“Biodiversity” was coined by E. O. Wilson in 1988 to describe all living organisms on the planet (Wilson 1988); it is a condensation of “biological diversity.” It is more commonly used to describe all species other than humans; this creates an unfortunate dichotomy between the voting and nonvoting species! Economic, aesthetic, and health benefits that biodiversity contribute to the human economy are termed “ecosystem services” (Daily et al. 1997; Daily et al. 2000). We are only just beginning to quantify these and are rapidly realizing how dependent the quality of human life and economic well-being is upon biodiversity. Ironically, the current increasing rates of species extinction may provide the sharpest way to quantify human dependence upon biodiversity.
Biodiversity’s greatest strength is its diversity; it includes huge organisms such as whales, and giant redwoods that live for centuries. At the opposite end of the spectrum are viruses and bacteria that are only visible under electron microscopy, including some of the most dangerous infectious agents on the planet: smallpox, anthrax, and influenza. The abundance of biodiversity is simultaneously its greatest problem; despite several centuries of biological investigation, we still have no exact idea of how many different species share the planet with us. Best estimates range between five and ten million; the number could be as high as one hundred million (May 1988; May 1990). Because we do not know how many species are currently extant, we do not know how rapidly they are going extinct. Our best estimates suggest we are losing biodiversity at around one hundred to a thousand times the normal background rate of extinction (May et al. 1995).
Although the sheer diversity of species inhabiting the planet seems almost overwhelming, taxonomists, ecologists, and evolutionary biologists continue to find ways to organize it from perspectives that reflect the underlying process of speciation and the birth and death processes that both determine abundance and reflect interactions between species in their major roles as consumers of, and as resources for consumption by, other species. New biodiversity is created by speciation when the processes of natural and sexual selection act upon two or more subpopulations of the same species that are partially isolated from each other. If selection operates with sufficient intensity for long enough, then the original species will diverge to form two subspecies that will eventually be incapable of breeding with each other. The amount of biodiversity on the planet at any time always reflects the interaction between speciation and extinction rates. Ultimately, all extinct and extant species evolved from a common ancestor that lived in the primitive organic soup of the world’s oceans sometime between two and five billion years ago. The branching processes that are driven by speciation create a natural organization of species into a hierarchical treelike system of branches and twigs that define groups of closely related species with a common evolutionary ancestor (that may be now extinct). The first person to classify all known biological species in this way was Linnaeus (Linnaeus 1735). Subsequent early classifications were based on the morphology of species; more recently this has been replaced by molecular trees that compare the DNA (or RNA) that contains the genetic code that drives the individual physical development of each individual of each species. Mutations in this code are the raw material upon which natural selection operates to modify structure and function and drive speciation. Subdivision of populations through time give rise to new species, which creates an evolutionary treelike structure for all species that have ever lived; the genetic relationships between species are defined by the branch lengths between them, and these define the total time since they shared a common ancestor.
On a broader geographical scale, speciation and dispersal have tended to spread species from their commonest origin in the tropics out into the temperate and Arctic zones. This pattern has in turn been modified by the slow underlying breakup and movement of the continents: thus the oldest plant groups are found in New Caledonia and northeastern Australia, while an arc through Antarctica follows the main trunk and many of the major branches of the evolutionary tree of plants to colonize South America. The ancient diversity of plants then decreases as a subset of the original botanical diversity crosses the Atlantic and passes through Europe and down into Africa, or across the Urals into Asia. The strip of rainforest along the coast of Queensland contains nearly all of the world’s most ancient plant families; in contrast, although the forests of Indonesia and the Fynbos of South Africa have a huge number of plant species, they have mainly evolved recently as massive radiations of closely related species from within very few plant families. The insects that feed on these plants show similar patterns of radiation; in contrast, the geographical patterns in the mammals and birds are less distinct, given their much greater ability to disperse more rapidly. Once these patterns of speciation and radiation were recognized, many creation myths were quickly tripped up, particularly the Christian one: it is essentially impossible to reconcile the simultaneous creation of ten million species with the observed underlying genetic and geographical structure of biological diversity that reflects sequential, hierarchical, and intrinsically timed evolutionary radiation from an ancestral ocean-dwelling, single-celled organism (Darwin 1859).
Although the amount of biodiversity seems bewildering, there are curiously beautiful and consistent patterns in the way the abundance and diversity of species seems to be organized: there are many more species in habitats close to the equator than at the poles, suggesting that sunlight, day length, and available moisture are important in determining the number of species that can be supported at any location. Similarly, we tend to find more species on continents and larger oceanic islands than we do on smaller and more isolated oceanic islands (MacArthur and Wilson 1967). Within any location there is always a mixture of a few abundant species and many rare species. This delights bird watchers and plant hunters, who ferociously seek to identify as many rare and uncommon species as possible. Yet, the relative abundance of plants, or birds, or insects, or bacteria at any location seems to follow an underlying statistical pattern that is remarkably consistent and almost predictable. If the numbers of individuals are plotted out as a frequency distribution, boxed into categories that sequentially represent a doubling of abundance, then the pattern is always very close to log-normal (the notorious bell curve), except with logarithmic classes of abundance along the x-axis. Ecologists have long been fascinated with the mechanisms that produce this pattern: once we know the number of species that occupy a region, then we can quickly ascertain their relative abundance (May 1975). The most parsimonious explanation has been provided by Hubbell’s Neutral Theory of Biodiversity (Hubbell 2001), which suggests that the observed patterns of relative abundance can be produced by a model that assumes all species are equal and have similar birth, death, and extinction rates (Hubbell 1997). Yet biologists know that these assumptions cannot be correct, as different species tend to have different birth and death rates and actively compete with each other. So two of the greatest questions facing ecologists who are interested in diversity are (1) what determines the number of species that can coexist at a particular location? and (2) how does this factor interact with the birth, death, and extinction rates of neutral theory to create the consistently observed patterns of species abundance? Ultimately, we will also need to know whether the observed species abundance relationship creates resilience in the ways in which biodiversity contributes ecosystem services to the human economy.
At any location, biodiversity is organized into a food web: the many plant species will be fed upon by slightly fewer species of herbivorous animals that can digest cellulose and other plant structural material. These in turn will be fed upon by even fewer predatory species. Throughout their lives, all these free-living species will be fed upon by a large diversity of parasitic species that slowly consume them from the inside out, rather than the outside in. These parasitic species may form between 40 and 90 percent of the species networked together within the food web at any location (Dobson et al. 2008; Lafferty et al. 2008). There will also be a large diversity of “decomposer” species that digest individuals that have died, or consume the excretory products that live individuals discard. We have no good estimate of the abundance of decomposer species, but a simple thought experiment tells us that the world would be a much messier place in their absence. We are just beginning to understand the structure of food webs; mathematically, it is one of the deepest and most intractable problems in science and arguably one requiring the most urgent attention. Central to the complexity of this problem is that the mathematical structure of all food webs always becomes increasingly less stable as the number of interacting species increases (May 1974). Mathematically, there have to be constraints on the intensity with which species interact with each other, constraints that interact with the geometrical structure of the web in ways that allow multiple species to interact, consume, compete, and coexist with each other. Ecologists have begun to identify some of these rules and geometries (McCann 2011). The current rates of extinction emphasize the urgency of fully understanding this problem before there are either no pristine webs left to study, or the reduction in vital services supplied to humans declines dramatically.