Racing Ahead in the Rainforest
It’s time to put a number of strands together—to weave an understanding of evolution in the rainforest. You know that the “raw material” of evolution is mutations; they occur in rough proportion to the rate at which an organism reproduces, so those organisms that reproduce quickly (like insects or bacteria) will have far more potential for variation due to mutation than, say, an elephant or a human being. You also know that the environment (including competition from other living things) selects those traits that make an organism more fit to reproduce. So changes in the environment are more likely to produce changes in species over time. Finally, you know that in the rainforest the supplies of key factors for productivity—sunlight and water, are in (almost) limitless supply. So does 1+1+1 add up to a remarkably rapid rate of evolution in the rainforest? Researchers have found that some tropical species evolve twice as fast as related temperate species. It’s still not clear whether this is because the rate of metabolism is higher (resulting in more frequent DNA replication and greater chance of mutation), or whether the higher biodiversity in tropical rainforests causes greater pressure from competition. A third factor might also be involved; even though the rainforest seems to be a vast expanse without barriers, in fact its very density might prevent organisms from moving far from their original locations. That would make genetic drift more likely to occur.*
On the Brink of Extinction?
Life has existed on Earth for about 3.6 billion years; for most of that time, variations and species have competed for limited resources. That might lead one to imagine that the history of life has been a continuous trend toward more and better species. But of course, that hasn’t occurred.
Most of the species that ever existed on Earth are now extinct. (Remember, “species” is a term invented in the 18th Century by humans, so it isn’t really as clearcut a distinction as it seems.) And there have been many periods of mass extinction on the planet. As a general rule (using our pretty arbitrary definition of the term) between 10 and 100 species per year have probably gone extinct for most of Earth’s history. But today, due to pressure from human development and overpopulation, as many as 27,000 species per year may be disappearing—most in tropical areas. (Remember, almost 2 million species exist there.)
Most of those that are disappearing are pretty tiny, of course. Of the 5000 mammal species known today, the normal extinction rate over the Cenozoic probably averaged about 1 species per 200 years. In the past 400 years we’ve lost ~90 species, and over 150 are critically endangered in this decade. The greatest rates of extinction are occurring in the rainforest. The U.N.'s Millennium Ecosystem Assessment has warned that extinctions are presently running at 100 to 1000 times the background rate there.
Some of the extinction is due to habitat destruction, like logging and pollution from mining and oil drilling. Some is coming from population pressure; native people use monkeys and other animals as sources of meat in their diet. But it's always a one-way street.
It’s hard to overestimate the effect that diminished rainforest biodiversity may have on Earth’s ability to sustain life. As you’ve seen in the past few pages, no species is an island—none lives alone. The elimination of a grazer may severely affect the survival of grasses--either because some species overrun the grassland, or because other grazers take over. The elimination of a tiny pollinator (like America’s honeybees) might result in a domino reaction that destroys countless species of plants and pollen eaters. More on biodiversity in a few pages.
Image credit: USGS