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What is Biodiversity?
In addition to the various species that exist today and their distinct evolutionary histories, biodiversity also refers to genetic variety both within and across species populations as well as the distribution of species over entire continents or seas as well as local habitats, ecosystems, and landscapes. Managers and policymakers must try to integrate the values of biodiversity into their strategies for managing land and water, so they must have a clear understanding of what biodiversity is and what it means. We cannot fully appreciate the quantities of information that need to be taken into account unless we have a thorough understanding of all the interrelated scientific aspects of biodiversity that are discussed in this chapter. Options for managing biodiversity are necessarily limited by a confluence of social and biological constraints. The biological knowledge of biodiversity that we give in this chapter serves as the foundation for the subsequent chapters 3 and 4, which address the "uses" and "value" of biodiversity.
There are numerous ways to use the term biodiversity. Views on biodiversity vary among economists, ecologists, ranchers, gardeners, mayors, and miners. Many times, when people talk about biodiversity, they substitute it for "wild places," "abundance of species," or even "large, furry mammals." However, biodiversity has a specific meaning for scientists working in fields related to biodiversity, including population biology, ecology, systematics, evolution, and genetics: "the variety and variability of biological organisms" (Keystone Center 1991; Noss and Cooperrider 1994; Wilson and Peter 1988). Similar definitions of biodiversity include "variability among living organisms from all sources" provided by the Convention on Biological Diversity. These categories are so wide that only specific levels of biological organization—genes, species, communities, ecosystems, and even our planet—can be taken into consideration in order to fully comprehend them.
Genes, Species, and Populations
Within species, there is genetic variation. We would lose a great deal of biodiversity if every species were condensed into a single, small population of individuals who are genetically similar. Even while the amount of species in various locations may not change as we go across an area, the species do. For example, a forest and its neighboring grassland may have nearly completely distinct species assemblages. Furthermore, a grassland's ecosystem functions are distinct from those of the adjacent forest.
Individuals of the same species living in the same area and interacting in a variety of ways, including interbreeding, make up a population. Although populations of the same species that live in various environments can share members, they frequently exhibit some degree of genetic differentiation, and the more genetic differentiation that exists between them, the more unique they become. Metapopulations are collections of geographically dispersed populations found in habitat patches spread throughout a terrain. In various habitat patches throughout a landscape, populations might go locally extinct; they seldom swap members, and when they do, it's usually dangerous and requires traveling across unfriendly terrain. Within a metapopulation, individual local populations undergo extinction, and in cases where habitat is left unoccupied, other local populations will eventually recolonize it.
Variation in Genetics and Adaptation
While a large portion of genetic diversity may only be identified biochemically, some genetic variation can be clearly seen in the anatomical, physiological, behavioral, and life-history traits—or phenotypes—of members of a community. The foundation of both general phenomena, such as slow phenotypic changes over a geographic transect when environmental conditions change, and local adaptations is genetic variation. Coevolution, in which species evolve adaptations in reaction to each other's adaptations, is likewise predicated on genetic variety.
Examples of adaptive evolution within species abound. Although the common mussel has a large reproductive output and high rates of genetic exchange, populations are genetically differentiated over surprisingly short distances, ranging from a few meters to several kilometers, throughout its extensive continuous range off the eastern coast of North America (Koehn and Hilbish 1995). The common yarrow is a composite plant native to California that can survive in a wide range of environments, from the high Sierra Nevada to the Pacific Coast. It exhibits unique, habitat-specific morphologies based on genetic variation (Clausen and others 1958). The genome organization of Drosophila flies varies greatly depending on seasonality, habitat, elevation, and regional geography (Dobzhansky 1970).
Diversity Metrics
Managers have to make decisions, one of which is how to evaluate biodiversity. How can we tell if there has been a change in biodiversity? To evaluate biodiversity, scientists employ a variety of techniques.
Statistical indices of species diversity can be used to compare biodiversity across different areas (Magurran 1988; Pielou 1975). The overall number of species and the relative abundances of all species (evenness) in a sample are the two metrics that are combined in most indices. These indices have drawn criticism due to the possibility that comparable index values could represent significantly different sample compositions. A particular index value may be attributable to many fewer but more common species (e.g., high relative abundance of numerous species) or it may reflect a high species richness (many uncommon species).
Within-area diversity, or formally alpha diversity, is the simplest way to evaluate diversity because it just counts the number of species in a specific area. Ecologists refer to this metric as species richness; by using the terms rich or poor, they suggest that there is no economic value. The number of species in a given area is determined by their presence alone, not by their abundance.
Species Counts May Not Be Accurate
We can be positive of only one thing for many groupings of species (such mites, nematode worms, and aquatic fungi): we do not know many, if any, of the species. As a result, we are able to quantify the quantity of some species in a region but not all of them. We only have partial inventories of the species present for a small number of regions. For instance, a list of all the fungi, lichens, bryophytes, vascular plants, mollusks, arthropods, amphibians, mammals, fish, and birds that have been identified for the Pacific Northwest, where there is much debate over old-growth forests, is not comprehensive due to differences in our understanding of various taxa. Our knowledge of insects and fungus is far from full, yet we are familiar with every bird and animal. Given the sheer number of species and groupings that were inventoried, even that sample stands out.
Species numbers are typically expressed as counts of readily observable or identifiable species. Although there are many birds in Costa Rican forests, it is unknown if there are as many other species—like fungi—in the same proportion. Though not usually, areas abundant in one species group are also frequently rich in another. Many bird species may be found on remote islands, but there may be few or no animals or amphibians present.
The Area Counted and Species Counts
Greater numbers of species are found in larger areas than in smaller ones; the United States contains more species overall than the state of Tennessee. Species diversity, however, does not simply increase in proportion to the area. If an area is half the size of another area, it may contain 85% of the species found in the bigger area. Therefore, without knowing the biological context, we cannot use the number of species per unit area as a metric of biodiversity. Therefore, it is possible that we imply that Costa Rican or riparian habitats within western US rangelands are more diversified than other places of comparable size when we say that they are richer in species.
Terrains as Biological Diversity
Biodiversity is sometimes used to mean "a diversity of landscapes" in conversations. A location with both grassland and forest is thought to be more diversified than one with solely grassland. Boulder, Colorado has a diversified ecosystem because of ponderosa pine savannas, grasslands, wetlands, and riparian woods (see the Boulder case study in chapter 3). A multitude of marine, estuarine, riparian, and terrestrial "habitat types" can be found in Camp Pendleton, in addition to a great number of threatened and endangered species (see the Camp Pendleton case study in chapter 1). Compared to an area of comparable size (West Virginia, for example), Costa Rica has significantly more life zones (see the case study in this chapter).
Different species relationships are indicated by terms such as forest and grassland. There is a lot of between-area variability in grassland and forest edges. Boundaries between associations frequently line up with observable variations in the environment's physical characteristics and in ecological processes like the cycling of nutrients. Three issues arise when managers use landscape terminology to characterize biodiversity:
Methods of Ranking and Rating Based on Biology
Biologists use many metrics to evaluate how important biodiversity conservation is. Some focus on the preservation of species, while others uphold the functionality of communities or ecosystems.
From the standpoint of biological systematics, species differ in their importance for maintaining and conserving biodiversity. To determine such a value, various methods have been employed. A generalized hierarchical strategy can be applied, progressing from genetically dissimilar sister populations to groups at different taxonomic levels along a genealogical to phylogenetic continuum. Populations of a species with spatially variable genetic distinctiveness would be more valuable than genetically more homogenous populations. Comparably, a related species that is more genetically dissimilar from another would be of greater benefit to a specific protected species than one that differs only marginally. A phylogenetic ranking of taxa can employ that form of ranking; species with a higher evolutionary distance would be more valuable because the objective is to preserve the highest level of biological variety. When there is enough relationship data available, the approach can be refined (Faith 1994). Long-branch taxa have the most value under such a framework.
