Herbivores increase the global availability of nutrients over millions of years.

Abstract

Can the presence of herbivores increase global nutrient availability? Animals disperse vital nutrients through ecosystems, increasing the spatial availability of these nutrients. Large herbivores are especially important for the dispersal of vital nutrients due to their long food passage times and day ranges, and large herbivores from past periods (the Pleistocene) may have increased nutrient concentrations on the continental scale. However, such results have been demonstrated theoretically but not yet empirically. Models suggest that the Pennsylvanian subperiod (323-299 million years ago), with no tetrapod terrestrial herbivores, would have had fewer, less-well-distributed nutrients than the Cretaceous period (145-66 million years ago), with the largest terrestrial herbivores ever-the sauropods. Here, I show that these models are supported empirically by remnant plant material (coal deposits) from the Cretaceous (N = 680), which had significantly (P < 0.00001) increased concentrations (136%) and decreased spatial heterogeneity (22%) of plant-important rock-derived nutrients compared with the Pennsylvanian subperiod (N = 4,996). Non-biotic physical processes, such as weathering rates, cannot account for such differences, because aluminium-a nutrient not important for plants and animals, but weathered in a similar manner to the above elements-showed no significant difference between the two periods, suggesting that these large changes were driven by plant-herbivore interactions. Populations of large wild herbivores are currently at historical lows; therefore, we are potentially losing a key ecosystem service.