Biological diversity

Cave paintings attest to our species’ enduring interest in biological diversity. Our early ancestors appear to have enjoyed the aesthetic qualities of nature as well as recognising its importance as a source of food and fuel. Poets and artists throughout history have sought inspiration from the natural world. But the first scientific enquiries about biological diversity emerged when explorers returned with descriptions of sharp contrasts in the variety and number of plants and animals in different parts of the globe. Baron von Humbolt, who travelled in South and Central America between 1799 and 1804, asked why more species are found in tropical regions than in temperate zones. The question he posed continues to be fiercely debated and his answer — climate — remains one of several plausible explanations. A second pervasive pattern, which ecologists also still struggle to fully understand, is the species–area relationship. Early Victorian naturalists first noted that more species are found in larger areas. It turns out that every tenfold increase in area leads, approximately, to a doubling in the number of species harboured. This two-pronged approach of describing patterns of biological diversity, and then endeavouring to understand the reasons why the composition and variety of the biota varies from place to place, remains popular. But the threat of unprecedented rates of species extinction has added new urgency and led to a surge of interest in the function of biological diversity, particularly in terms of the services provided to humankind, and in its conservation. Although the concept of biological diversity has a venerable history, the term itself is of surprisingly recent origin and first entered the scientific literature in the 1950s. Its more widely used contraction, biodiversity, came to prominence after being adopted as the title of E.O. Wilson’s influential 1988 book. Biological diversity is simply the variety and abundance of organisms at a given place and time. The Convention on Biological Diversity (CBD) definition is often cited: “‘Biological diversity’ means the variability among living organisms from all sources including, inter alia, terrestrial, marine and other aquatic systems and the ecological complexes of which they are part; this includes diversity within species, between species and of ecosystems.” Defining biological diversity may be straightforward, but measuring it is not. The different organisational levels embodied in the CBD definition need to be tackled in different ways. Within species diversity, for example, can be measured using molecular methods — DNA microarray technology is one approach that is proving increasingly invaluable here. At the other end of the scale, ecosystems are usually evaluated in terms of the numbers of species, or endemics, they support. Setting aside the difficulties of delineating ecosystems, and defining species — a non trivial issue for many asexual and prokaryote organisms — surveying large areas can be a Herculean task, particularly where invertebrates are involved. In practice, most of the interest in biodiversity measurement has been directed at the CBD’s between-species category. This is the type of biological diversity under investigation when farm trials of genetically modified crops are conducted to evaluate their effects on wildlife, or when the bird faunas of, say, oak wood and pine forest are compared. It is a universal characteristic of ecological communities that some species are extremely abundant, others only moderately common and the remainder, often the majority, rare. This means that community diversity can be partitioned into two components: species richness (the number of species present), and species evenness (the distribution of species’ relative abundances). Biological diversity is obviously linked to species richness. But diversity is also assumed to increase as assemblages become more even (Figure 1). This is a meaningful assumption, as it appears that the ability of an assemblage to resist change or recover from a perturbation is related to its evenness as well as to its richness. It also means that diversity can be described in many different ways. Some investigators opt for estimates of species richness. Although these are intuitively easy to understand, because the number of recorded species is correlated with sampling intensity, as well as with