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For A Predator-Free Life, Avoid The Tropics

author

Stephen Luntz

Freelance Writer

clockMay 18 2017, 19:00 UTC
Plasticine caterpillar

Life's tough when you are caterpillar whose shade of green doesn't match the leaves, but it's even tougher in the tropics. Chung Yun Tak

You're far more likely to be eaten by a tiger or bitten by a snake in tropical jungles than attacked by a bear in the frozen north. This long-standing casual observation, that the world lives faster and dies younger closer to the equator, has been given rigorous endorsement for the first time, albeit only for caterpillars.

The extra energy made available by the Sun near the equator leads to a vast profusion of life, at least when water is also available. Although Mediterranean climates match them for plant diversity, tropical rainforests and coral reefs host easily the biologically richest and most diverse animal concentrations. At the poles, meanwhile, species are few and far between, although the summer can see a profusion of migratory birds, and those species that have adapted to the cold can be present in great numbers.

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These observations would be expected to affect the number of predator-prey interactions, but this has not been confirmed, and indeed some studies of plant-animal interactions have found surprisingly weak evidence for an equivalent effect.

Researchers from 35 scientific institutions came together to finally get some firm numbers on whether the anticipated increase in predation rates exists. Led by Professor Tomas Roslin of the Swedish University of Agricultural Sciences, they investigated the risk plasticine caterpillars face of being attacked by predators at 31 locations across six continents. The authors report in Science that for every additional 1º latitude away from the equator, the risk of being attacked fell by 2.7 percent. At Zackenberg, Greenland (74.3º North), a caterpillar's risk of attack each day was just 13 percent of a twin at the equator.

Latitude was not the only important thing. Should you be reincarnated as an arthropod and can't migrate polewards, it's best to get high, as Lewis Carroll apparently knew. Every 100 meters (330 feet) of altitude produced a 6.6 percent reduction in predation, although the study never went above 2,100 meters (7,000 feet), where the daily risk was 24 percent of that at sea level.

In order to survive, species vulnerable to predation might be expected to shorten their developmental stage, feasting rapidly on the profusion of plant matter available in warm, wet climates, or develop better defensive mechanisms. The authors recommend further study, but found the faster growth rates of tropical caterpillars don't go close to making up for the extra dangers they face each day.

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Interestingly, the safety provided by altitude or latitude was from other invertebrates. Attacks from birds and mammals didn't show a relationship to location, raising questions as to whether larger models, too big to be eaten whole by insects or spiders, might not show the same pattern. Perhaps you'd better worry about those polar bears after all.

The safest place studied for caterpillars, despite the lack of camouflage, was Zackenberg, Greenland. They also got a nice view. Tomas Roslin

natureNature
  • tag
  • altitude,

  • caterpillars,

  • predator-prey relations,

  • tropical dangers,

  • latitude

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