Research on biodiversity-ecosystem function relationships

Recent research and vision

Given the many ongoing threats to global biodiversity, it is worth considering whether, or to what extent, ecosystem functions depend on biodiversity per se. At the root of this question is whether declines in biodiversity will create negative feedbacks that accelerate ecosystem decay as global change progresses. Understanding such a process would enable us to better predict how communities and ecosystems will respond to global change. And from a practical standpoint, the relationship between biodiversity and ecosystem function could provide important motivation for biodiversity conservation… if biodiversity is indeed important for ecosystem function.

In my recent work, I examined this question in the context of bee diversity and plant pollination. I found that, due to diet complementarity among wild bee species, diverse plant communities do indeed depend on a diverse bees assemblage. Notably, even locally rare bees can be important pollinators for some plant species (Simpson et al 2022; see below for more details). This work was purely observational, however, and does not consider demographic consequences to plants of pollinator visitation rates or pollinator diversity.

In my future work, I would like to more explicitly consider the role of diversity in food web resilience. While the classic biodiversity-ecosystem function (BEF) approach is compelling for its simplicity, I think it relies too heavily on implicit assumptions. For example, what functions are we measuring and why are some functional outcomes preferred over others? I think the root of the BEF question is, as I stated at the top, about whether biodiversity loss will destabilize ecosystems, creating negative feedbacks that accelerate ecosystem decay. Ultimately, “does biodiversity support biodiversity?” It is about more than functional rates—it is about food webs and food web resilience.

To advance the BEF field, I would like to address this root question more directly. In particular, how do common functional rates (e.g. productivity or nutrient cycling rates) relate to food web structure? Are there feedbacks between food web structure (e.g. diversity, complementarity) and functional rates that affect foodweb resilience?

Past projects and collaborations

Importance of bee diversity, including rare bee species, for pollination function

Many studies have asked whether pollinator diversity enhances pollination function. Historically, however, such studies have focused on crop monocultures or, in a few cases, average pollination rates across multiple plant species. With such an approach, it is typically pollinator abundance, not diversity, that is the primary driver of pollination function. As a result, locally rare bees necessarily appear unimportant for function because they provide a low proportion of overall visits. In a diverse plant community, however, many plants need to be pollinated simultaneously. If different pollinator species are important to different plant species, then pollinator diversity may be much more important for plant communities than it has appeared in previous studies.

In this study, I used 11 plant-bee visitation networks to assess the importance of bee diversity and rare bees to pollination function (Simpson et al 2022). Using a rarefaction-like analysis, I asked how the number of “functionally important” bee species—i.e., dominant plant visitors—changed as we moved from considering only a single plant species to a community of plant species. Indeed, because of complementarity in bee diets, many times more bee species appeared functionally important when we considered plant communities as a whole. Further, a substantial portion of functionally important bees were locally rare.

Importance of wild bees to crop pollination services

Most of the worlds crops are dependent on animal pollinators, especially insects. The importance of wild insects, however, relative to domestic honey bees, has remained an open question. This is an important economic question because it will determine the appropriate strategy to maximize crop yield. It is also an important question for conservation because the importance of wild bee abundance or diversity to crop yields will determine the extent to which crop pollination services can motivate insect conservation. Some prior work has suggested wild bee abundance, if not diversity, to be a better predictor of crop yields than honey bee abundance but, taken as a whole, the literature has been inconclusive.

In this study, led by James Reilly, we utilize the newly-available Crop Poll database to more definitively answer this question (Reilly et al, 2024). The Crop Poll database is an open database of pollination and yield data from hundreds of studies of 35 crop species from six continents. This analysis roughly doubles the size of previous meta-analyses. We found insect diversity and visitation frequency to positively affect crop yield, but visitation frequency (a proxy for abundance) was more important than diversity. Overall, honey bees and wild insects contributed about equally to crop yield.