Two sorts of biocrust community assemblies: one beneath the shrubs and the other outside their canopies

Ecologists always wonder about how the different species in a given ecosystem assemble, what makes some to be able to be there and others not, and how important are the environmental conditions regarding that of the interactions between the neighbours. This is an important question as it helps us to predict which species combinations “work” together, or what to expect if we modify the land use or if the environment becomes warmer and drier. In other words, it helps to foresee how our natural environment will look like in the future and which species are most in danger and require more active and larger efforts for their conservation. In this study, we saw that the factors determining how species assemble change depending on where you look at. If growing beneath the shade and protection of a shrub, communities are determined mainly by biotic interactions, whereas they are determined by how adapted they are to the environment otherwise. This dual mechanism of community assembly was suggested already by studies looking at plant-plant interactions, where the identity and the traits of the species living beneath shrubs are widely different to those of species growing outside, and the former are far less adapted to current environmental conditions than the later (e.g., Schöb et al. 2013). Taking together this line of evidence with some of our own studies reporting a shift in the way species interact depending on where they grow (Soliveres et al. 2011), we decided the time was ripe to test for the existence of this dual community assembly mechanism properly.

We did our study in Australian biocrusts. Biocrusts are communities formed by lichens, mosses, cyanobacteria and fungi which live associated to the soil surface in almost every place devoid of vascular plants. These organisms are very important for preventing soil erosion, fix atmospheric nitrogen and modify the carbon cycle and water movement in drylands. Indeed, they are considered the “living skin of Earth” by some passionate ecologists (Bowker et al. 2018). Their importance is perhaps best placed in context when one thinks on the heavy dust storms unleashed in drylands once biocrusts have been degraded by overgrazing or other activities. Thus, understanding how biocrusts react to environmental changes, will help us predicting which species will be more or less successful, and which would be the “properties” of the ecosystem (soil erosion, nutrients and water cycling, etc), under contrasting environmental or management scenarios.

We knew that biocrusts are sensitive to grazing, and that different biocrust species have contrasting preferences for sunny or shady places; and therefore react differently to the presence of vascular plants. Because of the latter, biocrusts may react quite strongly to the widespread increase of woody vegetation in many drylands across the planet. However, we did not know how these two main environmental changes occurring in drylands worldwide (overgrazing and woody encroachment) could affect, when acting in tandem, the species richness and composition of biocrust communities. Our main finding is that these environmental changes interact: when growing in open spaces, biocrusts are quite sensitive to grazing; however, beneath shrubs, biocrusts are mainly determined by how neighbouring species compete. Interestingly, biocrusts also have a huge variety of chemical components, which partly use to compete fiercely for available space (as tourists for a location for their towels in a crowded beach). This chemical variety seems to also determine how they compete, and we found that it makes it difficult for any given species to be able to beat all the rest. The latter type of competition (in which there is not any species that can dominate) is called intransitive competition, and there is a growing evidence that this may be more extended and general that once we thought.

Thus, our study shows how multiple facets of biocrusts diversity respond to two major environmental changes in drylands: overgrazing and shrub encroachment. It also adds further support to the extent of intransitive (i.e., non-hierarchical) competition in natural communities. More importantly, our work illustrates that trying to infer a single community assembly process in any community might be a mistake, since these assembly processes are heavily responsive to changes by keystone species (and perhaps to extreme climatic events or land use changes too, although this is another story). How general these dual mechanisms are, and whether or not can we draw general patterns regarding their response to extreme events, the interaction between global change drivers, or the loss of keystone species, we think, will help to better forecast the future of our natural communities.

Santiago Soliveres, co-author.

I am a Ramón y Cajal researcher currently working on the effects of global change on our ecosystems, mainly in drylands, and trying to understand the consequences of these changes on biodiversity and ecosystem functioning. Amongst the major global change factors, I am particularly interested in climate change, habitat fragmentation and pollution. I am currently working on trying to understand the interactions of these factors on plant and soil communities.  Most of my work with David Eldridge, and this one is no exception, is about the consequences of shrub encroachment and overgrazing in drylands, and in particular in Australia where these are major environmental issues (or at least they were before the catastrophic season of wildfires that is devastating the country).

I got involved in Ecology due to my curiosity to understand the world we live in, and because natural ecosystems are one of the most exciting, complicated and interesting “study problems” a scientist can face, and it´s lots of fun to try understanding them. The best thing of being an ecologist is to work in the thing you love with the people you like; the worst thing is the bureaucracy characterising Spanish academia. My only advice is to do the stuff that motivates and excites you, and the rest will come, I was never worried about the CV but about answering the questions that popped in my mind, and if you can help anyone on your way…do it! After work, I like to spend my time with a beer in my hand and friends next to me, hiking above or snorkelling below, or chasing cats and enjoying a walk with my wife. The paper I´m most proud of is one of Miguel Berdugo (Berdugo et al. 2017), a former PhD student and an excellent researcher. He opened a beautiful and very interesting research line by focusing on the non-linearity of the responses to drylands to increasing aridity. I´m really excited to see how this develops in the future!

Berdugo, M. et al. (2017). Plant spatial patterns identify alternative ecosystem multifunctionality states in global drylands. Nature ecol. Evol., 1: 0003.

Bowker, MA. et al. (2018). Biocrusts: the living skin of the earth. Plant & Soil, 429, 1-7.

Schöb, C., et al. (2013). Variability in functional traits mediates plant interactions along stress gradients. J. Ecol., 101, 753-762.

Soliveres, S. et al. (2011). Microhabitat amelioration and reduced competition among understorey plants as drivers of facilitation across environmental gradients: towards a unifying framework. Persp. Plant Ecol. Evol. Syst., 13, 247-258.