CASCADING EFFECTS OF LARGE HERBIVORE LOSS ON LIANA-TREE DYNAMICS IN AN AFRICAN SAVANNA

Abstract

Large herbivores are crucial regulatory components of global ecosystems, acting to increase community diversity and mitigate ecological disturbances at both micro and macro scales, yet prevailing thought has suggested that, at the individual level, herbivores adversely affect plants via consumption and physical damage. However, recent work highlights various direct and indirect benefits of herbivores on plant species and indicates that herbivore loss may have negative net effects on local flora. One particular ecological threat posed by herbivore loss is the release of rapidly growing, competitively dominant plant species that are normally controlled by herbivory. Using observational surveys and manipulative experiments in a series of herbivore exclosure plots in central Kenya, we monitored the effects of herbivore exclusion on a common opportunistic plant type - lianas (woody vines). Evidence from the neotropics shows that liana infestation negatively affects tree fitness; we therefore measured the relationship between lianas and savanna trees, using Cynanchum viminale as a focal liana species. We found that herbivore loss leads to increases in both the incidence of lianas and the severity of liana infestation on trees. The tree fitness surveys showed that increased liana infestation is associated with reduced fruit production among host trees. A mean-field model motivated by this fieldwork reveals that loss of herbivores causes the liana-tree system to shift toward a stable equilibrium at which liana infestation is double its control levels. Together, our empirical and theoretical results demonstrate the importance of herbivores in maintaining stable plant community composition and present a novel mechanism by which large herbivores indirectly benefit savanna trees.