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The Extinction Vortex
To clarify the concept of an extinction vortex, take a hypothetical example of a population ground-nesting jungle-fowl
from Southeast Asia. Throughout their range, their native forests have been fragmented by logging and human development.
Some fragments of forest are still large enough to support seemingly healthy, but small populations of jungle-fowl.
In one location a population of 60 birds capable of reproduction, 30 females and 30 males, appears to be thriving.
Every year an average hen successfully rears one chick that survives infancy, for a total of 30 new adult birds
added to the population, while 30 birds (50% of the adult population) are consumed by predators or hunted by man.
Time Elapsed
Major Events
Population
Initial
fragmentation
60
1 year
drought
42
2 years
flu
21
3 years
dogs
16
4 years
chance
8
5 years
5
:
11 years
3
12 years
2
13 years
0
The extinction vortex begins simply enough with an unusually long dry season. The drought takes its toll on the
jungle-fowl population, especially the young. By the time monsoon rains begin, the population stands at 45: 20
females, 25 males. Over the course of the next year, the females have the same number of average young, one each,
to give a total of 20. But 23 (about 50%) junglefowl are consumed in the course of the year leaving the population
standing at 42: 17 females, 25 males.
At this time, peasants encouraged by a government settlement policy clear a patch of degraded forest land for subsistence
agriculture. With them, the peasants bring livestock including chickens, closely related to jungle-fowl, and dogs.
The chickens harbor an avarian flu, to which they are immune, which quickly spreads among the junglefowl population
of the nearby forest fragment. The flu decimates the jungle-fowl; at year end 21 remain: 9 females, 12 males.
The presence of dogs increases predation of the jungle-fowl to 66% of the adult population every year so after
another year the population stands at just 16: 6 females, 10 males.
The population of females is steadily declining either coincidently or perhaps because females are not as agile
as males in escaping predators. In any case, demographic stochasticity spells the beginning of the end for the
jungle-fowl. It might be the impact of the dogs disturbing nest sites or maybe just a stroke of bad luck for the
jungle-fowl: only 3 females reproduce in the next year. By the end of the year the population is down to 8; 2 females,
6 males.
Now it is probably only a matter of time before the species goes extinct in the forest fragment. With the presence
of dogs, the death rate of the jungle-fowl exceeds their birthrate. By the end of 5 years the population is down
to 5: 2 females, 3 males. As the population density is low, predation rates by dogs drop slightly and the adult
population of birds fluctuates between 4 and 8 for the next several years. However the species is on the brink
of disappearing. All it takes now is one chance event.
The end comes quite rapidly. The population dips to 3, and the single remaining adult female is crushed by a tree
limb that falls during a thunderstorm. Neither of her three chicks survive without her protection. The two male
birds wander the forest looking for the mate they will never find. After the final male jungle-fowl is eaten by
a python, the species is extinct from the forest patch.
The final death of the jungle-fowl of the Asian forest patch is not primarily responsible for the extinction of
the species. The extinction of the jungle-fowl was made possible by its isolation. The small population became
more susceptible to chance events that further reduced its size and made it more susceptible other factors.
"Rainforest" is used interchangeably with "rain forest" on this site. "Jungle" is generally not used.
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Between June 2000 and June 2008, more than 150,000 square kilometers of rainforest were cleared in the Brazilian Amazon. While deforestation rates have slowed since 2004, forest loss is expected to continue for the foreseeable future. This is a look at past, current and potential future drivers of deforestation in the Brazilian Amazon.
