Polar bears aren't exactly living large these days. Not only is their habitat shrinking due to global warming, but so are their genitals — thanks to industrial pollutants.
A paper published in online edition of the journal Environmental Science and Technology detailed the bears' most recent plight. Researchers with the National Environmental Research Institute in Denmark looked at the genitals of 100 polar bears from Greenland and found that the higher the levels of certain industrial pollutants in their systems, the smaller their genitals, and therefore the less likely they were to be able to successfully reproduce.
Polar bears ingest these toxins from the fat of seals and other marine mammals. Although areas of the Far North, like Greenland or Canada's Arctic, may seem pristine and far from polluted cities, long-lasting toxins are carried there by atmospheric and ocean currents. Once there, they accumulate in the food chain, eventually concentrating in the fat of marine mammals.
While dramatic stories about shrinking genitals may make headlines, far less attention is being focused on the role of top predators, like polar bears, in the overall functioning of an ecosystem. That role is actually essential. If these large carnivores disappear, it can dampen the entire ecosystem's capacity to adapt to change — especially big changes like global warming.
On the surface, losing a big predator might seem beneficial for other creatures in an ecosystem, which would otherwise be preyed upon by the giant carnivores. But the impact of big predators is much more complex. In nature, every creature has a role to play. The presence of large carnivores actually acts as a "top-down" influence that can benefit the entire food chain.
A recent paper published in the journal Trends in Ecology and Evolution provides an excellent example of this function. Researchers looked at the effect of wolves on the food chain in Yellowstone National Park. The gray wolf had been driven to extinction in the park in the 1920s, but was reintroduced in 1995. Before their reintroduction, elk mortality in the winters had been dropping because winters had been getting shorter and less severe — likely due to global warming. This meant that less and less carrion was available for scavengers like coyotes, eagles and ravens, which depend on carcasses for food.
However, once the wolves were established, they became the primary source of elk mortality throughout the year — increasing the availability of food for carrion-dependent scavengers. Computer models of food chain dynamics, coupled with empirical evidence, have enabled researchers to conclude that, "the presence of wolves will enable scavengers to adapt to the effects of global warming over a larger timescale than if the wolves were absent."
In other words, top predators like polar bears or wolves can help entire ecosystems adjust to a changing climate. The paper concludes: "Their results clearly show that restoring top predators could be crucial for buffering the effects of global warming and also for reducing uncertainty in an increasingly unpredictable and warmer world."
Large carnivores are under increasing pressures from global warming, agricultural and urban expansion, unsustainable hunting, logging and pollution. They also tend to reproduce slowly, and they need large spaces to survive. But if their presence can act as a buffer against change for entire ecosystems, then it makes practical sense to be working to conserve them and restore their abundance.
That isn't happening right now. Carnivores, including polar bears, which are at the top of food chains, have been routinely rejected for listing under Canada's Species at Risk Act, in spite of the recommendations of the scientific body responsible for Canada's threatened species. As is usually the case with this toothless legislation, politics trumps science. That has to change, because protecting large predators may prove to be an effective way to buffer entire food chains against the growing threat of global warming.