A Global Analysis of Non-Trophic Impact of Herbivores on Seedling Models Under Contrasting Levels of Defaunation

Anthropogenic changes are altering large and medium sized mammal densities in forest ecosystems across the world, causing significant declines in their abundance and impacting their many contributions to the functioning of ecosystems and their services to humanity. Although animal-forest trophic interactions (i.e., when animals consume plants or plant parts, as in pollination or seed dispersal) are well understood within the scientific community, non- trophic animal impacts (e.g., trampling on the vegetation) are under-studied. Yet, the few studies available are beginning to show that non-trophic (not consumptive) interactions could significantly impact the current and future functional integrity of forest ecosystems.

In order to contribute to close that knowledge gap, I conducted a global analysis to determine the influence of non-trophic damage on forest ecosystems by collecting studies conducted at a global scale and investigating the extent of non-trophic damage under differing levels of mammalian defaunation. The hypothesis of this research was that, if non-trophic damage is an important and consistent occurrence in forest ecosystems, areas in which large and medium- sized animals are present should have higher non-trophic damage than areas that have been defaunated. To test the hypothesis, I compiled studies in which artificial seedlings were used to quantify animal non-trophic damage on plants of the understory. In my literature search, for each study I compared animal impact data (i.e., mortality of artificial seedlings) from two conditions (treatments): i) a defaunated treatment that represented areas that had an absence or low numbers of large and medium sized animals, and ii) a control group that represented areas with intact faunal communities. The difference in percentage damage to artificial seedlings per site was statistically evaluated using a linear mixed effects model. The results show that out of 16 sampled sites, 14 showed higher mortality of seedlings in control treatments which, overall, experienced a 7-times higher non-trophic damage compared to defaunated treatments. These results suggest that the structure of understory plant communities will be affected by the negative anthropogenic impact on animal populations. I posit that the functional integrity of forest ecosystems may be at risk if these animals are lost. Therefore, their protection is imperative.