We associate the tropics with danger, where life lives fast and dies young confronted by a profusion of lifeforms that wish to eat it. However, a recent discovery has marine biologists puzzled: large fish, including sharks, actually pose more of a threat to those below them in the food chain in temperate waters than in tropical waters. The explanation for why this is remains elusive, but the crystal clear waters in the tropics now look like an even more tempting place for a dip.
The closer you get to the equator, the more life there usually is, a rule that applies both on land and in the oceans. Tropical rainforests and coral reefs burst with species, fueled by the extra energy pouring in from the Sun. It makes sense for this to lead to more interactions, including predator-prey relationships, since there are simply more things that could eat you – a view that has dated back at least as far as Darwin.
Dr Marius Roesti of the University of British Columbia thought that, like so many things we take for granted, it was time to apply some science to this one using datasets on baited deep-sea fish lines in the open ocean. Covering 900 million attacks by large fish predators from 1960 to 2014, the data is not only an impressively large sample but it also dates back far enough to test human-induced disruptions.
In Nature Communications, Roesti reports that fishing baits were most likely to be taken in temperate areas and in places poor in species diversity, the exact opposite of what was expected. The pattern held throughout the world's oceans, although it was stronger in the Southern Hemisphere, with baits being taken around 65 percent faster between 30 and 40 S than within five degrees of the equator. Predation rates did fall sharply as latitudes increased above 40, however.
Inevitably, over-fishing means it now takes a lot longer to get a bite from a large fish, but Roesti found this has barely affected the relative rates between different climate zones. Roesti and co-authors don't understand why the pattern they found occurs, writing that “Evolutionary ecologists are now charged with explaining why the eco-evolutionary processes thought to generate and maintain diversity are not always strongest in the most diverse regions on Earth.”
Biologists did have some warning that our theories of predation being highest in the tropics might be flawed. Several attempts have been made to test the theory on land and the results have been confusing and contradictory. However, Roesti notes, these used sample sizes far smaller than the one he drew on, making unexpected findings easy to dismiss.