Assessment of Grassland Ecosystem Conditions in the Southwestern

Michele Merola-Zwartjes Chapter 4: Biodiversity, Functional Processes, and the Ecological Consequences of Fragmentation in Southwestern Grasslands What is Biodiversity?_______________ the number of species that occur in a defined area; and (2) species evenness, a measure of the extent to Concern over accelerating extinction rates and loss which the individuals of the different species present of species diversity on a global scale was the subject are equally abundant. Diversity is thus a measure of E.O. Wilson’s seminal volume Biodiversity (Wilson of species richness weighted by relative abundance. 1988). This work essentially transformed the term Technically the most highly diverse communities “biodiversity” into a household word as a short-hand are those with the greatest species richness, each for species diversity—or more simply, the full array component species being equally abundant within the and variety of living organisms on Earth. But the term community. In reality, such a situation is unlikely biodiversity means much more than the complement to occur, as many species (such as top predators) of plant and animal species that one expects to find in are naturally less numerous in the community. some given area. The term now encompasses not only Furthermore, from a management standpoint it is the diversity of species, but their genetic structure, the often the more rare species in the community that interaction of the biotic and abiotic components of the environment at the ecosystem level, and at an even are of interest, rather than the common species that higher level the array of communities and ecosystem tend to dominate weighted indices of diversity. The processes and functions that make up the landscape term “biodiversity,” then, as used by many biologists, or regional level of biological diversity. Such a detailed most often refers to the more simple measure of exploration of biodiversity is beyond the scope of this species richness, and that is how the term is used section. in this discussion. Biodiversity as used here refers In the interest of (relative) brevity, the following to natural or native biodiversity. The distinction is discussion will focus on the more limited definition important because the introduction of exotic plants or of biodiversity, concentrating on the variety of plant animals may technically increase diversity (at least and animal species of the New Mexico and Arizona in the short term) by adding to the overall species grasslands. richness of a given area. However, as discussed later, A few points of clarification on terminology: The introduced species usually interfere with normal com- most common indices of species diversity (such as munity and ecosystem functions at some level and Simpson index, Shannon-Weiner index) are a function often eventually replace the native species, thereby of two parameters: (1) species richness, or quite simply leading to a net loss of native biodiversity. USDA Forest Service Gen. Tech. Rep. RMRS-GTR-135-vol. 1. 2004 49 Why is Biodiversity Important? prevent many species from being successful in this environment, in fact the opposite is true. The highly There are two main schools of thought on this variable precipitation, temperature extremes, and subject. One theory is commonly referred to as the elevational gradients created by the basin and range “rivet hypothesis” (Ehrlich and Ehrlich 1981). This topography have provided numerous opportunities hypothesis proposes that each species plays some small for adaptation and evolution. Small-scale variations but important role in the ecosystem – like the rivets in soils, aspect, and moisture all affect microclimate that collectively hold an airplane together. After some and resource conditions, and this variety of conditions number of species (rivets) are lost, a critical threshold in turn offers a diverse array of niches available for is crossed and the system (plane) rapidly begins to exploitation, resulting in increased floral and faunal disintegrate. The fundamental assumption is that species richness. Species diversity in the Southwest greater diversity (more rivets) results in increased has been further enhanced by its biogeographic history. stability of the system. The flora and fauna of distinctive regions historically The other theory may be called that of “functional isolated during the Pleistocene (for example, Great redundancy.” This idea holds that communities com- Plains, Chihuahuan Desert, Sonoran Desert, Mojave prise a few functional groups of ecologically equivalent Desert, and Great Basin Desert) have converged fol- species, to the effect that several species may be lost lowing glacial retreats, leading to the characterization with little impact because there are several “back-up” of the desert Southwest as a “biological melting pot” species in place that are capable of carrying out the same function in the ecosystem (Walker 1992). (Parmenter and others 1995). For Southwestern grass- Although sources of great debate, there is not lands in particular, biodiversity is further enhanced a great deal of hard data to back up either theory. by the complex, mosaic nature of their distribution. Studies in grasslands to date, however, tend to sup- Grasslands in this region are interspersed with shrubs, port the rivet hypothesis. Grassland plots with greater woodlands, and riparian areas, leading to high species species diversity were found to be more resistant to diversity due to the presence not only of grassland drought and to recover more quickly than less diverse specialists, but also more generalist species from plots (Tilman and Downing 1994). Another grassland adjacent habitats that may utilize the grasslands for experiment showed that plant productivity and soil other purposes (Parmenter and Van Devender 1995). nitrogen utilization both increased significantly as a Highly patchy interspersed habitats, such as sand function of plant species diversity, leading the authors dunes, playa lakes, and lava flows, add to biodiversity to conclude that “the establishment and functioning of by the presence of specialized and/or endemic plant these grassland ecosystems depended on their species and animal species distinctive to these areas (such richness” (Tilman and others 1996:720). as fringe-toed lizards in sand dunes) (Parmenter and How, then, to manage for biodiversity in grassland others 1995). Brown and Kodric-Brown (1995) argue ecosystems? With so little evidence, the assumption of that biodiversity along the Arizona and New Mexico functional redundancy seems somewhat reckless. Even borderlands may be among the greatest on the continent if there is some degree of ecological overlap between due to the biogeographic confluence in this region. species, we have no idea if the “backup” system is as Underlying the considerable biodiversity of the efficient as the primary one (Odum 1992). The most Southwest at the level of species richness is a high prudent course of action in Southwestern grasslands degree of genetic variability. Rangeland plants as a – or any ecosystem—is to ensure the ecological integ- group exhibit high levels of genetic diversity, probably rity of the system by managing for the conservation of due to a combination of adaptation to diverse eco- maximal native biodiversity, or, in the oft-quoted words logical conditions and frequent hybridization between of Aldo Leopold, “to save every cog and wheel”—or, to interfertile species (Nevo and Beiles 1989, Wayne and see the context of his remarks (Leopold 1953): Bazzaz 1991). Modern grassland plants have survived The last word in ignorance is the man who says historical cycles of great climatic variation; high levels of an animal or plant: “What good is it?” If the land of genetic variability have given these plants the mechanism as a whole is good, then every part is good, ability to adapt and persist throughout such oscilla- whether we understand it or not. If the biota, in the course of aeons, has built something we like but do tions (Tausch and others 1993). Genetic diversity is not understand, then who but a fool would discard also high in Southwestern animals, both vertebrates seemingly useless parts? To keep every cog and wheel and invertebrates. Pocket gophers (Geomydiae), for is the first precaution of intelligent tinkering. example, are common mammals in Southwestern grasslands. Although the species often are difficult to Biodiversity in the Southwest________ distinguish visually, they are some of the most geneti- cally variable mammals known; species may differ not Although at first glance it might seem that the only in terms of the alleles represented, but in the climatic extremes of the desert Southwest would numbers of chromosomes carried as well (Parmenter 50 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-135-vol. 1. 2004 and others 1995, Patton and Sherwood 1983). Despite one considers grasses in terms of their contribution the fact that Southwestern mammals are relatively to range quality, Stubbendieck and others (1986) well known, modern laboratory techniques continue to suggest 94 species of grasses in their list of the 200 identify new species on the basis of genetic differences most important range plants in North America. Some between populations. Within the last few decades of the more common native grasses of the Southwest several new species have been identified in New include several species of gramas (such as black Mexico, including a grasshopper mouse (Onychomys grama [Bouteloua eriopoda], blue grama [B. gracilis], arenicola),

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