Interacting effects of multiple herbivore guilds on the fitness and demography of a savanna tree.

Plant populations are limited by a diverse array of herbivores that restrict growth and survival throughout the plant’s life cycle. Few studies, however, simultaneously quantify the impacts of multiple herbivore guilds on the lifetime performance or population growth rate of plants. In African savannas, large ungulates such as elephants are important drivers of woody plant population dynamics, while the potential impacts of smaller, more cryptic herbivores such as rodents, have largely been ignored. I integrated a five-year manipulation of rodent densities into a large-scale ungulate exclusion experiment to quantify the impacts of three herbivore guilds - wild ungulates, domestic cattle, and rodents - on all life stages of a widespread savanna tree, Acacia drepanolobium. I used demographic modeling to determine the overall influence of each guild on tree population dynamics, and to determine the importance of different demographic stages in limiting population growth under contrasting consumer communities. I showed that wild ungulates dramatically reduced population growth (λs reduced from 1.06 to 0.98), shifting the population trajectory from increase to decline. The processes that drive these effects, however, were strongly mediated by rodents. The impact of wild ungulates on population growth was predominantly driven by their negative effect on tree reproduction when rodents were excluded, and on adult tree survival when rodents were present. By limiting seedling survival, rodents also reduced population growth (decrease in λs from 1.06 to 1.01); however, this effect was strongly dampened where wild ungulates were present (decrease in λs from 0.98 to 0.97). I suggest that these complex interactions between disparate consumer guilds can have important consequences for the population demography of long-lived species, and that the effects of a single consumer group are often likely to vary dramatically depending on the larger community in which interactions are embedded.Science, Faculty ofZoology, Department ofGraduat