Rapid decline of a grassland system and its ecological and conservation implications

Abstract

One of the most important conservation issues in ecology is the imperiled state of grassland ecosystems worldwide due to land conversion, desertification, and the loss of native populations and species. The Janos region of northwestern Mexico maintains one of the largest remaining black-tailed prairie dog (Cynomys ludovicianus) colony complexes in North America and supports a high diversity of threatened and endangered species. Yet, cattle grazing, agriculture, and drought have greatly impacted the region. We evaluated the impact of human activities on the Janos grasslands, comparing changes in the vertebrate community over the last two decades. Our results reveal profound, rapid changes in the Janos grassland community, demonstrating large declines in vertebrate abundance across all taxonomic groups. We also found that the 55,000 ha prairie dog colony complex has declined by 73% since 1988. The prairie dog complex has become increasingly fragmented, and their densities have shown a precipitous decline over the years, from an average density of 25 per ha in 1988 to 2 per ha in 2004. We demonstrated that prairie dogs strongly suppressed woody plant encroachment as well as created open grassland habitat by clearing woody vegetation, and found rapid invasion of shrubland once the prairie dogs disappeared from the grasslands. Comparison of grasslands and shrublands showed markedly different species compositions, with species richness being greatest when both habitats were considered together. Our data demonstrate the rapid decline of a grassland ecosystem, and documents the dramatic loss in biodiversity over a very short time period concomitant with anthropogenic grassland degradation and the decline of a keystone species.

Figure 1. Location of the Janos region showing current land uses and natural vegetation cover in 2005.

The Janos region covers 1 millon hectares in northern Mexico, bordering the United States. The main plant communities are grasslands, shrublands, and temperate forests. This region still maintains one of the largest prairie dog complexes in the world, and the Janos Biosphere Reserve has been designated to protect this biologically important region.

Figure 2. Changes in the El Cuervo prairie dog colony in 2002 (left), and of the same area in 2006 (right).

These photos show the rapid loss of prairie dogs within the largest colony of the Janos grasslands, following two decades of intensive land use and drought. Note the sparse coverage of annual grasses and forbs and the lack of perennial grasses, which is characteristic of degraded grasslands in Janos. These plants are only available during the rainy season and most of the year the area is bare ground.

Figure 3. Change in area of the Janos-Casas Grandes prairie dog complex from 1988 to 2005.

Extent of prairie dog colonies in 1988 and 2005, and location of the sampling sites 1992–2004: pitfall trap grids and transects for herpetofauna, point count transects for birds, Sherman trap grids for small mammals, and spotlighting transects for medium and large mammals (see Table A3 for specific sampling periods for each group).

Figure 4. Effect of prairie dog presence/absence on maintenance of the grassland ecosystem.

A) Prairie dog colony in Janos, Chihuahua, Mexico, note the absence of woody plants. B) Prairie dog expansion into ephedra shrubland by physically damaging the invading shrubs. Visible in the picture is the extensive browsing of shrubs, and several burrows at the base of the shrub which exposes the roots (see also map in Fig. 2D). C) Polygon of the southern portion of the prairie dog colony of Los Ratones, which was covered by grassland in 1988, showing a 43% advance of honey mesquite (Prosopis grandulosa) and ephedra (Ephedra trifurca) shrubland after the colony was poisoned between 1988 and 1990. D) Sixteen percent expansion of the La Báscula prairie dog colony into ephedra shrubland between 2000 and 2005.

Figure 5. Vertebrate species richness in grassland and shrubland habitats.

Total number of shared and unique species of all vertebrate groups combined, birds, small mammals (including lagomorphs), herpetofauna, and carnivores on the grassland and shrubland habitats over all sample periods (1994, 1995, 1996, 2000, 2001, 2002, 2003, and 2004; see Table A3 for specific sampling periods for each group).

Figure 6. Total abundance of vertebrate species in grassland and shrubland habitats.

Total abundance of herpetofauna, birds, small mammals, rabbits, and carnivores on the prairie dog grassland and shrubland habitats over all sample periods (1994, 1995, 1996, 2000, 2001, 2002; N = 8 for each sample period). Asterisks (*) indicate significant differences in abundance between the habiats at P<0.01.

Figure 7. Differences in species composition between grassland and shrubland habitats.

Detrended Correspondence Analysis (DCA) ordinations based on species composition of herpetofauna, birds, and small mammals. Multi-Response Permutation Procedure (MRPP) demonstrates that the species compositions of these vertebrate groups is significantly different between the prairie dog grasslands and the shrublands (1994, 1995, 1996, 2000, 2001, 2002; N = 8 for each sample period).

Figure 8. Relative abundance of vertebrate species in grassland and shrubland habitats.

Relative abundance of herpetofauna, bird, and small mammal species on the prairie dog colony grasslands and the shrublands over all sample periods (1994, 1995, 1996, 2000, 2001, 2002; N = 8 for each sample period). Asterisks (*) indicate significant differences in abundance between the habitats at P<0.05.

Figure 9. Decline in vertebrate species density over a ca. 10-year period (1992–2004).

Mammal and bird species in the Janos prairie dog grasslands showing dramatic declines in densities over time. (Note prairie dog densities are compared from 1994–2004.) Of the 33 bird species that were sampled, only those that exhibited a 2-fold or greater change over time are shown here.