Sudden shifts in ecosystems

The field of sudden shifts in ecosystems is blooming and challenging the classical theory assumption that nature responds in a smooth way to gradual change. Theoretical developments and empirical analyses and observations have provided evidence that small increases in human and climatic pressure may trigger sudden shifts between healthy and degraded ecosystem states once a threshold has been surpassed (Scheffer et al. 2001, Rietkerk et al. 2004, Schröder et al. 2005, Bestelmeyer et al. 2011). This implies the possibility for rapid, large, and not easy to reverse ecological and economic losses and explains the effort that is being dedicated over the past few years to finding early warning signals (Dakos et al. 2012, Kéfi et al. 2014). Sudden shifts affect a wide range of aquatic and terrestrial ecosystems (Scheffer and Carpenter 2003, Rietkerk et al. 2004) but knowledge advance has been less developed for terrestrial systems, including drylands, where the factors and mechanisms, pressures, and interactions that control and drive sudden shifts remain largely unknown.

Potential for sudden shifts in drylands

Drylands cover 41% of the Earth’s surface and provide ecosystem services to over 38% of the global population including some of the poorest and most vulnerable on the planet (Millennium Ecosystem Assessment 2005). Currently, 10% to 20% of the world drylands are already degraded, and ongoing population growth and climate change are expected to exacerbate desertification risk (Millennium Ecosystem Assessment 2005). Modelling experiments suggest that drylands may experience sudden shifts from functional to degraded states in response to smooth increases in human and climatic pressures in these areas such as grazing and drought (Rietkerk et al. 2004, Kéfi et al. 2007). These shifts are hypothesized to be governed by plant-plant interactions and (ecohydrological) feedbacks between vegetation and water redistribution, which despite being widely accepted are still poorly understood and quantified by empirical data. CASCADE aims to empirically investigate the main mechanisms and processes involved in some of these ecohydrological feedbacks, specifically those between plant spatial pattern, water redistribution, and plant functioning, and how they may be modulated by plant diversity.

Ecohydrological feedbacks and sudden shifts in drylands

Ecohydrological observations in dryland communities have suggested that a decrease in the cover and size of vegetation patches increases the hydrological connectivity of bare-soil areas (i.e. runoff-source areas) and the global losses of water and nutrients from the system, which may in turn reduce plant productivity and further decrease plant cover, completing a positive degradation feedback (Bautista et al. 2007, Mayor et al. 2008, Turnbull et al. 2010, García-Fayos et al. 2010, Bestelmeyer et al. 2011) (left feedback loop in the figure below). In contrast, at the plant patch scale, the coarsening of plant pattern (i.e., increasing of bare-soil connectivity) would result in a relative increase in resource inputs to the individual patches, which may increase patch productivity and growth, which in turn increases plant cover and decreases bare-soil connectivity (Puigdefábregas and Sánchez 1996, Ludwig et al. 2005, Urgeghe et al. 2010) (right feedback loop in the figure below). Despite the interplay between these opposite feedbacks is increasingly recognized as a critical mechanism underlying dryland functioning and potential for sudden shifts (Ludwig et al. 2005, Turnbull et al. 2012, Mayor et al 2013), the quantification of their relative strength and of their individual or combined impacts in dryland functioning is currently lacking. Further, among a number of critical factors, plant diversity is expected to modulate these feedbacks (Bautista et al. 2007, D’Odorico et al. 2012), but this role is still largely unknown.

Assessing dryland ecohydrological feedbacks

In order to disentangle the various components of the ecohydrological feedbacks that relate plant pattern, resource availability and productivity in drylands, as well as the independent role of critical factors that control these feedbacks, CASCADE has followed a fully manipulative experimental approach combined with field observations.

»Experimental approaches to the analysis sudden shifts gives an overview of the experimental approaches and a summary of the results

The three experiments are described in detail in

»Spatial pattern and resource distribution in drylands

»Feedbacks between plant pattern and resource conservation: a mesocosm experiment

»Patch-scale effect of resource redistribution on plant performance

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