Extinction is not new. It has occurred continuously since life began, and there have been a number of extinction crises in the past. However, the current paroxysm is the first great extinction brought about by human activity (although we suspect that prehistoric humans were involved in the extinction of a number of large mammal species, and perhaps some species of fish and shellfish) (See Part I, D1.) The “natural” rate (i.e., the rate not affected by human activity) is about one species per year. The current rate is much higher, but since we do not know how many species there are, it is impossible to know with exactitude how many are lost annually.
Tropical rainforests are, with coral reefs, the most endangered ecosystems in the world, and are vulnerable to a substantial loss of biodiversity. Deforestation depletes biodiversity by destroying habitat, by separating contiguous areas of rainforest from each other, by interfering with plant reproduction, and by exposing organisms of deep forest to “edge” effects (Part II, G5). Logging does not simply remove a few trees from the forest. When canopy trees are cut, many smaller forest trees and plants dependent upon them for shade or support or moisture vanish. Animals dependent upon trees or other vegetation for food, shelter, water, and breeding sites also disappear. Only those animals (generally the larger ones) which can migrate to contiguous forest areas survive. Plants in cut-over areas often cannot be pollinated, or if they are, their seeds fall upon unsuitable open areas where they cannot survive. In addition, many rainforest species are restricted to relatively small areas and are found no where else. When the areas in which these species reside are logged or burned, they will disappear.
Logging activities push roads into previously untouched forest. Even if logging is selective, the logging roads and tracks made to pick up the cut trees cause damage to soil and plant life. Erosion is common along these roads. And since roads create openings where none previously existed, the forest now has a long “edge,” where conditions are very different from a normal forest interior (there is more light, temperatures are higher, humidity is lower). As we have seen, species adapted to the conditions within a forest interior cannot survive along the newly-formed edges. If areas are clear cut, the problems are exacerbated. (For a more detailed discussion of these issues, see Part VII; Part II G5f.) Additionally, the loggers hunt animal populations previously untouched. And the roads act as conduits into the forest where people can penetrate relatively easily. People follow the logging roads in search of agricultural or grazing lands, and they deforest more areas along these roads. And they, too, hunt, for subsistence and for sale of “bushmeat” to the burgeoning cities of the tropics. Modern tools make these colonists much more efficient than formerly, for they now have shotguns, chainsaws, and outboard motors.
Ancillary consequences of deforestation are common. Pollution, silting and other damage which results from logging lead to declines in biodiversity. In Malaysia, more than half of river fish species disappeared after logging activities, for example (Ryan, 1992). Amphibian populations in tropical areas have declined or disappeared under the multiple stresses of habitat loss, pollution and disease.
Many of the species being lost are vital to the preservation of our ecosystems, or to significant parts of them. Others may have potential value to humans in terms of food or medicine. It is shocking and surprising to learn that 85% of the food which humans eat comes from only 20 types of plants, and that two-thirds of it comes from just three: rice, maize and wheat. Humans must find new sources of food so that, should some pestilence strike one of these vital crops, famines would not occur. Some of these food sources might yet be found among the species which are thoughtlessly being extinguished in rainforests.
As has been pointed out, biodiversity losses of today are comparable to the great mass extinctions of eons past. After the Cretaceous extinction, 65 million years ago, it took between five and ten million years for mammals and coral reefs, and 25 million years for other marine organisms to regain species richness equivalent to previous eras. After the Permian extinction (250 million years ago), when half of the marine invertebrate families were lost, twenty million years were required for recovery (Myers, 1988a). Recovery from the current human-induced extinction will doubtless require even more time, since we are eliminating both animals and habitat. Where can new large species evolve? Most endangered now are large mammals, since they are the easiest to hunt, the lowest in numbers, and require the largest habitats and ranges. The organisms which will remain will be a more homogeneous lot, “generalists” like humans, organisms which can survive in contact with humans and in the chinks we have left for them. They need to be able to reproduce rapidly, disperse widely, and live under difficult conditions. They must be “weeds!” We are selecting for rats, cockroaches, invasive grasses – all of which are tough and can live in disturbed environments. These “weedy” species represent only a small fraction of the rich variety of organisms which exist today (but there are fewer every day). They will soon form a much higher proportion of existing organisms, as the gene pools which allow for evolution are rapidly shrinking.
Examples of biodiversity loss
a. The case of mahogany(Swietenia macrophylla):
Mahogany historically has been logged unsustainably, with timber companies and loggers extracting all available trees and moving to new areas when the local supply becomes exhausted. By 1735, mahogany had become rare in Jamaica and loggers moved on to Central American countries for their supplies. Two species of mahogany (the Caribbean and Pacific coast mahoganies) are now virtually extinct; the supply of mahogany now comes from a third, South American species. This species will no doubt go the way of the others unless incentives for its preservation are provided, since logging in the tropics is done with little government oversight and few incentives to operate sustainably. CITES (the Convention on International Trade in Endangered Species of Wild Fauna and Flora, an international agreement which regulates international trade in specified species of endangered plants and animals) and regulatory bodies have been of use in the protection of certain species of animals, but not of plants and trees. Involved countries (which profit from mahogany trade) have blocked the listing of mahogany by CITES several times, although mahogany regenerates slowly, logging of this species is virtually unregulated, and the mahogany supply is dwindling rapidly (Gullison, et al., 2000). Boycotts of mahogany products have been effective locally but not globally, and have not provided sufficient incentive for preservation. Very recently, CITES, bowing to great international pressure, has listed mahogany as an endangered species.
b. The case of Borneo:
Here are the results of research on the changes in biodiversity in a particular area of Malaysian Borneo (Bennett and Dahaban, 1995). One year after logging began in one area surveyed, there was no significant change in the total number of species, but some species, such as the Burmese brown tortoise, the rail babbler, and the four-striped ground squirrel, had disappeared from this area, and populations of ungulates, primates and hornbills (despite their protected status) were significantly lower. Hunting of many of these animals was far above sustainable levels. Squirrels thrived, since the removal of their larger food competitors left them greater food supplies (fruits, seeds). Some species – tree shrews, magpie robins, and bulbuls – species not previously found in the forest, but which are tolerant of disturbed areas, appeared to replace the species dependent upon undisturbed forest. Two to four years after selective logging, primate and mammalian species diversity was lower, except for ungulates. Hunting did not so much reduce the number of species present (although some species disappeared) as it did greatly diminish the abundance of animals. Generally, then, with the advent of selective logging and shifting cultivation, there were shifts in species, with edge-tolerant and colonizer species replacing primary forest species. There are frequently unintended and unanticipated consequences of forest removal. For instance, flying foxes are the only known pollinators of some forest trees in Borneo, but as they are heavily hunted, some tree species will disappear. Civet cats are major seed dispersers necessary for forest maintenance and regeneration, but as they are hunted to near extermination, the forests will be unable to sustain themselves. We know little of the eventual consequences of removing large animals from the forest ecosystem, and even less about forests’ needs for various smaller animals, or insects or plants.