Fundamentals of Conservation Biology. Malcolm L. Hunter, Jr.. Читать онлайн. Newlib. NEWLIB.NET

Автор: Malcolm L. Hunter, Jr.
Издательство: John Wiley & Sons Limited
Серия:
Жанр произведения: Биология
Год издания: 0
isbn: 9781119144175
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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 = −∑pi log pi, where pi is a measure of the importance of the ith species.

      Evenness = H/Hmax where Hmax 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.

      Often, being precise and quantitative will reveal solutions to a difficult problem, but using quantitative indices of diversity can be misleading when maintaining biodiversity is the goal. Consider the following three lists of species, each one representing (in very abbreviated form) a sample of the species found in three different types of ecosystems.

Forest Marsh Grassland
Black oak Reed‐grass White prairie‐clover
American toad Painted turtle Horned lark
Eastern racer Red‐winged blackbird Black‐footed ferret
Scarlet tanager Muskrat
Raccoon

      If someone were asked which of these tracts is most important from the perspective of maintaining biodiversity, one measure of biodiversity – species richness – would suggest that the forest be chosen. However, if you knew that the black‐footed ferret is one of the rarest mammals in the world and that all the other species listed are very common, you might well select the grassland tract. Why?

      The simple answer is that all species may count the same when tallying species richness, but conservationists almost always consider additional information such as the likelihood of a species becoming extinct, its role in an ecosystem, and more. Consequently, not all species are equal from a conservation perspective. We will return to this issue in other chapters, but we need to build a foundation here by considering how conservation decisions are shaped by patterns of diversity and risk of extinction at different spatial scales.

      (Apple2499/Shutterstock)

Schematic illustration of the distribution of four hypothetical lizard species showing alpha diversity (within an ecosystem, a plus b), beta diversity (among ecosystems, a/b plus c), and gamma diversity (geographic scale, a–c plus d).