In many desert systems, the physical activity of animals dramatically alters soil ecological processes at a range of spatial scales (Huntley & Inouye 1988). This disturbance by animals (biopedturbation) is a potential driver of ecosystem processes in deserts and may be equally important as abiotic processes (Lavelle 1997; Whitford 2002). Soil disturbance results when animals ‘engineer’ the soil surface while foraging or constructing burrows, nests or resting sites. Although there are many studies of the small-scale effects of ephemeral structures such as foraging pits and seed caches on ecosystem processes (e.g. Dean & Milton 1991; Longland 1995; James, Eldridge & Hill 2009), few studies have addressed the functional role of disturbances across broad scales such as communities or landscapes.
Animal structures that are relatively large in relation to the engineer who created them and persistent over decadal time scales, have a marked influence on plant and soil processes operating at landscape scales. For example, decomposing seed and organic matter within the larder hoards of banner-tailed kangaroo rat (Dipodomys spectabilis) mounds (Mun & Whitford 1990) form patches of higher soil nitrogen, which are preferred sites for desert annuals (Krogh et al. 2002). Their tunnels are also sites of greater breakdown of faeces and urine, support diverse desert fungi and bacteria (Hawkins 1996; Whitford & Kay 1999) and develop into nutrient-rich patches. Conversely, while the ejecta mounds (mounds formed from excavated soil material) of American badgers (Taxidea taxus) are nutrient-poor, the soil surrounding the mounds has greater moisture due to enhanced run-off from the mounds (Eldridge 2009), supporting a diverse plant community (Platt 1975).
In the Chihuahuan desert of north-western Mexico and south-western United States, substantial increases in the extent of woody plants such as mesquite (Prosopis glandulosa) and creosote bush (Larrea tridentata) have largely been responsible for the ‘desertification’ of extensive areas of former black grama (Bouteloua eriopoda) grasslands into dense shrublands (Buffington & Herbel 1965; Gibbens et al. 2005). Apart from widespread soil loss, desertification is accompanied by altered soil chemistry, particularly changes in the spatial distribution of nutrients resulting from the replacement of resource-rich closely spaced grass tussocks with resource-rich, widely spaced shrub islands (Schlesinger et al. 1996). Although some communities such as mesquite coppice dunes are severely degraded, others such as tobosa grass (Pleuraphis mutica) playa remain virtually unchanged.
Desertification affects animal populations by changing habitat characteristics and/or resource availability. Examples include increased foraging efficiency of Myrmecocystus ants that forage on homopteran-produced honeydew in mesquite-dominated shrublands (Forbes et al. 2005), or the loss of banner-tailed kangaroo rats from eroded creosote bush shrubland due to extensive erosion and loss of grasses (Kerley, Whitford & Kay 1997; Schooley, Bestelmeyer & Kelly 2000). Desertification-induced changes in animal populations can even induce feedback processes that tend to maintain or reinforce the desertified state. Such situations occur when rodents preferentially remove grass tillers from desertified grasslands, preventing a return to an intact grassland (e.g. Kerley, Whitford & Kay 1997; Bestelmeyer, Khalil & Peters 2007). Changes in animal populations will invariably result in changes in the density or size of their structures, affecting soil physical and chemical properties and processes, and altering plant community structure by changing annual plant communities on and off their structures (Mun & Whitford 1990). Changes in the density of animal-produced structures may be more influential and persistent than changes in the number of animals, given the greater relative importance of engineering effects over trophic-level effects such as herbivory in resource-limited environments (Wilby & Shachak 2004).
Given the likely effects of such widespread changes in plant community structure associated with desertification, we would expect desertified grasslands to be characterized by a markedly different community of soil-disturbing animals, and therefore a different spectrum of soil disturbances, compared with intact, undesertified grassland. We would also expect that the impacts of desertification on soil-disturbing animals would have flow-on effects on desert plant communities. Further, the loss of functional desert grasslands may be partly attributable to reductions in, or loss of, ecosystem services provided by these animals (Whitford 2002).
Our study predicts therefore that a change from intact grasslands to shrub-dominated ecosystems (desertification) will be associated with a change in composition and a reduction in diversity and abundance of structures used by successive generations of soil-disturbing animals. While it is generally acknowledged that animal activity is critical for the functioning of arid ecosystems (e.g. Jones, Lawton & Shachak 1997), we are unaware of any empirically based studies that have sought to test this general proposition among a range of communities, organisms and trophic groups. Our discussion here focuses on longer-lived disturbances across six vegetation communities in two desertification states (desertified, undesertified). Specifically, we examine (i) changes in the spectrum of animal structures in relation to desertification state, (ii) the effects of mounds of banner-tailed kangaroo rats on creosote bush shrubs in former black grama grassland, and (iii) effects of a range of ant species on soil chemistry and the ensuing effects on plant communities. We contend that changes in the populations of soil-disturbing animals alter the degree and nature of their disturbances, and thus have dramatic effects on desert plant communities. Using these examples we argue that the loss of desert animals and their attendant functions are key mechanisms for reinforcing the maintenance of desertified grasslands.