A mechanistic framework for studying indirect effects of large vertebrate herbivores on pollinators

Abstract

Globally, large herbivores (e.g., cattle, elk) graze over 2.6 billion hectares of land. These lands can also be used to conserve pollinators that rely on similar resources, specifically diverse plant communities. Pollinator conservation will benefit from management in lands that are used for livestock grazing and wildlife conservation. However, maximizing ecosystem services provided in these areas is often nuanced and difficult. To improve our ability to support multiple ecosystem services in grazing lands, we reviewed published literature to investigate the mechanisms of indirect effects of large herbivores on pollinators via their resources (food plants, nesting sites). We used a framework from previous research on indirect effects of insect herbivores to explore and interpret how plant responses mediate large herbivore effects on pollinators through three mechanistic categories: resource abundance and availability, plant appearance, and plant chemistry. Using the broader conceptual model, we conducted a targeted literature review that found ~ 95% of studies investigating pollinators and large herbivores focused on resource abundance and availability. Consequently, more research is necessary to understand how large herbivores impact pollinators through multiple mechanisms. Future research could also test responses with both large herbivores and insect herbivores to determine their combined ecological consequences. This research will provide insights for managing large herbivores and pollinators simultaneously, while connecting concepts of pollinator ecology and grazing ecology. Filling research gaps on the mechanisms of indirect effects of large herbivores on pollinators will ultimately improve management of multiple ecosystem services and our ability to conserve declining pollinator species.