BIODIVERSITY: AN INTRODUCTION

BIODIVERSITY: AN INTRODUCTION
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Warren Y. Brockelman

OVERVIEW

“Biodiversity” is a term which covers all the systems of living things, including DNA, genes, individuals, species, communities, and ecosystems. All systems are necessary for life to persist on Earth. The recent concern about biodiversity stems mainly from the realization that the Earth is in the midst of a major spasm of species and habitat extinction, and the promise that biotechnology and genetic engineering hold for the improvement of our lives and our very survival, provided that we save as many species as possible.

The first living things, probably bacteria-like anaerobic heterotrophs, probably arose as early as 3.8 billion years (3.8×10⁹ years) ago, early in the history of the Earth, when physical and chemical conditions became suitable. The earliest primary producers were aquatic cyanobacteria whose fossils appear about 3.6 billion years ago. When planktonic plants became common in the seas, they started producing the free oxygen that accumulated in the atmosphere and made possible more anaerobic metabolism.

Vascular plants, amphibians and early reptiles evolved into diverse types after about 300 Mya (million years ago), and increased in size . Dinosaurs dominated the Earth from 245 Mya to 65 Mya, at which time a large extinction event occurred, probably caused by collision of a large asteroid with the Earth. At least four other major extinction events are evident in the fossil record before that time, when most of the species on Earth disappeared. Between such catastrophes, extinction occurs naturally at a more steady rate of about 1 species per million per year.

How many species are there on Earth at present? Estimates now range from 5 to over 15 million species, but most species in tropical regions have not been described or scientifically studied. Only about 1.5 million species have been given valid scientific names. About 2/3 of all species are insects; vertebrates comprise only about 50,000 species and vascular plants about 270,000. Our knowledge of invertebrates, fungi and microorganisms is still very limited.

At present, habitat destruction and pollution are causing thousands of species to disappear per year, at a rate which is more than 1000 times the natural background rate. It is becoming apparent that the majority of species will become extinct before we will be able to study or use them. Humans began causing extinctions of large mammals and birds at least 50,000 years when they colonized Australia, and later the New World, Madagasgar, New Zealand, and the South Pacific Islands.

While extinction may be a rapid process, the evolution of new species and whole communities is not; it requires millions of years. Evolution involves changes in populations through time, and also speciation—the divergence of a population into two separate lineages. Speciation requires separation and isolation of parts of a population sufficiently long to allow natural selection or random genetic changes to result in two distinct populations, which will not normally interbreed with one another in nature.

The early naturalists and philosophers envisioned living forms as fixed types. Charles Darwin realized, however, that species populations must contain individual variation that is heritable in order for evolution by natural selection to occur. We now understand that this variation is caused by changes (mutations) in the nucleotides of DNA which makes up genes, and by chromosomal rearrangements.

By measuring the amount of difference in DNA between species and higher groups, we are able to date with reasonable accuracy the times of divergence between groups. This is allowing us to increase the precision of our phylogenetic classification systems, which now need to be revised in the light of new molecular evidence.

The study of biodiversity at the species level and above involves the study of populations that exist in their ecological communities. Their interactions determine the stability and diversity of communities: herbivores and plants, predators and prey, competitors, trees and seed dispersers, pollinators and plants, etc. High diversity has been claimed to lead to increased stability, but it is more likely that both stability and diversity are maximized by long periods of evolution in communities, during which species “coevolve,” resulting in higher ability to find resources, avoid severe competition and avoid extirpation by predators and grazers.

Species diversity in local areas can be modeled as an equilibrium which represents a balance between local extinction, and colonization of immigrants as new species disperse in from outside the areas. Although the model is an oversimplification, it has considerable support from studies of faunas on islands, and species diversity in isolated patches forest which behave like islands.

Extinction rates will increase as forests become fragmented and species population sizes become smaller. As populations become isolated, dispersal between them is reduced. The species at most risk of extinction are those that have limited geographic ranges, low rates of population increase, poor dispersal ability, low natural density, and those that are hunted for their valuable horns, ivory, antlers, bones, skin, meat, etc.

It is now highly likely that we will lose at least half the species on earth in the next few centuries due to tropical forest destruction and fragmentation, hunting, pollution, and damming and canalization of rivers. We should attempt to save as many species as possible because many are directly useful to us, maintain ecosystems on which we depend for survival, and because it is simply wrong for us to destroy other species. With the extinction every species, unique genetic information and variability is lost that is potentially useful to humans, information which cannot be recreated.

We can slow the extinction of species by many different methods. The major goals that are being pursued are: reduction of human population increase, conservation of natural ecosystems in “protected areas,” reduction of rural poverty, special breeding programs for critical species, reduction of pollution and disturbance of rivers, and diversification of forestry and agriculture to reduce our dependence on monocultures.

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