Читать книгу Fundamentals of Conservation Biology - Malcolm L. Hunter Jr. - Страница 31

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
Red maple	20	20	60
Yellow birch	10
Richness	4	3	3
Evenness	0.92	0.88	0.99
H	0.56	0.39	0.47

H = −∑p_i log p_i, where p_i is a measure of the importance of the ith species.

Evenness = H/H_max where H_max is the maximum possible value of H.

The ecological importance of species richness seems quite evident, especially if you consider the loss of richness through extinction. Similarly, most conservation biologists would be concerned about any process that reduced evenness, because this would mean uncommon species are becoming less common, while common species are becoming more common. To return to our jury metaphor, this would be analogous to losing a man from the jury that only had two men. Richness and evenness are often combined into a single index of diversity using mathematical formulae (Table 2.2) but, as we will see in the next sections, such indices are of limited utility.