The Overlooked Role of Facilitation in Biodiversity Experiments

Understanding the functional role of biodiversity in an ecosystem is an essential component of predicting the consequences of biodiversity loss. Experimental studies have consistently shown that the loss of biodiversity can lead to a loss in ecosystem functioning (BEF relationships). Our ability to predict the consequences of biodiversity loss in understudied ecosystems, and in a global change context, requires a deeper mechanistic understanding of BEF relationships. Here, we highlight three categories of facilitation that can be important drivers of BEF relationships: indirect biotic interactions due to pathogens and mutualists; abiotic interactions due to nutrient enrichment; and abiotic interactions due to microclimate amelioration. We demonstrate how increased environmental severity, abundance of specialist pathogens, and biological nitrogen fixation rates likely drive increased facilitation and, thus, the strength of the BEF relationship, across ecosystems. Past research has demonstrated that decreased biodiversity often reduces ecosystem productivity, but variation in the shape of biodiversity–ecosystem function (BEF) relationships begets the need for a deeper mechanistic understanding of what drives these patterns. While mechanisms involving competition are often invoked, the role of facilitation is overlooked, or lumped within several less explicitly defined processes (e.g., complementarity effects). Here, we explore recent advances in understanding how facilitation affects BEF relationships and identify three categories of facilitative mechanisms that can drive variation in those relationships. Species interactions underlying BEF relationships are complex, but the framework we present provides a step toward understanding this complexity and predicting how facilitation contributes to the ecosystem role of biodiversity in a rapidly changing environment. Past research has demonstrated that decreased biodiversity often reduces ecosystem productivity, but variation in the shape of biodiversity–ecosystem function (BEF) relationships begets the need for a deeper mechanistic understanding of what drives these patterns. While mechanisms involving competition are often invoked, the role of facilitation is overlooked, or lumped within several less explicitly defined processes (e.g., complementarity effects). Here, we explore recent advances in understanding how facilitation affects BEF relationships and identify three categories of facilitative mechanisms that can drive variation in those relationships. Species interactions underlying BEF relationships are complex, but the framework we present provides a step toward understanding this complexity and predicting how facilitation contributes to the ecosystem role of biodiversity in a rapidly changing environment. facilitation that is mediated through changes in the abiotic environment (e.g., vapor pressure deficit, soil porosity, soil moisture, or nutrient enrichment). facilitation that results from the activity of a higher order trophic interaction (e.g., bacterial, rhizobial, or arbuscular mycorrhizal fungal communities). occurs when an increase in the density of species b increases the performance of species a. occurs when species have unique and complementary resource requirements that can allow some species to stably coexist; these groups of species can be more productive and capture available resources more comprehensively than any species in monoculture. occurs when higher diversity mixtures have a higher statistical probability of including particularly productive species. When those species that are more productive in monoculture are also better competitors in mixture, higher diversity communities can be more productive than lower diversity communities. the case where an individual species grows more in mixture than it does in monoculture, after accounting for differences in the proportion of seed planted. For example, corn seed in monoculture might be planted at 100%, while corn seed in a two-species mixture might be planted at 50%. If corn grows 100 g per unit area in monoculture, but greater than 50 g per unit area in a two-species mixture, this is considered species-specific overyielding.