A soil ecological perspective to trophic rewilding
Walter S. Andriuzzi
Department of Biology and School of Global Environmental Sustainability
Colorado State University
We all know that our planet has become less wild. Agriculture, forestry, mining, urbanisation have all turned natural ecosystems into places often poor in biodiversity. Through hunting and fishing, whether for subsistence or sport, we have made many species rare, and others extinct. However, in some parts of the world projects have been undertaken to reverse this process and “rewild” our planet. Rewilding is the introduction of animal species that are missing from an ecosystem. Trophic rewilding means that the aim is to restore the trophic interactions in that ecosystem, that is, to return the food web to its original state by restoring populations of large herbivores and/or top predators (for instance, elk and wolves), or of species that are known to have cascading effects on the food web, such as beavers. In most cases it is a re-introduction of species that disappeared in more or less recent times because of human activities (such as hunting, habitat loss, and so on), but there are also ambitious (and controversial) proposals to introduce species that have never been in the ecosystem in question, at least not during the historical record. Individuals and organisations advocating rewilding have started to influence policy in various countries, and many ecologists are therefore devoting their careers on finding out whether rewilding works and predicting its costs and side-effects.
Given that the goal of trophic rewilding is to restore the functionality of wild habitats prior to human intervention, one would expect that its proponents strive to have an encompassing view of the ecosystems they seek to restore. Such a view would take into account the potential interactions between the (re)introduced beasts and the other species already in the system; not merely the plants that the elk eat or the small mammals and birds that the wolves may prey on, say, but also the microbes, the invertebrates, and so forth – including of course soil biodiversity. Alas, this has not been the case so far. In our invited review on soil biological responses to rewilding – part of a thematic issue on rewilding just published in the journal Philosophical Transactions of the Royal Society B – we found that soil organisms and processes have been rather tangential to the scientific debate on trophic rewilding. More precisely, there is some research on the impact of rewilding on belowground processes and properties that are directly relatable to soil fertility and plant growth (for instance nutrient content, mineralization); there is a little research that is more or less directly relevant to the impact of rewilding on some soil organisms (for instance mycorrhizal fungi, dung beetles); and there is practically no research on the impact of rewilding on most soil fauna and microbes, although some informed guesses can be made based on available knowledge of how vertebrate herbivores affect soil communities (which we discussed previously on this blog). To put it bluntly, trophic rewilding has largely forgotten about soils. We are happy to report that this has started to change, at least as can be gathered from the other contributions to the journal issue: for example, Cromsigt and co-authors explicitly consider the role of soil processes in mediating the possible impact of rewilding on climate change, and Van Klink and co-authors focus on the possible effects on the arthropods, many of which are soil or litter fauna. This being said, there is clearly a need for a great deal more research on the subject. Since soil communities are hugely important in ecosystem functioning, how can we predict the real impact of trophic rewilding if we don’t know how the soil microbes and fauna will respond?
This is particularly crucial because soil organisms will not just passively react to rewilding. Their ecological responses will feedback on the plants and large animals aboveground, and could potentially facilitate rewilding, or cancel its benefits. One specific example we discuss in our review is whether trophic rewilding in sub-arctic ecosystems will help fighting climate change. Some ecologists argue that the introduction of large herbivores in those regions will push the ecosystem to trap and store more CO2, thereby acting as a C sink. This would make trophic rewilding not only a way to restore biodiversity and wild nature, but also a tool to mitigate climate change. Other ecologists are more cautious, on the ground that there are too many uncertainties on how the ecosystem will react to rewilding. While we don’t aim at “picking sides” in this debate, our review certainly goes more in the latter direction. Firstly, knowledge on how rewilding may impact carbon cycling is still sketchy, partly because the effects on the organisms which together with plants drive carbon cycling – soil microbes and fauna – have not been studied enough. Secondly, some existing studies actually suggest that re-introducing large mammals to cold climates may lead to less carbon storage, for instance due to enhanced soil respiration and priming of soil organic matter decomposition.
On the other hand, in our review we also discuss how the responses of soil organisms could also facilitate trophic rewilding. For example, soil invertebrates that act as ecosystem engineers, such as termites, earthworms and dung beetles, help the formation of particularly fertile patches of vegetation, and this in turn influences how herbivores use the landscape and contributes to regulating their populations. We indeed suggest that, parallel to re-introducing big animals, the restoration of soil organisms, particularly keystone ones such as ecosystem engineers and mycorrhizal fungi could be an asset to rewilding projects. In short, there are both caveats and unexplored synergies which a soil-focused perspective can bring to trophic rewilding, and we hope that our critical review will push experts in the field to engage more with soil ecologists and vice versa.