TROPICAL RAINFORESTS: Rainforest structure

Structure and Character

Keystone Species

Hypothetical Example to Illustrate the Complexity of Rainforest Relationships and the Removal of a Keystone Species

The following is a hypothetical, over-simplified situation in a forest that contains the following species: a fruiting tree called the Zam tree, agouti, jaguar, small rodent, leaf-cutter ant, butterfly, lizard, tree frog, tree snake, beetle, bird in order to demonstrate the effect of the loss of a cornerstone species on the entire system. The forest, known as la Selva Verde, is bordered on the north and west by a half-mile wide tributary of the Amazon river and on the south and east by savanna and scrub land. The savanna is the remnants of a previous agricultural site abandoned after three years when the poor tropical soils could no longer support the crops. The agricultural site was created after a commercial mining company constructed a road to access its bauxite mine. In the savanna, a few colonists have established a small village and rely on subsistence hunting and agriculture from newly cut and forested areas to the south. The forest still covers a relatively substantial area, 50 square miles. However, much like an island, it is isolated, from the greater forest. Over time small isolated ecosystems like la Selva Verde are more vulnerable to decimation.

The intricate relationships between the aforementioned species are immeasurable and still largely unknown, but a few interactions are well studied enough to be noted. Keep in mind that for clarity, relationships in this example are oversimplified and unlikely in nature (i.e. leaf cutters would not specialize on one species of plant). The fruiting Zam tree is pollinated by a particular species of butterfly which has adapted a specialized tongue just long enough to penetrate the deep flower, so it feeds only on the nectar of the flower of the Zam tree species. In turn, a unique species of beetle lays its eggs exclusively in the feces of the butterfly.

After pollination the tree produces seed pods containing hard seeds which fall to the floor and are consumed and dispersed by the abundant agouti. When the agouti lose track of their nut stashes, the seeds germinate. The agouti, the rodent, the monkey, and the river fish populations are enough to support two adult jaguar and one sub-adult. The forest also supports the widely dispersed, sparsely populated monkeys which feed on various fruits and flowers of the rainforest canopy, but not the hard seed of the Zam tree. A small bird species, along with the tree frog, feeds on insects, including the leaf-cutter ants which exclusively use the leaves of the Zam tree. The lizard feeds only on the leaf cutter ants and the butterfly and in turn is preyed upon by the tree snake which also feeds on the frog. In addition, a large ground-dwelling rodent, also preyed upon by the jaguar, feeds on grubs it finds in rotting wood.

As apparent, the species of the rainforest are highly dependent on one another for survival. When the agouti population is reduced, in most part due to subsistence hunting by the neighboring colonists, a devastating chain reaction begins. Immediately, the three jaguars - most dependent on agouti meat for sustenance in this particular forest - leave the forested patch: one crossing the river, and two slaughtered by colonists as they wander the deforested savanna in search of game. With the loss of the jaguar, the small rodent population increases marginally and begins to eat the already rare beetle larvae, making it even scarcer. Also benefiting slightly from the loss of the jaguar are the monkeys whose population increases from 250 to 300 individuals. So after a decade the only species lost from the forest are the jaguar and the agouti.

Sixty years down the road, the forest has become a reserve in a country-wide effort to protect national biodiversity and is safe from further human interference. However, all is not well in la Selva Verde rainforest: the Zam tree, without the agouti, is unable to disperse its seeds which simply die in the shade of the canopy and one night, six decades later, the last remaining Zam tree falls during a typical tropical thunderstorm. The tree was already weakened by age and an unusually dry season perhaps induced by local deforestation. The Zam tree falls without successfully reproducing.

The death of the tree is responsible for the downfall of other species that were dependent on the tree for survival. Without their precious leaves, the leaf cutter ants decline and the colony disappears as the population drops below the minimum threshold. The butterfly, specialized for the pollination of the tree's flower, also perishes causing the beetle to become extinct. The loss of the butterfly, the beetle, and the ant species spells doom for the lizard whose extinction causes the disappearance of the lizard-eating tree snake which cannot subsist solely on tree-frog feeding. Benefiting from the loss of their main predator, the adaptable frog population skyrockets not affected by the disappearance, unlike the lizard, of the other three insect species. The bird population holds steady, despite the loss of insects, feeding on other forest insects. The net result from the elimination of a single keystone species, the agouti, meant the eventual extinction of at least eight species from the forest patch.

Although the forest in this example was purely hypothetical, it illustrates what is actually occurring in isolated forest patches across the world. More than two-thirds of the world's rainforests exist as such fragmented remnants under 1 million acres (400,000 ha). It may serve as a global model, as worldwide, species disappear. Some of these species may be cornerstone species, whose disappearance could cause the forest system to crash. We can only speculate what would be the effect of such a loss.

Continued: Rainforest biodiversity

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