Some Like It Cold

Antarctic fur seal

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Overall global biodiversity is highest in the tropics and decreases toward the poles as temperatures decline. A new study provides a large-scale meta-analysis showing that marine mammals and birds exhibit the opposite trend and theorizes that metabolic differences between cold– and warm-blooded animals may explain this pattern.

The team of researchers, led by biologist John Grady, now at the National Great Rivers Research and Educational Center, first compiled previously published data on nearly one thousand species of cold-blooded (ectothermic) and warm-blooded (endothermic) marine species to identify global distribution patterns. The data showed that the number of ectothermic marine species is highest in the tropics, but endothermic marine species, such as birds and mammals, are most abundant in colder waters. When the team examined phylogenetic diversity, which corrects for large numbers of closely-related species, this trend was even stronger.

To explain these patterns of abundance, the researchers created theoretical models that predict the ability of each species to move and feed in cold or warm water. These models were partly informed by previous research that indicated that endotherms’ top speeds are not affected by temperature, whereas ectotherms are able to move faster in warmer water.

According to the models, this difference suggests that in cold water, endotherms are more likely to be able to out-swim their ectothermic predators and prey, increasing their survival rates and food consumption rates relative to their ectothermic counterparts. In warm water, ectotherms are more likely to be successful. The researchers theorized that this temperature-driven asymmetry promotes greater diversity of endothermic marine mammals and birds in cold polar waters.

These predictions have conservation implications for marine endotherms, as rising sea temperatures may allow ectotherms to better catch their endothermic prey, evade their endothermic predators, and out-hunt their endothermic competitors. The scientists also point out that, because of this imbalance, warmer water is “predicted to exert substantial additional constraints on mammal and bird populations independent of food production or habitat conditions and may alter the balance of marine endotherms and ectotherms across the globe.” (Science)