阅读理解
Most groups
of plants and animals are richer in species and more plentiful near the equator.
In the ocean, that holds true for cold-blooded predators(掠食者). But warm-blooded predators are more diverse
toward the poles and noticeably missing from several warm hot spots. Why?
John Grady,
an ecologist, and his team considered the possibility—warm-blooded animals need
a lot to fuel their metabolism(新陈代谢). Perhaps colder waters are just richer
in small fish? But they found that at higher, colder places, there isn't actually
much more food around. It's more that warm-blooded animals are eating a much bigger
share of it than their cold-blooded competitors.
The real
explanation is simple. An animal's speed, swiftness, and intelligence depend on
its metabolism, which in turn depends on its temperature. Since birds and mammals
can keep heating their bodies in icy conditions, they remain fast and attentive.
By contrast, the fish they hunt become slower and duller. At some tipping point
of temperature, seals, dolphins, and penguins start out swimming their prey(猎物). They become more likely to come upon
targets and outpace the cold-blooded predators of their own.
In Grady's
words, "Warm-bodied predators are favoured where preys are slow, stupid and
cold." That's why sharks and other predatory fish dominate near the equator,
but colder waters are the kingdom of whales and seals. By keeping food to themselves
in the poles, these creatures can then specialize on specific types of prey, which
makes them more likely to split into separate species. The killer whales of the
North Pacific, for example, include mammal-eating transients and fish-eating, year-round
residents.
But the
world is changing. It's likely that the surface of the oceans will warm by 2 to
3℃
within this century. Grady's team estimates that every time the ocean's surface
warms by 1℃, populations of sea mammals will fall by 12%, and populations
of seals and sea lions will fall by 24%.
But "predictions
are hard," Donna Hauser from the University of Alaska Fairbanks notes. "Polar
bears are losers of a warming world, but some populations are still doing well.
Some groups of whales have changed the timing of their migrations; others are hunting
in deeper, colder waters. These changes might make sea mammals more adaptable to
changing climates. Maybe they just need to find the places where fish remain slow,
stupid and cold."
