Biodiversity plays a crucial role in moderating the harshness and unpredictability of the desert environment. Functional groups arc recognizable and conspicuous in their effects on ecosystem processes. Particularly vivid examples include the cffects of burrowing vertebrates and invertebrates on soil and hydrological properties, the influence of woody plants on the soil, microclimate and food resources for animals, and the distinct cffects of plants with different canopy architectures, rooting depths and photosynthetic pathways on the dynamics of soil water. While there have been few experiments directed explicitly at the issue, there is considerable evidence of differences among species within a growth form having measurable consequences on the rate of some ecosystem-level process. The general implication is that diversity at the species level serves to ameliorate the effects of unpredictability or heterogeneity of the environment, while diversity at the functional group level increases the total use of resources (and therefore total ecosystcm productivity).
Human influences have altered diversity in desert systems both directly and indirectly. These alterations have often involved members of functional groups demonstrated to have significant effects on ecosystem function, for example, changes in the relative abundance of woody plants, reduction of burrowing vertebrates, or introduction of phieatophytic woody plants to desert riparian areas. While there is less documentation available for the impact of human activity reducing specks richness within funclional groups, we suspect that more arid systems will be more vulnerable, due to their overall lower diversity. The evidence suggests that any trend toward decreasing species richness within groups will lead to further losses of species (those which depend directly or indirectly on the eliminated organisms) and to increasing variance in ecosystem structure and function -decreased buffering of the environmental variation so typical of arid lands.
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