Even though diversity typically decreases from the tropics to the poles, in the frigid waters of the high latitudes, warm-blooded marine mammals and birds thrive, both in number and species richness. The findings of a new related study help to explain a long-standing ecological paradox: Why does the biodiversity pattern of marine mammals and birds - which exhibits greater species richness at the poles than in the tropics - deviate from that of most other life on Earth? What's more, the results highlight new challenges that animals like whales, seals and penguins will likely face as polar sea temperatures continue to rise. Most of the planet's biodiversity is concentrated near the tropics and forms a well-documented latitudinal gradient of increasing diversity toward the equator. This diversity gradient is a pattern widely observed in nearly every form of animal, plant and insect life on the land and in the oceans. However, for marine mammals and birds, the polar opposite is true. The tropics are home to very few warm-blooded mammals and bird species, like seals, whales and penguins, and only dolphins show high diversity in the warmer waters of tropical seas, according to the authors. Why this group of animals exhibits such a different pattern of diversity has remained unknown. To address this puzzle, John Grady and colleagues compiled a distributional database of 998 top marine predators, including whales, sharks, fishes, seabirds and reptiles, which revealed striking differences in the biogeography of both warm- (endothermic) and cold-blooded (ectothermic) predators. Grady et al. then modeled predation rates, metabolism and water temperature among these species and found that, overall, warm-bodied predators are favored where prey are "slow, stupid and cold." They have a metabolic advantage when it comes to hunting prey in these conditions - becoming faster than their cold-blooded counterparts, as water temperatures decrease. Their metabolism allows them to generate heat to elevate temperatures in the eyes and brain in some cases, to improve sensory abilities as they hunt. In a related Perspective, Nicholas Pyenson discusses the study in more detail.
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Journal
Science