Leaf litter. Image credit: Léa Beaumelle (CC BY 4.0)
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.
