Historic mass extinctions

So far there is no evidence for the massive species extinctions predicted by the species-area curve in the chart below. However, it is possible that species extinction, like global warming, has a time lag, and the loss of forest species due to forest clearing in the past may not be apparent yet today. Ward (1997) uses the term "extinction debt" to describe such extinction of species and populations long after habitat alteration:

Decades or centuries after a habitat perturbation, extinction related to the perturbation may still be taking place. This is perhaps the least understood and most insidious aspect of habitat destruction. We can clear-cut a forest and then point out that the attendant extinctions are low, when in reality a larger number of extinctions will take place in the future. We will have produced an extinction debt that has to be paid. . . We might curtail our hunting practices when some given population falls to very low numbers and think that we have succeeded in "saving" the species in question, when in reality we have produced an extinction debt that ultimately must be paid in full. . . Extinction debts are bad debts, and when they are eventually paid, the world is a poorer place.

For example, the disappearance of crucial pollinators will not cause the immediate extinction of tree species with life cycles measured in centuries. Similarly, a study of West African primates found an extinction debt of over 30 percent of the total primate fauna as a result of historic deforestation. This suggests that protection of remaining forests in these areas might not be enough to prevent extinctions caused by past habitat loss. While we may be able to predict the effects of the loss of some species, we know too little about the vast majority of species to make reasonable projections. The unanticipated loss of unknown species will have a magnified effect over time.

The process of extinction is enormously complex, resulting from perhaps hundreds or even thousands of factors, many of which scientists (let alone lay people) fail to grasp. The extinction of small populations, either endangered or isolated from the larger gene pool by fragmentation or natural barriers like water or mountain ranges, is the best modeled and understood form of extinction. Since the standard was set by MacArthur and Wilson in their masterwork The Theory of Island Biogeography (1967), much work has been done modeling the effects of population size and land area on the survival of species.

The number of individuals in a given population is always fluctuating due to numerous influences, from extrinsic changes in the surrounding environment to intrinsic forces within a species' own genes. This population fluctuation is especially a problem for populations in isolated forest fragments and species that are critically endangered throughout their range. When a population falls below a certain number, known as the minimum viable population (MVP), it is unlikely to recover. Thus the minimum viable population is often considered the extinction threshold for a population or species. There are three common forces that can drive a species with a population under MVP to extinction: demographic stochasticity, environmental stochasticity, and reduced genetic diversity.

Demographic stochasticity involves birth and death rates of the individuals within a species. As the population size decreases, random quirks in mating, reproduction, and survival of young can have a significant outcome for a species. This is especially true in species with low birth rates (i.e. some primates, birds of prey, elephants), since their populations take a longer time to recover. Social dysfunction also plays an important role in a population's survival or demise. Once a population's size falls below a critical number, the social structure of a species may no longer function. For example many gregarious species live in herds or packs which enable the species to defend themselves from predators, find food, or choose mates. In these species, once the population is too small to sustain an effective herd or pack, the population may crash. Among species that are widely dispersed like large cats, finding a mate may be impossible once the population density falls below a certain point. Many insect species use chemical odors or pheromeres to communicate and attract mates. As population density falls, there is less probability that an individual's chemical message will reach a potential mate, and reproductive rates may decrease. Similarly, as plant species become rarer and more widely scattered, the distance between plants increases and pollination becomes less likely.

Environmental stochasticity is caused by randomly occurring changes in weather and food supply, and natural disasters like fire, flood, and drought. In populations confined to a small area, a single drought, bad winter, or fire can eliminate all individuals.

Reduced genetic diversity is a substantial obstacle blocking the recovery of small populations. Small populations have a smaller genetic base than larger populations. Without the influx of individuals from other populations, a population's genome stagnates and loses the genetic variability to adapt to changing conditions. Small populations are also prone to genetic drift where rare traits have a high probability of being lost with each successive generation.

The smaller the population, the more vulnerable it is to demographic stochasticity, environmental stochasticity, and reduced genetic diversity. These factors, often working in concert, tend to further reduce population size and drive the species toward extinction. This trend is known as the extinction vortex. See the box on the right for an example of an extinction vortex.

Some mathematical ecologists have suggested that population fluctuations may be governed by properties of chaos making the behavior of the system (the fluctuation of a species's population size) nearly impossible to predict due to the complex dynamics within a given ecosystem.

CURRENT EXTINCTION ESTIMATES Estimate and Method of estimation % Global Loss per decade 10 million sp. Annual Loss 30 million sp. Annual Loss Source 0.2-0.3% annually based on tropical deforestation rate of 1% annually 2-3% 20,000-30,000 60,000-90,000 Wilson (1989, 1993) 2-13% loss between 1990 and 2015 using species area curve and increasing deforestation rates 0.8-5.2% 8,000-52,000 24,000-156,000 Reid (1992) Loss of half the species in the area likely to be deforested by 2015 8.3% 83,000 250,000 Raven (1988) Fitting exponential extinction functions based on IUCN red data books 0.6-5% 6,000-50,000 18,000-150,000 Mace (1994)

Tropical species are not only threatened directly by deforestation, but also by global climate change. Even if species survive in protected reserves, they may perish as a result of rising ocean levels and climactic changes. Many tropical species are used to constant, year-round conditions of temperature and humidity. They are not adapted to climate change even if it is as small as 1.8F (1C). Changes in seasonal length, precipitation, and intensity and frequency of extreme events that could occur should the Earth warm may strongly impact biodiversity in seasonal tropical forests and cloud forests. Studies show that unusual weather conditions—such as those under el Niño and la Niña—can cause population fluctuations of many forest animals. Should the frequency and intensity of such extreme events reach the level where whole populations are unable to recover to their normal level between events we could see localized extinctions and serious changes in the ecosystem. Climate changes could especially impact some sensitive ecosystems like cloud forests, which would be drastically affected by any lifting of the cloud cap. One often-overlooked consequence of increased temperatures is the spread of disease among wild animals. For example, there is a good chance that avian malaria and bird pox will be spread to Hawaiian upland forests by mosquitoes currently limited to elevations below 4,800 feet (1,500 m) due to temperature constraints. The spread of these diseases to upland forest would probably mean the extinction of several endangered bird species.

Many forest communities have survived global climate change in the past by "migrating" north or southward. However, today, because of fragmentation and human development, there are few corridors of wild territory for migration. Highways, parking lots, plantations, housing developments, and farms impede the slow, but steady movement necessary for many communities to survive changing climate conditions. Unable to escape the changes, many species within these communities will have to cope or face extinction. One of the contributing factors to the worldwide decline in amphibian populations may be the gradual climate change over the past 100 years, which when coupled with the increase in UV-B radiation, may have weakened their defense to a previously harmless fungal infection. This fungus has been detected on dead or dying frogs in locations around the world.

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Global climate change may have had an impact on the extinction of North American megafauna at the end of the ice age some 10,000 years ago. One of the leading theories for the demise of these mammals—which included such wild beasts as giant sloths, mammoths, sabertooth cats, and oversized horses and rhinos—is that habitat fragmentation, caused by global climate change, split species into small populations, making them more vulnerable to extinction. As the last glacial interval came to a close and the great ice sheets receded, an additional factor came into play: the presence of hungry human hunters. Models (the Moisimann and Martin model of 1975, amended by Whittington and Dyke in 1989) suggest that by merely killing off 2 percent of the mammoth population every year, year after year, the entire species would be doomed to eventual extinction some three or four centuries down the road. These natural (climate change) and unnatural (human) influences working in concert surely condemned to extinction some of the most magnificent creatures ever seen by man. Today we are facing a similar situation, only this time we may be responsible for both factors, the global climate change and the overexploitation.

Extinction of a large number of species is highly likely because of the intricate relationships between species. David Quammen (1981) explains:

The educated guess is that each species of plant supports ten to thirty species of dependent animal. Eliminate just one species of insect and you may have destroyed the sole specific pollinator for a flowering plant; when that plant consequently vanishes, so may another twenty-nine species of insects that rely on it for food; each of those twenty-nine species might be an important parasite upon still another species of insect, a pest, which when left uncontrolled by parasitism will destroy further whole populations of trees, which themselves had been important because . . .

The complexity of the rainforest makes it impossible to anticipate when and what species will disappear.

Besides losing unique species that have lived on the planet for longer than we have and have every right to exist as we do, we are losing an incredible pool of genetic diversity which we could harness to help our own kind. As each species is lost, a unique combination of genes which has been produced over the course of millions of years, is lost and will not be replaced during our time. We head toward a future impoverished of the magnificent beasts that we remember learning about as children: ferocious tigers; armored rhinos; brilliant macaws; colorful frogs and toads. As these species vanish from the globe, the world is truly a poorer place. E.O. Wilson, one of the greatest biologists of our time, estimates that a 20 percent extinction rate of all species is possible by the year 2022 (Wilson 1992). Estimates of species loss each year range greatly as shown by this table.

Biologist Alfred Wallace on Biodiversity Loss

• Where are rainforests located?