The joint effects of biotic and abiotic biodiversity drivers

Background/Question/Methods

Fast worldwide biodiversity decline has propelled research on its potential environmental and biotic causes. However, most biodiversity studies focus on the influence of one single driver, while ecosystems are exposed to a multitude of potential biodiversity drivers.

We define two types of potential biodiversity drivers. Abiotic drivers are environmental variables that, when exceeding tolerance limits, may reduce fecundity and/or survival. This process is often referred to as environmental filtering. Biotic drivers represent a plethora of intra- and interspecific interactions that may also affect fecundity (competition-induced resource limitation) and/or survival (predation/mutualism).

By considering predator abundance and prey abundance as two environmental variables affecting prey survival and predator reproduction, we are able to construct a simple biodiversity model that unifies biotic and abiotic biodiversity drivers into one mathematical framework. By doing so, we are able to simulate biodiversity for various combinations of biotic and abiotic drivers and for various levels of dispersal limitation.

Results/Conclusions

In absence of environmental filtering, our model predicted that predator diversity was lowered by bottom-up control. We define bottom-up control as a situation where prey abundance constrains predator fecundity, but predator abundance does not increase the prey’s mortality rate. As expected, an environmental variable impacting on prey/predator reproduction resulted in a biodiversity reduction of prey/predators. In addition, our model suggested that these prey diversity reductions resulted in negative knock-on effects on predator diversity in bottom-up controlled systems by affecting the abundance of rare predators.

According to our model, top-down control (predator fecundity is unconstrained by prey abundance, but predator abundance increases the prey’s mortality rate) reduced prey diversity. When exposed to environmental variables affecting predator fecundity, positive knock-on effects on prey diversity were predicted.

The joint effects of top-down control with environmental variables impairing prey fecundity suggest nonlinear net effects when biotic and abiotic drivers combine. The same conclusion was drawn when bottom-up control coincided with impaired predator fecundity. We conclude that the effects of environmental variables on prey and predator reproduction propagate across trophic levels and that net effects of biotic and abiotic stress are not additive.