Plant-Animal Interactions and Defaunation in Tropical Forests: How Animal Communities and Anthropogenic Disturbances Drive Patterns in Seed Predation, Seedling Damage, and the Regeneration of Tropical Forest Trees

Abstract

The biotic forces that shape plant communities across ontogenetic stages drive patterns in survival, vegetation structure, and species diversity. In tropical forests, many of these forces are facilitated by interactions with animals, which can either promote or inhibit plant reproduction. Disruptions to these interactions – such as defaunation resulting from hunting and logging – can generate broad changes in tree recruitment, forest structure, and carbon storage, with demographic filtering at the seed and seedling stages responsible for many of the effects. Research to date has largely focused on a subset of prominent interactions (especially seed dispersal), while concurrent disruptions to other less-studied ecological processes may drive changes of opposite directionality for individual species or entire communities. With a limited understanding of seed predation, seedling establishment, and seedling physical damage and survival – particularly in Central African forests – it remains difficult to predict the outcomes of defaunation for tropical forest plant communities. In this dissertation, I use a combination of literature reviews, field-based experimental methods (including telemetric seed tags, seed and seedling exclosures, and artificial seedlings) and statistical analyses to assess 1) the role of plant-animal interactions and the influence of hunting on the regeneration of timber trees across tropical forests; 2) patterns of secondary dispersal and seed fate for two tree species in northeastern Gabon; 3) the role of seed traits and both seed density and distance from the parent tree in driving patterns of seed mortality and seedling establishment for ten tree species in northeastern Gabon; 4) the impacts of hunting on seed predation and seedling establishment for eight commercially important tree species across a defaunation gradient in northeastern Gabon; and 5) the role of physical damage by vertebrate trampling, rooting, and digging to artificial seedlings in intact and hunted and/or logged forests in Peru, Gabon, and Malaysian Borneo. I conclude that 1) hunting is likely to disrupt plant-animal interactions and tropical forest timber regeneration, but that these effects can be ameliorated given appropriate management; 2) seed fate is dependent on seed size and the identity of the seed predator, with evidence of scatterhoarding and secondary dispersal in northeastern Gabon; 3) seed traits, not density or distance from the parent tree, drive patterns in seed mortality and seedling establishment in northeastern Gabon, with vertebrate seed predation a stronger force than other mortality factors; 4) hunting-induced defaunation drives increased rodent seed predation and decreased seedling establishment of commercially-important tree species; and 5) vertebrate physical damage to seedlings is a consistent force in forests across the tropics, and hunting significantly reduces its strength. This dissertation highlights the important roles of wildlife in tropical forest ecological processes as well as the degree to which these interactions can be disrupted through hunting-induced defaunation, and emphasizes the value of appropriate management and continued comparative research across tropical forest regions.