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Want to avoid predators? Head to the poles

American Association for the Advancement of Science

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IMAGE: A plasticine caterpillar glistening with moisture while probing for predator attacks in the forest of Tai Po Kau, Hong Kong. This material relates to a paper that appeared in the... view more 

Credit: Chung Yun Tak

The pressure of predation increases towards lower latitudes and elevations, a new study modeling herbivore arthropods across the globe finds. This is the same pattern seen with respect to biodiversity, which has been known to be richer closer to the equator and at lower elevations. A widespread view among scientists is that biotic interactions become increasingly strong at lower latitudes. However, assessing such a theory can be challenging because of the complexity of large-scale patterns, which are often pieced together from data obtained by a variety of methods and protocols. Taking a more simplified approach, Tomas Roslin and colleagues modeled the predation rate of caterpillars across an 11,660-kilometer latitudinal gradient spanning six continents, also analyzing changes that occur across elevation. They measured predation risk by monitoring the fates of 2,879 model caterpillars molded from green plasticine over 4 to 18 days. The authors report that for every 1° of latitude away from the equator, the daily odds of a caterpillar being attacked decreased by 2.7%. Predation rates also decreased with increasing elevation, where every additional 100 meters of elevation decreased the odds of predation by 6.6%. The authors explain that, to counteract increased predation at low latitudes and evolutions, caterpillars could have evolved to shorten their development times, thereby reducing the lifespan over which they are vulnerable to prey; however, in an initial assessment of whether this was happening, the researchers found development times weren't impacted by latitude the way predation rates were. They conclude there is indeed an overall increase in pressure on larval herbivores at lower latitudes.

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