Field experiments that varied the composition of both plant litter and the organisms that break it down have revealed that, across ecosystems, lower biodiversity slows the rate of litter decomposition. See Letter p.218
Despite our increased awareness of biodiversity loss, and attempts to respond to it, the global rate of species extinction does not seem to be slowing1. Concern about how these losses may affect the way that ecosystems function has led to decades of experiments looking for connections between the properties of ecosystems and the numbers and types of species that live in them. Using field experiments and meta-analyses, ecologists have determined that decreased diversity reduces the efficiency of resource capture by communities and the conversion of those resources to biomass2. Most biomass created by plants is not consumed by herbivores, but becomes 'litter' after a plant dies. Do the effects of diversity die with the plants? On page 218 of this issue, Handa et al.3 suggest that the answer is no, by showing that both the diversity of dead plants and the diversity of organisms that decompose them contribute to determining how quickly this material is recycled back into ecosystems.
Although experiments and meta-analyses have given us confidence in our predictions of the effects of mixing together living plants, experiments in which different litter types are mixed rarely show predictable results4,5,6. Meta-analyses of these studies have been limited by different experimental designs, site-specific effects and the potential influence of the type rather than the number of species. Handa and collaborators attacked this problem using a series of coordinated field experiments, at sites ranging from the sub-Arctic to the tropics, in which they manipulated the diversity of both plants and detritivores — the invertebrates and microorganisms that break down litter. They found that a reduction in the diversity of either slows the rate at which litter is decomposed, regardless of the location of the experiment (Fig. 1).
Decomposition rates are an important determinant of the global carbon budget, affecting not only the incorporation of vegetation carbon into soils, but also the early stages of its release back to the atmosphere. That Handa et al. found effects of diversity on this process across biomes and ecosystems suggests a remarkable consistency in the consequences of biodiversity loss.
The authors manipulated detritivore diversity by varying the types of container the litter was decomposed in, and thus the organisms that could access and break down the litter. They show that the diversity of these organisms had even more influence than the composition of the litter community itself — the more types of detritivore, the faster the litter was decomposed. This finding provides experimental support for the results of a meta-analysis5 that concluded that the effects of consumer diversity on decomposition are generally more important than the effects of resource diversity.
Handa and colleagues' study is also one of the first to provide evidence for a potential mechanism driving the effects of higher litter diversity: the movement of nitrogen between different litter types. Laboratory studies had previously shown that such nitrogen movement was possible7, but whether it could occur in natural settings, and whether it would lead to changes in decomposition, was unknown until now.
Although nitrogen movement between leaves had been predicted to follow a simple gradient — from leaves with more nitrogen to leaves with less — it may instead be driven by carbon quality. If microbes grow quickly on litter from plants with high-quality carbon, this would increase the microbes' need for nitrogen, and when it is not available locally they may access it from the neighbouring litter. Handa et al. found evidence for nitrogen movement between litter types across biomes and ecosystems, and always between the same two litter types (from nitrogen-fixing litter species to rapidly decomposing ones). This is an exciting result that will increase our ability to predict the effects of biodiversity loss on carbon cycling and storage.
However, there is still no direct evidence for nitrogen translocation — in the latest experiments, it was inferred from patterns in nutrient loss or gain as the litter decomposed. The researchers report that when the nitrogen-fixing and fast-decomposing litter species are decomposed together, the amount of nitrogen present in one goes up while the amount in the other goes down, which is best explained by nitrogen moving between species. But it is possible that the nitrogen moving into the litter is coming from elsewhere, including the surrounding soil. Given that the predictive power of Handa and colleagues' results hinges on knowledge about underlying mechanisms, a key next step will be to confirm the implied mechanism with direct tests in the field. One possibility would be to perform experiments using litter with radiolabelled nitrogen in the field, which have so far been done only in the lab.
We are increasingly aware that understanding the impacts of biodiversity loss requires recognizing that not all species are at equal risk of extinction8. The identity of species lost has been shown to have large effects on plant productivity9 and decomposition3, which results in a range of potential outcomes for different extinction scenarios. Thus, being able to accurately predict the effects of species loss at a broader scale will require both generalizable rules linking certain types of species to the effects of their loss, and knowledge of the mechanisms that cause those effects.
Handa and colleagues' study brings us closer to that goal by providing three things: general patterns linking litter and detritivore diversity to decomposition; evidence for a mechanism driving that link; and evidence that these patterns and mechanism are the same regardless of whether they occur in an Arctic stream or a forest in the tropics. With this information in hand, we can strive towards linking patterns to realistic extinction scenarios, and thus predict probable outcomes of biodiversity loss.