TROPICAL RAINFORESTS: Disappearing Opportunities
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, poor farmers and colonists encouraged by a government settlement policy clear a patch of degraded forest land for subsistence agriculture. With them, the poor farmers bring livestock including chickens, closely related to jungle-fowl, and dogs. The chickens harbor an avian 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 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.
Continued: Extinction