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genes, ecosystems are large and conspicuous, and thus anyone with the most rudimentary understanding of ecology appreciates the value of a lake, a forest, a wetland, and so on. Nevertheless, ecosystems can be hard to define in practice. Where do you draw the boundary between a lake and the marsh that surrounds it when many organisms are moving back and forth between the two? This sort of problem can complicate the role of ecosystems in biodiversity conservation. Conservation biologists often advocate protecting examples of all the different types of ecosystems in a region, but how finely should differences be recognized? Is an oak–pine forest ecosystem that is 60% oak and 40% pine appreciably different from one that is 40% oak and 60% pine? If you look hard enough, every ecosystem will be unique. The rationale for protecting ecosystem diversity also differs. Some conservationists advocate protecting ecosystems for their intrinsic value as independent, biological entities of many interacting species, whereas others think of protecting ecosystems simply as an efficient way to protect most of the species that form the ecosystem.

Structure and Function

      The definition of biodiversity provided above emphasizes structure – forms of life and levels of organization – but sometimes ecological and evolutionary functions or processes are also included in a definition of biodiversity. For example, The Wildlife Society (1993) defined biodiversity as “the richness, abundance, and variability of plant and animal species and communities and the ecological processes that link them with one another and with soil, air, and water” (emphasis added).

      The diversity of ecological functions is enormous. First, each of the Earth's millions of species interacts with other species through ecological processes such as competition, predation, parasitism, mutualism, and others. To take a close example, consider the staggering array of interactions between our human bodies and the more than 10,000 species of microorganisms living in or on us, each also interacting with one another (Huttenhower et al. 2012). Second, every species interacts with its physical environment through processes that exchange energy and elements between the living and nonliving world such as photosynthesis, biogeochemical cycling, and respiration. All of these functional interactions must total in the billions. The diversity of evolutionary functions is even more complex. It includes all these ecological processes because they are key elements of the conditions under which individuals survive and reproduce or do not (that is, natural selection), in addition to processes such as genetic mutation that shape each species’ genetic diversity.

Functional biodiversity is clearly important. For example, a management plan designed to keep an ecosystem intact will almost certainly fail in the long run unless the processes of evolution, especially natural selection, continue, allowing the species that comprise the ecosystem to adapt to a changing environment. Sometimes, focusing on a functional characteristic, for example, the fire regime of a grassland rather than the species that comprise the grassland (Fuhlendorf et al. 2012), is the most efficient way to maintain biodiversity of an ecosystem. Nevertheless, conservation biologists usually focus on maintaining structural biodiversity rather than functional biodiversity for two reasons. First, maintaining structural biodiversity is usually more straightforward because it is easier to inventory and protect species than their interactions with one another. Second, if structural diversity is successfully maintained, functional biodiversity will probably be maintained as well. If we can maintain a species of orchid and its primary insect pollinator together in the same ecosystem, then we will almost certainly have a pollination interaction between the two. Similarly, if we can maintain the orchid’s genetic diversity, we will probably have orchid evolution. The converse is not necessarily true. It is easy to think of circumstances where some major ecological processes are maintained, but structural diversity is severely degraded. For example, many of the forests of Puerto Rico are dominated by exotic trees that have largely replaced the native species, but they still perform as forests in terms of photosynthesis, biogeochemical cycling, and hydrologic processes (Lugo 2004).

      In short, both the structural and functional aspects of biodiversity are important; however, if genetic, species, and ecosystem diversity are successfully maintained, then ecological and evolutionary processes will probably be maintained as well.

Measuring Biodiversity

      It is easy to provide a simple definition of biodiversity such as “the variety of life in all its forms and at all levels of organization,” but this is only a starting point. To monitor biodiversity and develop management plans, we should have a quantitative definition that allows us to measure biodiversity at different times and places.

The first step in measuring biodiversity is to determine which elements of biodiversity are present in the area of interest. Ideally, we would have a complete inventory, including genes, species, and ecosystems. In practice, logistical constraints commonly limit us to a partial list of species, often listing only vertebrates and perhaps vascular plants and a few large invertebrates. (Sometimes a list of ecosystems is compiled, although the basis for distinguishing among the different types is often unclear; we will focus on the species level of biodiversity here for simplicity.) Lists can be tallied to provide a crude index of biodiversity. In Table 2.1, for example, ecosystem A is easily recognized as more diverse than B or C because it has four species instead of three. This characteristic is called species richness or just richness, and it is a simple, commonly used measure of diversity.

Table 2.1 Hypothetical lists of species for three ecosystems.

Ecosystem A	Ecosystem B	Ecosystem C
Black oak	Black oak	Black oak
White pine	White pine	White pine
Red maple	Red maple	Red maple
Yellow birch

Ecologists also recognize a second component of species diversity called evenness, which is based on the relative abundance of different species. In Table 2.2 ecosystem C is more diverse than B because in C the three species have similar levels of abundance, or high evenness. The concept of evenness is not as intuitively obvious as the idea of richness. It may help to think of a jury that has five women and five men versus one that has eight women and two men; the five plus five jury is more diverse because it is more even. Note that no ecosystem ever has perfect evenness; typically there are some common species and many more uncommon species. Concern arises when uncommon species become even rarer, potentially to the point of local extinction.

Table 2.2 Abundance of species (number/hectare) in three ecosystems and measures of richness, evenness,and the Shannon diversity index (H), one of many ways to combine richness and evenness quantitatively. (Magurran 2004)

Ecosystem	A	B	C
Black oak	40	120	80
White pine	30	60	60
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