Biodiversity at multiple trophic levels is important for maintaining ecosystem functions and services. However, we lack knowledge on the interactions among these three facets of ecosystems. In addition, land-use intensification might affect these interactions by disrupting certain links or enhancing others. Therefore, understanding how land use intensity affects biodiversity and how this will affect ecosystem functioning and services is an important question to address if we are to manage landscapes to maximize both biodiversity conservation and ecosystem services supply.
Using data on 300 grassland and forest plots distributed across a gradient of land use intensity, we compare positive and negative correlation networks and identify the key trophic groups important for ecosystem functions and the ecosystem services most dependent on the maintenance of ecosystem functions and biodiversity.
We hypothesize that positive correlation networks can be considered as mutualistic networks and therefore, should be highly connected, nested and robust. On the other hand, negative correlation networks should behave as antagonistic and have a modular structure. In addition, we expect that increasing land use intensification will affect network properties by reducing connectance and shifting from a nested to a modular structure.
Results/Conclusions
We found that the number of nodes, links, and the density of both forest and grassland networks decrease with land use intensity in positive correlation networks, while the pattern is the opposite in negative correlation networks, confirming our hypotheses.
However, we found opposite mechanisms behind the correlation networks in each habitat. In grasslands, we observed a bottom-up control, driven by soil fungi pathogens in positive correlation networks and by symbionts in negative correlation networks. We also detected the key role of carbon and enzymatic activities in relation to ecosystem functions and services. In forest, we observed a top-down control, driven by secondary consumers and omnivores in both positive and negative correlation networks. We also identified the importance of available phosphorus and the trade-off or dependence relationship of the forest charismatic plants with other functions and services.
Our work provides the first results applying correlation networks to disentangle the relationships between biodiversity-functions-services linkages. We show that this novel approach is useful to unravel the role of each component of the system, detect thresholds of land-use intensity before a compositional shift, and identify keystone groups or functions that should be maintained in order to supply ecosystem services.