Biodiversity mediates the effects of stressors but not nutrients on litter decomposition

Abstract

Understanding the consequences of ongoing biodiversity changes for ecosystems is a pressing challenge. Controlled biodiversity-ecosystem function experiments with random biodiversity loss scenarios have demonstrated that more diverse communities usually provide higher levels of ecosystem functioning. However, it is not clear if these results predict the ecosystem consequences of environmental changes that cause non-random alterations in biodiversity and community composition. We synthesized 69 independent studies reporting 660 observations of the impacts of two pervasive drivers of global change (chemical stressors and nutrient enrichment) on animal and microbial decomposer diversity and litter decomposition. Using meta-analysis and structural equation modeling, we show that declines in decomposer diversity and abundance explain reduced litter decomposition in response to stressors but not to nutrients. While chemical stressors generally reduced biodiversity and ecosystem functioning, detrimental effects of nutrients occurred only at high levels of nutrient inputs. Thus, more intense environmental change does not always result in stronger responses, illustrating the complexity of ecosystem consequences of biodiversity change. Overall, these findings provide strong evidence that the consequences of observed biodiversity change for ecosystems depend on the kind of environmental change, and are especially significant when human activities decrease biodiversity.

eLife digest

Ecosystems are at their healthiest when they have a high level of biodiversity – that is, a wide variety of different species all living together. But human activity is changing the environment and putting ecosystems under pressure. One of the places this is most evident is in the communities of organisms responsible for breaking down dead plants.

These organisms – called decomposers – are highly sensitive to pesticides, metals and other chemical stressors, as well as excess nutrients, such as nitrogen, released by industry and farming. Exposing decomposers to these chemicals can change both the number of individuals of each species and the number of different species that are present. In other words, these chemicals can, respectively, alter both the abundance and diversity of decomposer communities. Controlled experiments in simplified conditions suggest that these changes in biodiversity affect ecosystem health. But, it remained unclear to what extent these results applied to real-world scenarios of environmental change.

To test the findings of controlled experiments, Beaumelle et al. investigated how chemical stressors and excess nutrients affect the breakdown of leaf litter – the debris of decomposing leaves that forms on top of soil. Previous studies suggest that the reduced biodiversity caused by chemicals should result in leaf litter breaking down more slowly. Whereas the loss in biodiversity caused by nutrients will increase the number of some decomposer species, causing leaf litter to break down faster or slower, depending on local conditions.

Beaumelle et al. tested these predictions by gathering the results from 69 independent studies conducted across the globe. The results showed that stressors caused the diversity and abundance of decomposers to decline, which reduced the breakdown of leaf litter, as expected. But, the outcomes of excess nutrients were more varied. Low levels of excess nutrients increased the breakdown of leaf litter, but at high levels slowed down the rate leaves decomposed. Furthermore, the effect excess nutrients had on biodiversity in decomposer communities changed according to the types of organisms in the ecosystem. This suggests that variations in biodiversity can impact ecosystems differently depending on the type of environmental change.

The breakdown of leaf litter plays a critical role in carbon balance, and this has knock-on effects for the Earth's climate. This work suggests that improving biodiversity is crucial to maintain the health of ecosystems, but successful strategies will have to be adjusted depending on the type of human impact (for example, chemical stressors or nutrient additions). These findings could help researchers design better approaches for boosting ecosystem health in the future.