The Real Biodiversity Crisis
Returning to Normal?
Some ecologists believe that species diversity will not rebound significantly as the natural environment becomes permanently impoverished. Vast tracts of wilderness, for example, may vanish from the earth in the not-so-distant future. If ecological diversity is lost in this way, some conservation biologists argue, speciation rates will be lowered permanently. And because species diversity represents a dynamic equilibrium between extinction and speciation, a lowered rate of speciation would undoubtedly create a lower equilibrium in species number. Thus some foresee a persistent speciation crisis and a plummeting number of species as the main threat to biodiversity in the long term.
Although this argument may hold for, say, the bigger mammals, which presumably require large areas for speciation, it is unlikely to apply to the many terrestrial organisms that are immobile or have small home ranges—in particular, plants and many invertebrates. Indeed, there is ample reason to believe that such life forms may not suffer in the long run. After all, fragmentation of their currently broad, continuous ranges may actually promote speciation rather than retard it. How so? Breaking up habitats will create populations that are isolated from one another, reducing the level of gene flow. Further, population sizes will be reduced, quickening the pace of genetic drift. The combination of these two effects provides an ideal template for speciation.
Species numbers may ultimately rebound for a second reason as well: because the disturbances people produce need not always decrease ecological diversity. In many cases, human actions may merely alter the character of habitats rather than eliminate them. That process might, in fact, foster speciation.
The study of island biogeography offers considerable support for this notion. Islands tend to have depauperate biota relative to continents, so ecological opportunity and thus rates of speciation are higher on islands than on continents. This has been seen for Hawaiian plants in the silversword family, for finches, for honeycreepers, for sparrows and for fruit flies. The link between rates of speciation and ecological opportunity is also apparent when one compares younger islands with older islands in the same archipelago. Older islands have more species than younger ones and so may offer less ecological opportunity. Consequently, speciation rates for older islands are lower than for younger ones. This relation has been clearly demonstrated for Hawaiian plants: Speciation rates on the youngest island in the chain, the big island of Hawaii, are 10 times greater than on the oldest island, Kauai.
Just as with the emergence of a new volcanic island from the sea, episodes of mass extinction offer particularly rich opportunities for the surviving groups. For example, the mass extinction at the end of the Cretaceous (65 million years ago) brought an end to the dinosaurs, but a massive diversification of mammals took place soon afterward. Similarly, with the dramatic decline of marine eurypterids (large arthropods) about 410 million years ago, the first large marine predators were lost. However, they were later surpassed in this role by certain fish, notably the placoderms, a highly diverse group that sported interlocking plates of armor. The placoderms underwent a spectacular radiation during the Devonian (between 410 and 360 million years ago), but at the end of this period all placoderms—large, small, marine and freshwater—went extinct. They were replaced by the actinopterygians (ray-finned fishes), which ultimately produced the teleosts, the dominant group of modern fish.