Herbivores fertilize planet

Are big animals just charismatic or do they play a critical role in our ecosystems?

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The paper in Nature Ecology & Evolution is here: http://go.nature.com/2xxZLpt

Are big animals just charismatic or do they play a critical role in our ecosystems? Theoretical work suggests that big herbivores are disproportionately important for the spread of nutrients and they can affect nutrient distributions at the continental scale. But we need empirical data to support the theory. An ideal way to test this would compare a world with the biggest animals ever to a world with no herbivores.

 I had been thinking about ways to test this idea and I was hoping to make use of the Grand Canyon which was near my university. Walking down the Grand Canyon takes you on a trip through time, from a period of large herbivores at the top to the period before multicellular life evolved at the bottom. I initially thought about measuring sediment chemistry along this gradient, but then realized I was more interested in fossilized plant chemistry. I realized that coal, which is fossilized plant material, could give the answers I wanted.  There are online databases listing coal chemistry from sites across the US. Coal from the US is generally from either the Carboniferous, prior to the evolution of tetrapod herbivores, or the Cretaceous, the period with the largest herbivores ever, the sauropods.

I was able to compare abundance and distribution of elements important to plants between these two periods and found as hypothesized that the plant important elements were more abundant and better distributed during the era of dinosaurs. However, elements not needed by plants or animals like aluminum showed no differences strongly indicating that there was a biotic aspect to the results.

Science can take you in unexpected directions which is part of the fun. The Carboniferous had no tetrapod herbivores, but it did have giant 2-meter long bugs! I remember spending a full day trying to calculate how far I would expect the biggest herbivore of the Carboniferous (a 2 meter centipede) would travel between food consumption and defection.                 

Despite the quirks, there are two important insights to this study. First, today we are rapidly losing our few remaining large animals (forest elephants have decreased by 60% in the last 10 years alone). Losing these animals will critically impair our future ecosystems as we lose their nutrient distribution capacity. Next there is an observed ecological rule called Cope’s rule that has shown that animals tend to get bigger over time.  Basically, it is a good evolutionary strategy to get big because then it is harder for predators to eat you.  Combining this rule, with what I have found in this study, that large animals are disproportionately important for the distribution of nutrients, indicates that the planet may have an intrinsic mechanism of increasing fertility over time. 

Chris Doughty

Assistant Professor, Northern Arizona University