Biodiversity

I

Introduction

Biodiversity, a contraction of “biological diversity”, term for variety or diversity within the biological world. In its widest sense, biodiversity is virtually synonymous with “Life on Earth”. The word was coined in 1985 and has since become very widely used in the popular media and in government and scientific circles.

It has become customary, partly as a matter of convenience, to consider biodiversity at three hierarchical levels that have special significance in human affairs: genes, species, and ecosystems. It is important, however, to recognize that these are just some of the ways in which biodiversity may be assessed, and that there is no precise definition of what the word means and thus no agreement on how biodiversity can best be measured.

The biological world can be regarded as a series of increasingly complex levels of organization: with the key molecules of life at one extreme, and communities of species within ecosystems at the other. Manifestations of biological diversity can be found at all these levels. Because biodiversity covers a range of concepts and can be considered at different levels and different scales, it cannot be reduced to a single measure. In practice, the diversity of species is central to the evaluation of diversity at other levels, and is a constant point of reference in biodiversity studies.

II

Species Diversity

The species is in many ways the basic currency of biological science, and the species is the focus of much of the research carried out by ecologists and conservation biologists. It is possible to count the number of species at any given sample site, particularly if attention is restricted to well-known organisms (such as mammals or birds); it is also possible to estimate this number in a region or country (but with increasing error as the area considered increases). This measure, known as species richness, provides one possible measure of how much biodiversity a site has, and a basis for comparing sites. It is the most straightforward and in many ways the most useful general measure of biodiversity.

Species richness varies geographically: warmer areas tend to support more species than colder ones; wetter areas more species than drier ones; less seasonal areas more than strongly seasonal ones; areas with varied topography and climate support more species than uniform ones.

Despite the importance of the species, there is still no one unequivocal definition of the term. Different criteria are used to classify species in different groups of organisms (eg. defining species of bacteria and species of bird are very different processes) and, often, different taxonomists will use different criteria for the same group of organisms and therefore recognize different numbers of species. However, these differences should not be over-emphasized; there is sufficient agreement for many purposes on how many species exist in well-studied groups such as mammals, birds, reptiles, amphibians.

Species number or richness, although a simple concept and practical to assess, is still an incomplete measure of diversity and has limitations when it comes to comparing the diversity of two different sites, areas or countries.

III

Species Endemism

Any given area contributes to overall global diversity both through the number of species present, and through the proportion of those species which are unique to that area. These latter are called endemic species. A species is endemic to some defined area if it is confined entirely to it (the term is derived from medical science, where a disease is described as endemic if confined to a certain area, and epidemic if widespread). For example, islands typically have fewer species overall than equivalent sized continental areas, but also usually have a higher percentage of species found nowhere else. In other words, other things being equal, they have lower species richness but higher species endemism. Assessing the relative importance of these two factors and therefore comparing the importance for biodiversity of islands and continental areas is not straightforward.

Areas rich in endemic species might be sites of active speciation, or of refuge for relict species, but whatever the theoretical interest, it is important for practical biodiversity management to be able to identify such discrete areas of high endemism. By definition, species endemic to a given site occur nowhere else. The smaller the area of endemism, the more at risk the endemic species will be through deterministic or chance population events. Whilst all may be vulnerable to the same episode of habitat modification, by the same token, all might benefit from the same conservation action. It is desirable to identify any such opportunities for cost-effective conservation action.

Endemism can also be defined in terms of national boundaries. This is of immense importance to the conservation of biological diversity because, almost without exception, conservation and management actions are applied and maintained in a national political context. This is so regardless of the source of scientific advice or of financial support for the actions undertaken.

IV

Other Aspects of Species Diversity

In addition to species richness and species endemism, a measure of biodiversity could attempt to assess the magnitude of differences between species. One way of achieving this relies on the information content of the classification or taxonomic system. Similar species are grouped together in genera, similar genera in families, families in orders, and so on until the highest level, the kingdom. This taxonomic organization is an attempt to represent genuine relationships between organisms, that is to reflect evolutionary history, so that species in the same genera are believed more closely related than species in different genera, and so on. Some higher taxa have thousands of species (or millions in the case of the beetles or Order Coleoptera), some only one. Species that are very different from each other (placed in different families or orders) by definition contribute more to overall diversity than species which are similar to each other (placed in the same genus). So, using this improved measure of biodiversity, if there were a choice of conserving two sites with the same number of species, it would be better to choose the site with more fundamentally different kinds of species than the one with many closely related species. Some scientists would take this argument further and suggest that diversity is better measured at higher taxonomic levels (for example, genera or families) than in terms of species.

The ecological importance of a species can also be significant in so far as some keystone species have an important role in maintaining the diversity of a whole community of other species. Keystone species would include decomposer organisms, top predators, pollinators, and so forth. In general, large trees greatly increase local biodiversity because they provide a range of specialist resources for other species (including nesting birds, epiphytes, parasites, fruit-eating herbivores, and a great many other organisms). However, as yet there is no way to quantify this kind of support role and compare its magnitude across different groups.