Colorado Rocky Mountains over the last 39 years (1974-2012). Furthermore, to investigate how changes in the flowering community might influence interactions with pollinators, we compare community-level patterns of flowering phenology with two years of data on nesting phenology and pollen provisioning for Megachilid bees.
We find that the overall patterns of community-level flowering have changed substantially over the last 39 years. The species-specific shifts in flowering phenology in our study system have lead to an increase in the duration of the flowering season by 50 days. This change in season length, with no overall change in total floral abundance, has contributed to an alteration of the temporal availability of floral resources. The spring peak of floral abundance is advancing at a faster rate than the summer peak in floral abundance, resulting in a scarcity of floral resources during the middle of the season. Our current measures of bee-nesting phenology do not mirror current flowering patterns, and instead are more in accordance with historic flowering patterns over the timeframe 1974-1984 (i.e. a baseline of community-level flowering phenology). Peak nesting activity (and pollen provisioning) for early-season Megachilid bees occurs at a time when floral resources are notably low, lagging behind the spring peak in floral abundance by approximately two weeks. However, nesting activity for late-season bees occurs during the summer peak in floral resources. These phenological patterns for plants and pollinators suggest that climate change can have strong effects on communities of interacting organisms via species-specific phenological shifts.