We have now seen that rate at which human-caused extinctions are occurring is different from the rate at which extinction occurred before we became so dominant.5 It should give us some pause to think that recovery from comparable extinctions in the past took 20 to 100 million years. But what are the immediate consequences of such extinctions? Geerat Vermeij [14] suggested three questions to be answered to understand the implications of extinction for human welfare.
- Which kinds of species are susceptible to extinction, and which are not?
- How will the extinction of species affect the communities in which these species lived and alter the evolutionary environment of surviving life forms?
- Can new species evolve on a human-dominated planet to replace the species that have disappeared and, if so, what will they look like?
Vanished species are unlikely to be a random subset of the biota.
- Karr (Barro Colorado) - ground-dwelling birds suffered higher rate of extinction than canopy dwellers [5].
- The proportion of species threatened differs dramatically among taxonomic groups in the United States (Table 5; [6]).
Correlates of extinction probability
- Local endemics (at whatever scale, remember Pimm and Askins)
- Low population density
- Large individual ranges
- Large body size
- High trophic level
- Endothermy - high metabolic rates
susceptibility to stress
- Biotic pollination - often associated with low population density, also susceptible to extinction of pollinator(s)
- Limited dispersal ability
- Low rate of population growth
If these speculations are correct, human caused extinctions have two properties:
- They are depleting biological diversity at a rate greater than at any time in the last 65 million years.
- The species that are going extinct are predominantly those adapted to special conditions of life in localized habitats.
The result is a more homogeneous biotic environment, one in which many of the same plants and animals are found worldwide - English sparrows, starlings, dandelions, wild oats. Most of the grasses you see on the hills of California are native to the Mediterranean. Native Californian grasslands are largely confined to serpentine outcrops. Nearly all of the vegetation in lowland areas of Hawaii is introduced from other areas in the tropics. Of the roughly 1,700 plants native to Hawaii, almost half are introduced (see [8] for a more careful analysis of this issue).
Whether it is a good thing for the character of our natural world to be changed in this way is something we must decide on non-biological grounds. Life will continue on this planet whatever we decide, but what we decide will have an enormous impact on what kind of life does survive.
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