Reduced precipitation frequency increases pollinator resources in an Australian old-field grassland

Background/Question/Methods

Predicted shifts in the timing and amount of precipitation with climate change have key consequences for the productivity of many grasslands globally. Furthermore, these ecosystem responses are predicted to have cascading effects on higher trophic level interactions, especially regarding timing and availability of floral resources for pollinators already at risk due to habitat degradation, disease and impacts from warming and drought. These impacts are of particular interest in areas of wildland-agricultural interface, where pollinators provide ecosystem services to crops while simultaneously relying on annual ‘weedy’ species that serve as un-intentional floral enhancements in old-fields and agricultural landscapes world-wide. To examine the impacts of altered precipitation on plant community structural and floral responses we applied three precipitation treatments, including a reduced frequency treatment (Frequency; rain summed over 22-day intervals), a 50% reduction in individual precipitation events (Drought), an ambient precipitation pattern applied under shelters (Ambient). We measured plant community structure 1 year after seeding target annual weedy species into an early successional old-field in a grassland community of Western Sydney Australia, while daily images were used for detecting flower abundance and identity between Nov 2019 and July 2020. Pollinator surveys were conducted approximately every ~ 6 weeks after seeding.

Results/Conclusions

We found that while the drought treatment did reduce overall soil moisture, the altered frequency treatment ultimately increased average soil moisture, especially during summer and autumn. This translated into an increase in the number of days that each treatment had flowers present during summer and autumn with 133, 85, and 105 total days under altered frequency, ambient, and drought respectively. Daily available flowers were also impacted by climate treatments with altered frequency plots containing almost twice as many flowers as ambient plots throughout summer, while drought plots contained half as many flowers as ambient plots. In this subtropical grassland, plants typically stay green and at least marginally active throughout the entire year. Old-fields therefore can provide off-season supplemental resources for pollinators within a larger agricultural landscape, assuming that environmental conditions allow for continued flower production. In our study, we found that a reduced frequency (but not quantity) of rainfall stimulated the overall availability of floral resources, while reduced quantity (but not frequency) reduced overall availability of floral resources. These findings suggest that while old-field flowers may provide necessary resources for pollinators when crops are not active, climate change impacts on water availability may constrain floral enhancement at the wildland-agricultural interface.