Impact of Grazing and Desertification in the Chihuahuan Desert: Plant Communities, Granivores and Granivory.(Statistical Data Included)

GRAHAM I. H. KERLEY [1]

WALTER G. WHITFORD [2]

ABSTRACT.--Livestock effects on plant communities through overgrazing (desertification) should affect the structure and functioning of semarid rangeland communities. We measured plant, granivorous ant and rodent communities and rates of seed removal by rodents and ants in grazed (by livestock) and ungrazed desert grasslands as well as mesquite and creosotebush shrublands to test hypotheses on the effects of grazing and desertification on ecosystem structure and functioning. In desert grasslands grazing reduced the cover of perennial grasses, particularly the dominant Bouteloua eriopoda, but the cover of forbs and shrubs did not differ between treatments. One species of perennial grass, Dasyochloa pulchellum, increased in grazed grasslands compared with grassland exclosures. Detrended correspondence analysis showed that grazing caused desert grasslands to shift in community structure towards the shrublands. There were more seed harvesting ant and rodent species in the creosotebush shrublands than in the grass lands and mesquite shrublands. Grazing had no effect on the diversity of ants or rodents within grasslands, and detrended correspondence analysis revealed no clear trends in granivorous ant community structure in the grazed and ungrazed grasslands or the mesquite and creosotebush shrublands. Ants removed more seeds than did rodents in the grassland sites but rodents removed more seeds than did ants in the creosotebush sites and seed removal rates by rodents and ants were the same in the mesquite sites, Our data support the hypothesis that livestock grazing leads to a shift from grassland to shrubland in the Chihuahuan Desert, with associated changes in the structure and functioning of faunal communities. Because grasslands support few species and low densities of rodents, seed harvesting ants are the most important granivores in these desert grasslands. On a larger scale, we therefore hypothesize that the observed dominance of rodents as seed harvesters in the Chihuahuan desert is a function of the desertific ation of desert grasslands to shrublands by livestock, and that associated feedback effects may complicate the regeneration of degraded communities.

INTRODUCTION

Degradation of desert grasslands to less productive shrublands (desertification) has encompassed a large portion of the landscape in the Chihuahuan Desert over the past 150 y and has been correlated with the introduction of domestic herbivores, chiefly cattle (Buffington and Herbel, 1965; Bahre, 1995). While the rate of vegetation change has slowed considerably, the shift from grassland to shrubland continues. Shrub dominated ecosystems are very resistant to climate stress (drought) and to attempts to restore the shrublands to grassland (Whitford et at., 1995; Roundy and Beidenbinder, 1995).

Documentation of desertification of Chihuahuan Desert grasslands has focused on changes in the composition and cover of the vegetation and on soil movement (Hennessy et at., 1983; Gibbens et al, 1983; 1992; Gibbens and Beck, 1988; Bahr, 1995). The consequences of the changes in vegetation and soils on other components and processes within the desertified ecosystems have received little attention. The few studies that have examined changes in the fauna attributable to livestock grazing have described changes in species composition and abundances of rodents, ants or birds (Grant et at., 1982; Bock et at., 1984; Heske and Campbell, 1991) with few attempts to investigate changes in ecological processes. In addition, many of these studies suffer from either a lack of replication or pseudoreplication, making interpretation of often contradictory findings difficult.

Granivory has been intensively studied in desert ecosystems as a range of taxa rely on seeds as resources, and the consequences of granivory have been shown to affect both plant communities and the coexistence of the granivorous taxa (Brown et at., 1979). Studies of granivory have focused on competition among rodents and ants and the importance of body size differences in structuring the granivorous ant and rodent communities (Brown et at., 1979). In North American desert shrublands, seed removal experiments have shown that rodents take more seeds than ants (Brown et al., 1975; Mares and Rosenzweig, 1978). Long-term studies manipulating granivorous ants and rodents have shown that the exclusion of these taxa results in changes in plant communities (Brown and Munger, 1985; Heske et at., 1993), indicating that granivory is important in structuring desert plant communities. Despite the large literature on granivory, few studies have examined the relative impacts of seed-eating ants and rodents in desert grassla nd habitats or the implications of desertification for this ecologically important process.

We investigated the effects of vegetation changes due to livestock grazing and effects of historical changes in vegetation on the granivorous rodent and ant communities and on the process of granivory. Our study focused on a set of grazed and ungrazed sites in an area that is a desert grassland with relatively low cover of shrubs. We compared the data from these sites to two different sets of shrubland sites which have been in the shrubdominated state for more than 50 y, but were historically grasslands (Buffington and Herbel, 1965). These shrub-dominated sites are considered to be alternate stable endpoints of desertification of Chihuahuan Desert rangelands (Whitford, 1995).

We hypothesized that vegetation cover and composition changes would result in changes in abundance and species composition of the granivorous rodent and ant communities. We tested our prediction that this would change the rates of seed removal by ants and rodents in the different habitats.

STUDY SITE

Study sites were located on the Chihuahuan Desert Rangeland Research Center and the adjacent U.S. Department of Agriculture Jornada Experimental Range, 40 km north-west of Las Cruces, NM. This region is part of the northern Chihuahuan Desert. Annual precipitation …