Indri lemur in Madagascar. Click image for more indri photos. (Photo by R. Butler)
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DEFORESTATION AND EXTINCTION
By Rhett Butler | Last updated July 22, 2012
The greatest loss with the longest-lasting effects from the ongoing destruction of wilderness will be the
mass extinction of species that provide Earth with biodiversity. Although great extinctions have occurred in the
past,
none has occurred as rapidly or has been so much the result of the actions of a single species. The extinction rate of today may
be 1,000 to 10,000 times the biological normal, or background, extinction rate of 1-10 species extinctions per
year.
So far there is little evidence for the massive species extinctions predicted by the
species-area curve in the chart below. However, many biologists believe 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 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.
EXTINCTION ESTIMATES MADE IN THE 1990s
| 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) |
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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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RECENT NEWS
For the most recent news article about extinction and biodiversity loss, check out The extinction blog
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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.
Estimates of species loss each year range greatly as shown by this table.
+ More information on extinction
Display articles >>
Review questions:
- Why is there a "lag time" for species extinction?
- Why do small populations have a lower probability of survival?
- How might climate change impact global biodiversity?
- Why are frogs dying around the world?
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WEEKLY NEWSLETTER
INTERACT
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Recent news
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