Bottom-up effects of a megaherbivore alter plant growth and competition regimes, promoting vegetation heterogeneity

Abstract

O_LIMegaherbivores are known to strongly influence multiple ecological processes, but their bottom-up impacts on vegetation via nutrient redistribution remain poorly understood, particularly in mesic ecosystems. Here, we investigated how woody plant communities are influenced by large nutrient inputs in the form of dung deposited by Asias largest herbivore, the Asian elephant (Elephas maximus), in the tropical forests of southern India. C_LIO_LIWe conducted field and mesocosm experiments on woody saplings to examine three mechanisms through which dung deposition by elephants can alter plant community assembly. Specifically, we tested if elephant dung input 1) creates hotspots of plant growth, and shapes plant communities by altering 2) the negative density-dependent effects of neighborhood competition, and 3) interference competition between plant functional types differing in nutrient limitation, namely nitrogen-fixers and non-nitrogen-fixers. C_LIO_LIOur findings show that dung deposition by elephants can generate fine-scale spatial differences in woody sapling communities by creating local growth hotspots and altering competitive interactions. We analyzed relative growth rate and final sapling size in the field experiment, and found that average-sized and large saplings receiving dung inputs were buffered against the negative density-dependent effects of neighborhood competition. In the mesocosm experiment, non-nitrogen-fixing species (which are nitrogen-limited) outcompeted nitrogen-fixers in accumulating biomass when supplied with elephant dung. These outcomes were associated with changes in their relative competitive strength which was stronger for non-nitrogen fixers under dung treatment and for nitrogen-fixers under control. Such bottom-up effects on plant growth and competition can be of substantially large magnitude, as we estimated that elephants in these forests create a total of 11000 such nutrient-rich sites /km2/year, with each elephant redistributing [~]130 kg Nitrogen/year through this pathway and each site receiving [~]20 g Nitrogen. C_LI SynthesisOur findings on the outcomes of sapling competition highlight the role of nutrient redistribution by megaherbivores as an underappreciated driver of species interactions that can alter plant communities at fine scales, effects that are widespread across megaherbivore habitats. Such bottom-up effects of megaherbivores, along with their top-down effects, have important conservation implications and can help in restoring species interactions and spatial heterogeneity in plant communities in defaunated habitats.