articlehighlights

The current extinction crisis, if unchecked, will disrupt evolution to a degree that:

• earth will see a proliferation of pests and a decline of large mammals
• the tropics will no longer be powerhouses for the evolution of new species
• the biodiversity losses will persist for millions of years

May 2001

The Toolache Wallaby (Macropus greyi) went extinct in Australia in 1943. Source: John Gould, F.R.S., Mammals of Australia, Vol. II Plate 19, London, 1863.

Human activities have brought the Earth to the brink of biotic crisis. Many biologists consider that coming decades will see the loss of large numbers of species^1-5… these extinctions

• will alter not only biological diversity
• but also the evolutionary processes by which diversity is generated

A simple consideration of time underscores the magnitude of the challenge to scientists and public alike⁶. Episodes of mass extinction documented in the geological record were followed by protracted intervals of rediversification and ecological reorganization; five million years can be considered a broadly representative recovery time, although durations varied from one extinction to another⁷. Suppose, too, that the average number of people on Earth during the recovery period is 2.5 billion (by contrast with the 6 billion today). Under these conditions, the total number of people affected by what we do (or do not do) during the next few decades will be in the order of 500 trillion. We are thus engaged in by far the largest “decision” ever taken by one human community on the unconsulted behalf of future societies.

The Core Concept

One of the first truisms absorbed by biologists is that evolution is not predictable. However, despite our inability to predict the products of evolution… we can make meaningful estimates about evolutionary processes as they will be affected by the depletion of biological diversity. [In other words] we may have little basis for predicting what large mammals might look like two million years from now, but much better reason to suppose that there will be very few of them.

The evolutionary dimension to the current biotic crisis has been vividly expressed by Michael Soule⁸: “Death is one thing, an end to birth is something else…” At least as important will be the alteration of evolutionary process, and for a period that is difficult to estimate but must surely measure in millions of years.

First-Order Effects: There will be several first-order effects stemming from the biotic crisis:

• a major extinction of species within the foreseeable future, estimated by some to remove between one-third and two-thirds of all species now extant^1,2,5,9

• a mega-mass extinction of populations, proportionately greater than the mass extinction of species, within the foreseeable future¹⁰

• alien invasions and other mixings of biotas^11-14

• progressive depletion and homogenization of biotas, with potential threshold effects on ecosystems^15,16

• biotic impoverishment generally, possibly including a decline of global biomass,^16-18

• gross reduction if not virtual elimination of entire sectors of some biomes, notably tropical forests, coral reefs, and wetlands, all of which have served as centers of diversification in the past^19-22

Further Evolutionary Effects: These first-order impacts will likely engender a series of further consequences, including although not limited to:

• fragmentation of species’ ranges, with disruption of gene flow^23-26

• decline in effective population sizes, depletion of gene reservoirs/pools^10,27,28

• biotic interchanges — introducing species and even biotas into new areas^11,14,29

• these impacts, in turn, might disrupt food chains/webs, symbioses, or other biological associations^30,31

These consequences could lead to further repercussions such as the following:

• An outburst of [select] speciation: It is unlikely that speciation will be evenly distributed among surviving lineages; it may be concentrated among particular clades or ecological types that thrive in human-dominated ecosystems [e.g., rodents]^32,33

• Depletion of “evolutionary powerhouses” in tropics: According to Jablonski²⁰, the tropics have been “the engine of biodiversity” for at least 250 million years. Today, we face the prospect of severe depletion if not virtual elimination of tropical forests, wetlands, estuaries, coral reefs, and other biomes, with their exceptional biodiversity and ecological complexity. Because some of these biomes appear, in some senses at least, to have served in the past as preeminent “powerhouses” of evolution^34,35, their decline could entail severe consequences for rediversification as the biosphere emerges from environmental crisis.

• Decline of biodisparity: Elimination of species is not the only measure of an extinction event. There can be declines, as well, in biodisparity [morphological and physiological variety].^36-38

• An end to speciation of large vertebrates: Even our largest protected areas will prove far too small for further speciation of elephants, rhinoceroses, apes, bears, and big cats, among other large vertebrates^28,39,40.

Lessons from the Past?

The geological record is replete with extinction events, their intensity ranging from the small and local to global mass extinctions that shattered Earth’s biological order. Inevitably, extinctions were followed by rediversification, directed in the case of the largest events by ecological reorganization.

The geologic record contains much evidence of bounce-back processes^38,41-46, but … In the present biotic crisis, it is hard to envision a scenario under which the factors that are driving the biosphere toward grand scale biodiversity loss will be mitigated in the wake of such loss. On the contrary, on any time scale we can envisage (and any scenario that does not involve early mass mortality for humankind), the situation becomes bad and then stays bad for some time to come. Thus, on the time scale of the human species, environmental disruption (or at least aspects of it) is permanent. Under these circumstances… the prospects for rediversification are limited.

Conservation Responses

• Should we be content simply to safeguard as much as we can of the planetary stock of species?
• Or should we pay equal if not greater attention to safeguarding evolutionary processes at risk?^47-49

Is it sufficient for us to maintain, for example, just the two elephant species we already have, or should we try to keep open the evolutionary option of further elephant-like species in the distant future?

This is an unusually significant question, with unusually significant implications for conservation strategies. Elephants, along with many other large mammals, are inclined to move around a good deal, a trait that enables them to maintain gene flow across large areas. As a result, their gene pools often tend to be fairly uniform [an elephant in East Africa may not be so different from one 4,000 km away in South Africa⁵⁰. Regrettably the remaining populations of elephants, substantial and extensive as they are, albeit fragmented and declining fast, are probably already below the minimum numbers to keep open the possibility of speciation⁵¹.

In marked contrast to elephants, with their slow breeding rates, many insect species have immense breeding capacities and rapid turnover rates. These latter attributes offer quick adaptability to environmental shifts, whereupon genetic changes are passed along promptly. These attributes not only leave many insect species well suited to survive the environmental upheavals of human activities, but they offer exceptional scope for speciation in comparatively short order. By contrast, elephants, together with other large-bodied species that reproduce slowly and hence possess restricted capacity for genetic adaptation, will be at an extreme evolutionary disadvantage. Does this factor imply that they should therefore receive all of the greater attention from conservationists or that, in a triage situation, they should rank lower in our priorities? Although this is a fundamental question, it has hardly been addressed.

These, then, are some of the issues that we should bear in mind as we begin to impose a fundamental shift on evolution’s course. We are “deciding” on evolution’s future in virtually a scientific vacuum — deciding all too unwittingly, but effectively and increasingly.

© 2001, National Academy of Sciences. Excerpts from paper “The biotic crisis and the future of evolution” published in Proc. Natl. Acad. Sci. USA, Vol. 98, Issue 10, 5389-5392, May 8, 2001. Reprinted with permission. See reprint policy.

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BioScience Article

“Global Conservation of Biodiversity and Ecosystem Services.”
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http://caliber.ucpress.net/doi/abs/10.1641/B571009

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