One of the first truisms absorbed by biologists is that evolution is not predictable. We can no more predict the future composition of communities than some Ordovician ecologist could have foreseen the Great Barrier Reef. However, despite our inability to predict the products of evolution
the trajectories of future morphologies or the innovations of future physiologieswe can make meaningful estimates about evolutionary processes as they will be affected by the depletion of biological diversity. 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 Soule (10): "Death is one thing, an end to birth is something else." In other words, impending extinctions will be far from the full final outcome of current environmental disruption. 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: (i) 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, 11); (ii) a mega-mass extinction of populations, proportionately greater than the mass extinction of species, within the foreseeable future (12); (iii) alien invasions and other mixings of biotas (13-16); (iv) progressive depletion and homogenization of biotas, with potential threshold effects on ecosystems (17, 18); (v) biotic impoverishment generally, possibly including a decline of global biomass (18-20); and (vi) 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 (21-24).
Further Evolutionary Effects. These first-order impacts will likely engender a series of further consequences, including although not limited to: (i) fragmentation of species' ranges, with disruption of gene flow (25-28); (ii) decline in effective population sizes, with depletion of gene reservoirs/pools (12, 29, 30); and (iii) biotic interchanges introducing species and even biotas into new areas, with multiple founder effects and novel competitive and other ecological interactions (13, 16, 31). These impacts, in turn, might disrupt food chains/webs, symbioses, or other biological associations (32, 33).
These consequences could lead to further repercussions such as the following six:
An outburst of speciation. As large numbers of niches are vacated, in conjunction with a splitting off of disjunct populations through habitat fragmentation, there may well be an outburst of speciation, even of adaptive radiation, albeit not remotely on a scale to match the extinction spasm (34-36). 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 (37, 38)?
Proliferation of opportunistic species. r-selected and generalist species, often appearing as opportunistic species, may proliferate, especially if there is preferential elimination of K-selected species that include natural controls of r-selected populations (32, 38). Could this proliferation lead to what has been characterized as a "pest and weed" ecology (39, 40)?
Depletion of "evolutionary powerhouses" in the tropics. Virtually every major group of vertebrates and many large categories of invertebrates and plants originated in spacious zones with warm, equable climates (41, 42). In addition, tropical species appear to have persisted for relatively brief periods of geologic time, implying high rates of evolutionary turnover and episodes of explosive speciation (21, 43, 44). According to Jablonski (22), 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 (45, 46), 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, the biota's manifest morphological and physiological variety (47-49). Biodisparity impoverishment can be assessed through the surrogate measure of loss of higher taxa or guilds, and, over the past 2000 years, the preferential elimination of species-poor genera has reduced biodisparity at rates even greater than those of species loss (48). Will the same pattern of non-random culling persist in the future?
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 (30, 50, 51). What knock-on consequences and ripple effects could there be for smaller species, indeed for biotas as a whole given, for example, the depauperizing impacts of the present-day decline of elephants (52)?
Emergent novelties. There may be many emergent novelties, although these are especially difficult to predict. For instance, there could be an explosive radiation within certain higher taxa, notably small mammals and insects able to thrive in human-dominated ecosystems. The question is not whether persistent lineages can evolve in unexpected ways, but rather to what extent the environmental constraints humans place on surviving populations will channel innovations toward properties we associate with pests.
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