It is difficult to find examples of extinction due to (fatal competition) natural selection alone. Charles Darwin observed that “neither the strongest nor the most intelligent necessarily survive; survival or evolution depends on response to changing conditions.” In other words, species extinction occurs when previous adaptations are no longer suitable to a changed environment. There seems to be a variable pattern of either 10my, or 26my, or 50my, or 100my, or 200my extraterrestrial-based extinction events. Differences in the severity of environmental change from the impact-related event are associated with corresponding differences in the severity of extinction. The case has been made for a constant pattern of impact-related major/massive extinction events, moderate/significant extinction events, and lesser/less-significant extinction events. The case has also been made for volcanism-related environmental change extinctions. These events also range in severity of eruption, with massive eruptions causing massive extinctions, to more frequent (constantly-occurring) eruption events causing noticeable, but small extinction events.
Environments are habitats. The degree in severity of extinction seems to parallel the degree of habitat destruction.
Scientists have found rather strong correlations between relatively recent climate change, extinction and biological speciation. In a turnover-pulse hypothesis, Elizabeth Vrba (Macroevolution, Diversity, Disparity, Contigency) suggested a decade ago that extinction is climate-based (Milankovitch cycle or tectonic-related) and extinction promotes greater possibilities for speciation. Climate dominates the environment.
Notable changes in climate cause noticeable changes in environments.
The ongoing ice age of the last 2½my was primarily caused by the photosynthetic depression of atmospheric greenhouse gases and biological carbon sink lock-up. At the present time, the Milankovitch Cycle plays the other major role in determining present climate conditions. Recall that the Milankovitch Cycle will play a major role in an upcoming notable icy Constant ExtinctionRetrospective 5555
Constant Extinction
11
56 Environmental Biology
extinction. The extent of the extinction will parallel the degree of habitat destruction. Climate change can be seen as a prolonged trend of increasingly-severe global cooling, which intensified 35mya with the rise of grasses and the corresponding drop in greenhouse gasses (notably CO2).
Recall the corresponding extra-terrestrial impact evidence from that time (34mya), as well as the evidence of a noticeable extinction event (Eocene extinction); ice formed at the Earth’s Polar Regions. There was an abrupt cooling 23mya, likely related to decreasing atmospheric levels of CO2 (cool wet grass rapidly buries carbon); this was associated with a climate-related noticeable extinction event (Oligocene). The India-Asia collision of 15mya became increasingly significant and added to the long-term trend of global cooling. Africa, previously covered by tropical rain forest, became much cooler and drier by 8mya. Global cooling was even more significant 5mya, a time of peak glaciation and another noticeable extinction event (Miocene).
Global cooling was most significant 2½mya, when the present ice age began. Earlier in this 2½my ice age, the extensive glaciation periods could remain for almost a million years. An advancing continental sheet of ice 3 km thick is a major habitat change; it wipes the slate clean of complex life in any area it occupies. It profoundly changes peripheral environments.
Nearby katabatic wind speeds may reach 200–300 km/hr. Glacial advancement crushes and freezes anything that stands in the path; melting can cause unimaginable flooding. Extensive glaciation profoundly affects sea level and available atmospheric moisture over the entire planet. With advancing glaciation, climate zones often shift faster than key residents (e.g. plants) can accommodate. Tropical environments are desiccated and repeatedly decimated (10% loss of area). Habitat destruction destroys resident populations; homeless populations cannot survive. Nothing else is more damaging to a population than the destruction of its habitat.
The most significant life change observed during the Pleistocene ice age of the past 1½ million years was species extinction (Levinton). Even in aquatic environments, shallow-water tropical invertebrates succumbed to the global cooling. The glaciations ended in the late Pleistocene (12000ya) with a final sudden cold spell and glacial advance. During the Pleistocene ice had periodically covered around 30% of the earth’s surface (partial snowball) with each 100,000yr Milankovitch cycle. The earth’s icy cap now only covers 10% of the surface, and it seems to be shrinking. The pattern that emerges here, and in at least some other intervals, is that there are more numerous, but sporatic, climate-based (noticeable but small) extinctions spaced from less than one to a few million years apart (up to about 10 million years). These climate-based small extinctions have strongly shaped the course of life up to the present. Although not as severe as the world-changing mass, moderate or lesser extinction events, sporadic, small, climate-based extinction events constantly occur, with variable
intensities. No environment is permanent; stable environments only last so long. The degree in severity of extinction seems to more or less parallel the degree of habitat destruction.
Constant extinction involves more than climate change. Shortly after the Pleistocene glaciations ended, the largest carnivorous predators, like the short-faced bear, saber-tooth cats, and dire wolves, which only ate the largest mammals (mammoths, mastodons, stag-moose, camels, horses, and other large mammals—over 100 species in North and South America), went extinct in a (mammal-record) short period of time. Any evidence of an extra-terrestrial impact is inconclusive; however, a cold spell and evidence of widespread fires in North America do allow for this possibility.
Still, Native Americans used fire frequently and extensively, modifying ecosystems to maximize human hunting and/or agricultural advantage (Mann, 1491). Widespread fires would have caused the destruction of existing environments and the creation of new environments; the largest herbivores (giant sloth-browser, mastodon-browser, mammoth-grazer of longer grasses or sedges) were vulnerable. Other grazers were attracted to fresh new growth, controlling grass-eating herbivore distribution. And as the habitat lost cover, the natural predator ambush would have been greatly compromised, making specialized predators the most vulnerable.
The extinction dates match man’s arrival; Europe, Asia, and Africa were less affected. Man’s environmental impact upon arrival in Australia 40,000ya is another dramatic example that illustrates a similar disappearance of megafauna. Relatively speaking, there has been a stable climate since the last period of Wisconsin glaciation (ending just10,000ya).
The climatic conditions of The Little Ice Age (1200–1900 AD) were irregular, but still, unusually stable. This climatic irregularity suppressed human populations, keeping human numbers well below one billion. It is said that there is supposedly a baseline “natural” extinction rate of one species per decade. Humans caused 2–3 species per decade to go extinct in the time period from 1600–1850 AD. Human populations increased to even greater numbers as The Little Ice Age ended; the past century ended with a moderate warming. This very recent moderate warming has created a climate optimum that has allowed human populations to dramatically increase over the past 100 years or so (10× over previous 500 years). Even stable environments experience ongoing population expansions and extinctions. Although it may take a very long time for development of a new species to occur, it doesn’t take very long, in our present time, to witness daily extinctions of numerous species. Humans and regional human population expansions are now pushing up to 20,000 species per year into extinction (mostly tropical—90% of all species are tropical).
Habitat destruction, exotic species introduction, genetic assimilation, pollution, disease, and inappropriate fishing or hunting of wild Constant Extinction 57
58 Environmental Biology
populations are all human-related causes of extinction today. The number of humans is still increasing, as new areas of wild habitats are occupied and/or modified. This, in turn, only increases the extinction of organisms that formerly occupied these habitats. This daily extinction is most certainly a form of constant extinction. Today’s daily eradication of environments is an accelerated form of the very low-level constant extinctions that occur throughout time, even in “stable environments.” The primary cause of extinction today is habitat degradation (destruction of ecosystems), which is itself caused by human overpopulation. The adaptive radiation following these very recent extinctions of environments is a continually-expanding human adaptive radiation.