Can biological invasions induce desertification?

A common form of land degradation at the desert margins involves the rapid encroachment of woody plants into historic grasslands and the acceleration of soil erosion as a result of the loss of grass cover (Archer, 1989; Schlesinger et al., 1990). The conversion of perennial grasslands into desert scrublands has been documented worldwide (Van Auken, 2000; Ravi, 2008) and has important implications for regional and global biogeochemical cycles, climate, biodiversity and food security (Schlesinger et al., 1990; Knapp et al., 2008). However, this is not the only possible mechanism of land degradation driven by shifts in dryland plant community composition. Here, we present a new desertification paradigm that includes the invasion of stable desert shrublands by exotic annual grasses (Fig. 1). The mechanism we propose suggests that two major drivers of global environmental change, namely biological invasions and climate change, may act in concert and amplify each other’s effect on land cover and soil resources. Both shrub encroachment and exotic grass invasions affect the spatial distribution of soil resources (e.g. carbon, nitrogen and water) through altering fire regime and soil-erosion rates. The conversion of native perennial grasslands into shrublands increases soil resource heterogeneity as a result of hydrological and aeolian transport processes that cause significant resource redistribution (Schlesinger et al., 1990). Our studies in the shrub–native grass ecotone in the northern Chihuahuan desert have shown that fires can counteract the formation of this heterogeneity, in that the interaction of fires with soil erosion favors a more homogeneous distribution of soil resources (Ravi, 2008; Ravi & D’Odorico, 2008). Through a nearidentical mechanism, invasion by exotic annual grasses can increase fire frequency, shrub mortality and soil loss, thereby destroying the heterogeneity of resources typical of desert shrublands and favoring the conversion into exotic (annual) grasslands. However, the long-term persistence of invasive grass cover may be restricted as a result of low-frequency recurrent droughts which displace these plant species that have not evolved in this regional climatic context (Salo, 1994; Martin-R et al., 1995). Drought-induced loss of vegetation cover is expected to be followed by even higher erosion rates and losses of soil resources. Thus, the process of degradation can be facilitated by the heterogeneity (shrub encroachment) and homogeneity (annual grass invasion) of resources, depending on the plant functional type inducing the change in resource distribution (Fig. 1). In the desert shrublands of North America, exotic grass invasion is a major environmental issue, in that these grasses are found to increase connectivity between shrub patches, triggering periodic fires in systems without a fire history (D’Antonio & Vitousek, 1992; Brooks & Pyke, 2001). Interannual variability of precipitation, a common feature of arid landscapes, is thought to be a key factor determining the dynamics of invasion by annual grasses. In water-limited systems, invasive annuals initially benefit from a nurse plant effect, in that they establish on the resource islands existing beneath the shrubs and can reach high densities relative to native species during favorable years (Brooks & Pyke, 2001). In years of high resource availability, the invasive grasses are able to out-compete native species (Martin-R et al., 1995; Davis et al., 2000; Arriaga et al., 2004). Furthermore, in these preferential sites, invasives find higher soil water contents that may allow them to survive short periods of low rainfall in greater densities than natives. Many invasive grasses are found to demonstrate a higher phenotypic plasticity (i.e. the ability to alter their morphology and physiological processes in response to environmental changes) than native grasses (Funk, 2008), even in low-resource environments, which allows them to survive in unfavorable environmental conditions (and sites). Together, these features probably allow invasive grasses to consistently maintain larger densities associated with their developing populations compared with native grasses. Any decrease in the native species’ composition and in the functional type composition could lead to a further increase in susceptibility of the ecosystem to invasion by exotic species (Zavaleta & Hulvey, 2004). In years of high precipitation, invasive grasses can spread into the interspaces, thereby establishing connectivity either between shrub islands or between shrubs and the sparse cover of native perennial grasses in the interspaces. Grass connectivity provides conditions favorable for the spread of fires typically induced by lightning in the summer months. It has been found that the effect of fires on soil properties results in the enhancement of soil erodibility. Our studies on fire-affected grasslands and shrublands in the southwestern USA have shown that fires increase wind-erosion rates in burned areas and that the increase in soil-erosion rates was considerably higher for shrublands than for grasslands (Ravi et al., 2007; Ravi, 2008). The erodibilty of soils under burned shrub patches increased considerably after fire, whereas this postfire change in soil-erosion rates was negligible under grasses and bare interspaces. This enhancement of erosion processes was