| Protecting population diversity
Most people have probably heard about how the earth is rapidly losing
biodiversity worldwide. We usually measure this loss by the high rate of species extinction now occurring compared to pre-industrial times. But there's more to protecting biodiversity than just preventing the extinction of species, and some researchers say that other measures of diversity may be better overall indicators of ecosystem health.
After a long, hard battle, Canada has finally joined the United States and Mexico, becoming the final North American country to enact legislation to protect endangered species. It's far from perfect, but it's a start. Protecting species at risk from extinction is a vital part of protecting biodiversity. Most scientists agree that maintaining such a diversity of life is crucial to the health of ecosystems – and ultimately to the health of humans as well.
However, if there were just a few individual members of every species left in the world, it would hardly make for a healthy planet. Indeed, maintenance of a small population of any species in a zoo or reserve hardly constitutes a healthy, flourishing species and is, in many ways, similar to extinction. Two other measures of diversity are also important – the diversity of groups within a species (population diversity) and the diversity of individuals within those groups (genetic diversity).
Although most of the focus on biodiversity loss has been on preventing the extinction of species, a recent article on the population diversity of mammals, published in the journal Science, points out that this narrow focus underestimates humanity's destructive influence on nature. That's because many mammals may be unlikely to become globally extinct, but their populations have become locally extinct or been decimated by human activities. This, in turn, may impair the overall health of local ecosystems because those animal populations fulfilled important ecological roles.
Researchers looked at 173 species from six continents, which together represent about four per cent of all known mammal species. Most of these mammals have lost more than 50 per cent of their original habitat due to human activity. With its dense human population, Southeast Asia showed the highest loss of mammal habitat, with most of the mammals studied there having lost between 75 and 100 per cent of their original range.
Similarly, other areas with high population densities – such as eastern North America – also had substantial mammal-population losses. When doing a film on grizzly bears for The Nature of Things, I was astonished to learn that they once ranged across Canada to Ontario and south to Texas. They were plains animals, living with bison, but they have now been pushed back to a few mountain areas.
In Africa, human population density does not correlate with mammal losses. The report's authors speculate that mammal decline on that continent is likely due to overgrazing of habitat by domesticated animals and hunting. Australia has also fared poorly due to overgrazing, agriculture and forestry practices as well as introduced species.
The researchers say the numbers in the report are crude and probably underestimated – it's difficult to measure the loss in detail because many less well-known species have not been studied or had their populations mapped over time. What's more, habitats have become so fragmented that although current maps show some species occupying large areas, they have in fact been relegated to small pockets within that range and lost much of their historical populations.
Species extinction is a major public concern. We feel for the plight of species on the brink, like pandas and tigers. Their loss would be tragic. But the insidious erosion of animal populations may have just as much, or more, of an impact on the functioning of our ecosystems. To be effective, conservation measures must take this into account.
Science Matters is a regular feature by writer, geneticist and television host David Suzuki. To discuss this topic with others, visit the discussion forum at www.davidsuzuki.org. |
