Climate change could lead to dramatic shifts in dryland ecosystems
Drylands cover about 41% of the Earth's land surface and host one in three humans inhabiting our planet. In these areas, life is highly conditioned by aridity, the balance between rain and evaporation of water from soil, plants and other surfaces.
A study by Dr Maestre of the University of Alicante and team has revealed for the first time that as aridity increases, dryland ecosystems undergo a series of abrupt changes. These changes happen in a non-linear fashion, and drastic shifts in essential ecosystem attributes occur. When certain aridity values are reached, the soil loses its fertility and eventually the entire ecosystem collapses, with dire consequences for local populations who directly depend on agriculture, cattle herding and wood harvesting for their livelihood.
More than 20% of global lands affected by 2100
‘One may expect that as aridity increases dryland ecosystems will gradually become less green and fertile, heading towards a more desert-like landscape,’ says Dr. Maestre. ‘However, there is another more concerning scenario, which is that these changes may not occur gradually, but drastically when certainly aridity levels or thresholds are crossed’.
According to climate forecasts, more than 20% of the emerged lands of the planet may cross one or several of the thresholds identified in this study by 2100 due to climate change. ‘Life will not disappear from drylands with forecasted aridity increases, but our findings suggest that their ecosystems may experience abrupt changes that will reduce their capacity to provide ecosystem services to more than 2 billion people, such as soil fertility and biomass production,' says Dr Miguel Berdugo, a postdoctoral researcher working at Dr Maestre´s lab and the lead author of the study.
Three phases in the cycle of change
The study identified three phases of accelerated ecosystem change in response to increases in aridity. First, when aridity levels cross values of around 0.5, ‘the ecosystem becomes limited by the lack of water. The vegetation changes and it becomes dominated by species adapted to more arid conditions and to drought, such as grasses and shrubs’, says Dr Maestre.
After these initial vegetation changes, when aridity values exceed 0.7, the soil becomes much less fertile. It loses its structure and becomes more vulnerable to erosion. Moreover, soil organisms that play essential roles in maintaining ecosystem functioning are radically affected, with large increases in the presence of pathogens at the expense of more beneficial organisms.
Finally, beyond aridity values of 0.8, an abrupt loss of diversity and plant cover take place. ‘The system collapses and becomes a desert. Plants cannot thrive in these conditions and can only survive taking profit of the ephemeral windows of opportunity that occur whenever it rains’, says Dr Maestre.
Minimising negative consequences
The findings of this study are of great relevance for understanding the impacts of climate change on dryland ecosystems. They could help establish appropriate mitigation actions in response to forecasted increases in aridity in drylands across the globe.
‘While we will not stop climate change, I believe we still can minimize its negative consequences on dryland ecosystems, which are essential to sustainable development’, says Dr Maestre. ‘By providing information on how multiple vegetation and soil properties change as aridity increases, and by mapping those areas most sensible to such increases, our results can be used to optimise monitoring and restoration efforts in drylands worldwide, to preserve their unique biodiversity and to hopefully avoid its desertification.
Read the full text article "Global ecosystem thresholds driven by aridity" published in Science 14 Feb 2020: Vol. 367, Issue 6479, pp. 787-790 DOI: 10.1126/science.aay5958
Further reading from UNCCD Library: drylands ; arid* : lands, areas, climate, environment, regions, zones, management