Modification of Natural Processes and Ecological Drivers
Changes in natural processes and ecological drivers (e.g., drought, fire management, ecological sustainability and integrity, or loss of keystone species) have influenced all habitats in New Mexico and the Southwest. However, some habitats are more resilient or resistant to these modifications. Aquatic systems, especially ephemeral habitats, may be considerably altered by drought conditions. Other ecosystems may have the ability to maintain or rebound to conditions of diversity, integrity, and sustainable ecological processes following such disturbances.
Climate Change and Drought
Climate change may occur in the Southwest from increased atmospheric concentrations of COB2B and other "greenhouse" gases. Effects may include increased surface temperatures, changes in the amount, seasonality, and distribution of precipitation, more frequent climatic extremes, and a greater variability in climate patterns. Such changes may affect vegetation at the individual, population, or community level and precipitate changes in ecosystem function and structure (Weltzin and McPherson 1995). They will likely affect competitive interactions between plant and animal species currently coexisting under equilibrium conditions (Ehleringer et al. 1991).
Plants respond differently to changes in atmospheric gases, temperature and soil moisture, in part based on their CB3 Bor CB4B photosynthetic pathways (Bazzaz and Carlson 1984, Patterson and Flint 1990, Johnson et al. 1993). For example, increases in winter precipitation favor tree establishment and growth at the expense of grasses. Increases in temperature and summer precipitation favor grasslands expanding into woodlands (Bolin et al. 1986).
Drought (an extended period of abnormally dry weather) is one of the principal factors limiting seedling establishment and productivity (Schulze et al. 1987, Osmond et al. 1987). Soil moisture gradients are directly altered by drought conditions. The distribution and vigor of some plant communities may be controlled primarily by soil moisture gradients (Griffin 1977, Pigott and Pigott 1993). Drought and climate change can potentially have a substantial effect on New Mexico's habitats.
For thousands of years, wildfires have been an integral process in New Mexico and southwestern forest and grassland ecosystems. Prior to 1900, naturally occurring wildfires were widespread in all western forests at all elevations (Swetnam 1990). From an ecological perspective, fire may be the most important disturbance process for many western forests (Hessburg and Agee 2003). Ecosystem processes and patterns are influenced and shaped by fire. These include soil productivity and nutrient cycling, seedling germination and establishment, plant growth patterns, vegetative plant community composition and structure, and plant mortality rates (Beschta et al. 2004).
Tree-ring and fire-scar data for the Southwest indicate that past fires were frequent and widespread (with an elevation range of variability) at least since AD 1700 (Swetnam and Baisan 1996). Within ponderosa pine and lower mixed-conifer forests and woodlands in New Mexico, naturally-occurring wildfires were frequently of low-intensity and helped maintain stands of older trees with an open, park-like structure (Moir and Dieterich 1988). Higher elevation, mixed conifer and spruce-fir forests (wetter forest types) exhibited less frequent fire return intervals and fires were generally stand-replacing fires of higher intensity, (Pyne 1984, Walstad et al. 1990, Agee 1993).
The extent to which fire occurred in southwestern grasslands varied geographically and is related to climatic variables such as seasonal and annual rainfall and physiographic variables such as elevation, slope and aspect (Archer 1994). Fire may have been rare in desert grasslands and limited in extent due to low biomass and a lack of continuity in fine fuels (Hastings and Turner 1965, York and Dick-Peddie 1969). In more mesic grassland and savanna systems where fire was a prevalent and recurring force, pre-historic frequency and intensity appear to have been regionally synchronized by climatic conditions (Swetnam and Betancourt 1990).
The elimination of high-frequency, low-intensity wildfires across New Mexico and the Southwest coincided with the reduction and/or elimination of fine herbaceous fuels caused by improper grazing practices (Savage and Swetnam 1990, Swetnam 1990, Swetnam and Baisan 1996). These grazing practices further reduced grass competition, thereby increasing tree and shrub establishment (Archer 1994, Gottfried et al. 1995), which further altered natural fire cycles. Since the early 1900s, systematic fire suppression efforts have further curtailed the natural fire regimes that historically kept ponderosa pine, mixed conifer and spruce-fir stand densities and fuel loads relatively low. Fire suppression allowed the development of ladder fuels and the accumulation of heavy fuel loads. Catastrophic, stand replacing crown fires are now the standard, rather than the exception as a result of these changes (Covington and Moore 1994).
Land management practices and fire suppression have had adverse effects on many New Mexico habitats through fragmenting, simplifying, or destroying habitats, and greatly modifying disturbance regimes (McIntosh et al. 1994, Hessburg and Agee 2003). These human-caused changes have created conditions that are outside of the evolutionary and ecological tolerance limits of native species (Beschta et al. 2004). Cumulatively, these practices have altered ecosystems to the point where local and regional extirpation of sensitive species is increasingly common (Rieman et al. 1997, Thurow et al. 1997). As a result, the integrity of many terrestrial and aquatic ecosystems has been severely degraded at the population, community, and species levels of biological organization (Nehlsen et al. 1991, Frissell 1993).
Ecological Sustainability and Integrity
When biotic and abiotic disturbances are modified or removed from New Mexico's ecosystems, plant and animal diversity and ecological sustainability are lost (Benedict et al. 1996). Ecological sustainability is essentially the maintenance (or restoration) of the composition, structure, and processes of the ecosystem over time and space (US Forest Service 2000). Likewise, ecosystem integrity incorporates the concept of functioning and resilience. It includes: 1) maintaining viable populations, 2) preserving ecosystem representation, 3) maintaining ecological processes, 4) protecting evolutionary potential, and 5) accommodating human use (Grumbine 1994). The loss of ecological sustainability and integrity will thus affect species that are closely tied to specific habitats or ecosystems.
Loss of Keystone Species
Keystone species, such as beavers (Castor canadensis), bison (Bison bison), and prairie dogs (Cynomys sp.), are species that have a large overall effect, disproportionate to their abundance, on the structure or function of habitat types or ecosystems. If a keystone species is extirpated from a system, other species that are closely associated with the keystone species will also disappear. In New Mexico, several keystone species have either been completely removed or have experienced significant population reductions in their historic range. With their removal or reduction in population levels, other species population levels variously decline or benefit.