Chapter 4: Biodiversity
The loss of biodiversity has been called by E. O. Wilson the "folly for which our descendants are least likely to forgive us." This chapter will review the concept of biodiversity and then describe general patterns in the global distribution of vertebrates and the reasons why we are losing biodiversity so rapidly. The purpose of the chapter is to give some indication of what we humans stand to lose if we let present trends continue. We start by giving some definitions of biodiversity, then discuss species diversity, focusing on vertebrates, followed by a discussion of species numbers in relation to biogeography. The final section of the chapter is devoted to extinction, the loss of biodiversity.
Biodiversity is a term used to describe the whole of biological diversity, i.e., all living things: plants, animals, and microorganisms and all their interactions with each other and their environment. The term includes three levels of organization:
1) Ecosystem diversity refers to geographically defined areas with sets of interacting populations of plants, animals, and microbes. Each ecosystem has its own set of organisms and interactions, which are often defined by humans in terms of the most distinctive organisms and processes: a hardwood forest ecosystem, a temperate lake ecosystem, a Pacific coast rainforest. Implicit in the recognition of ecosystem diversity is that natural systems are more complex and species-rich than human dominated systems and that by protecting natural ecosystems we are protecting many organisms and processes about which we know little. A forest, with its myriad species of trees, birds, mammals, insects, and other organisms is more diverse than a neighboring agricultural field with its single dominant plant species and near lack of wildlife.
2) Species diversity refers to the number of different species of all types in a given area or region. Because it involves counting things (a uniquely human obsession), species diversity is what is most commonly thought of as biodiversity. It is particularly useful for comparing different areas. The number of bird species in a hectare of tropical moist forest is much higher than the number in a hectare of temperate forest and, hence, the tropical forest is said to have a more diverse bird fauna.
3) Genetic diversity refers to the amount of genetic information (variation) carried within a population of organisms. It is genetic variation that allows a species to evolve. Evolution is constantly occurring in response to the constantly changing climatic and habitat conditions of vulnerable to population collapse or extinction than those with a lower degree of genetic variation because they can adapt to changed conditions.
HOW MANY SPECIES ARE THERE?
Historically, humans lived with little knowledge of the world outside their immediate environs. Early peoples had an intimate relationship with local plants and animals they relied on for their food, clothing, and housing needs (Chapter 1). Although this relationship required an appreciation for the seasonal rhythms and habits of local plants and wildlife, this knowledge did not reflect an understanding of the world on a larger scale. Europeans of several centuries ago believed that the organisms they experienced in their everyday lives were all that existed on the earth. The total number of species of plants and animals was thought to be in the hundreds, so an interested individual could know them all in colloquial terms. It was not until the mid-eighteenth century that a system for naming and classifying organisms was even developed, in response to the voyages of exploration that revealed to Europeans how diverse the world really was. Alfred Russell Wallace and Charles Darwin, who embarked on such trips, were especially important in demonstrating the rich diversity of life on earth and in explaining why llamas occurred only in South America, orangutans only in southeast Asia, and marine iguanas only on the Galapagos Islands.
A. R. Wallace was a British naturalist who lived, from 1848-52, in South America and then, from 1854-62, in Indonesia for the sole purpose of collecting and describing plants and animals not found in the museums of Europe. Charles Darwin was a young British naturalist who in 1831-36 sailed aboard the H.M.S. Beagle on a mapping expedition to South America. Both men made extensive collections of previously unknown organisms and took comprehensive notes on plants and animals in native habitats. In an attempt to explain patterns in the distribution and appearances of animals they saw, both men, independently, "discovered" the mechanism of evolution, natural selection. Their travels also documented the rich diversity of life, vastly increasing our appreciation for the number and variety of animals on earth.
Today, approximately 1.4 million organisms (including 400,000 beetles) have been described, named, and classified (Wilson 1988), but biologists do not know even within an order of magnitude (i.e., 10 times) the total number of species. Estimates of the total number of species range from 3 to 50 million (Erwin 1988) and the difficulties involved in arriving at an accurate estimate have been the subject of investigation and conjecture for decades (May 1988). Even 50 million species may be a low number, as we are just beginning to appreciate, for example, the diversity of life in the soil (a mass of animals less than 5 mm long), life on the sea floor, and life in the forest canopy. Indeed, it can be argued that whatever number it hit upon, it should be doubled based on nematodes alone. Nematodes are small worms that parasitize all organisms (even you!) and it is likely that each non-nematode species supports at least one distinct nematode species!
The nematodes, as a group, are largely undescribed, reflecting the fact that all groups of organisms have not been equally surveyed. Some large, conspicuous groups (e.g., birds, mammals) are thought to be 90-99% described (Wilson 1988). Even with these groups, though, we are still discovering new species and even new genera (groups of related species). For example, a new species of antbird (a small, dark, forest-dwelling songbird) was described from Peru in April, 1990; described in September, 1989; a new genus and species of Hawaiian honeycreeper was discovered on Maui in 1973. This latter species is already in imminent danger of extinction. Expeditions to poorly documented parts of the world are still turning up many new species of vertebrates; a biologist (James Patton) visiting the Andes Mountains in Columbia in July 1995 discovered, in two weeks, six new species of mammals: four rodents, a shrew, and a marsupial. Often even local diversity is overlooked. For example, the "red crossbill" is a favorite forest bird of bird watchers because of its bright color and ability to break apart pine cones with its unusual beak. However, the crossbill is not one species, as had long been thought, but seven species that can be told apart by eye with great difficulty (Benkman 1993). The birds, of course, have no problem telling each other apart; each has a distinctive set of calls. And just as advances in the technology of sailing ships allowed faunal surveys of many areas of the globe two centuries ago, new technologies, especially deep-diving submersibles, are allowing biologists access to regions previously unexplored. These surveys have discovered entire ecosystems with hundreds of new species in the deep sea near hydrothermal vents.
The effort to estimate the number of species is an interesting exercise, if for no other reason than to remind us that we humans are just one species among the millions. However, we will never really know how many species there are because the very term implies species are fixed entities. The founder of modern systematics, Carolus Linnaeus, indicated in his massive work Systema Naturae (1758) that species could be classified because each was an individual creation of God, fixed in it characters. Modern biologists more or less extended this concept by thinking of species as groups of interbreeding organisms with distinct morphological, physiological, behavioral, and ecological characteristics, each created in geographic isolation from other similar populations through a long evolutionary process.
Increasingly, we are realizing that species can be as dynamic as the landscapes in which they live. They can change through time and one species can give rise to hundreds of distinct local forms, each evolving in a few generations. For example, the threespine stickleback (Gasterosteus aculeatus), a small fish, has been called by evolutionary biologist Michael Bell "a superspecies made up of semispecies!" What we know as the species (the superspecies) is found in coastal regions throughout Eurasia and North America (Moyle 2002). Throughout its wide distribution it is easily recognizable to us as a threespine stickleback. The stickleback is a very widely distributed form because it can live in both salt and fresh water. The saltwater form can invade isolated coastal streams and lakes, where it settles down and becomes non-migratory (a genetic characteristic, indicating natural selection at work). In each of the lakes and streams the new form can develop distinctive characteristics and will not interbreed with any marine sticklebacks it might encounter. In a few hundred years (or less), in essence, a new species has evolved (the semispecies). If a natural disaster occurs (e.g. , the eruption of Mt. St. Helens), the local species may go extinct in a wink of a lava flow, and the process starts again. A conventional taxonomist could spend a lifetime describing each of the hundreds of isolated stickleback populations as a species, but there seems little point in doing so. More important is understanding and appreciating the beauty of the evolutionary process - and making sure it is allowed to continue.
The rest of this chapter will be concerned primarily with the results of the process of speciation: the distribution patterns of vertebrates (their biogeography). Although a relatively small proportion of the total number of described species (48,000 or about 3%), from our point of view, they are dominant over much of the earth's surface and comprise all the species that we commonly refer to as "wildlife."
Table of Contents
1. Roots of the modern environmental dilemma: A brief history of the relationship between humans and wildlife
2. A history of wildlife in North America
3. Climatic determinants of global patterns of biodiversity
5. Natural selection
6. Principles of ecology
7. Niche and habitat
8. Conservation biology
9. Conservation in the USA: legislative milestones
10. Alien invaders
11. Wildlife and Pollution
12. What you can do to save wildlife
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