Islands and introduced herbivores: conservation action as ecosystem experimentation.
Abstract
Overgrazing by exotic herbivores has a widespread impact on plant communities. We used the removal of exotic European rabbits, goats and donkeys from the San Benito Islands, Mexico, as an experimental manipulation to examine the importance of top-down and bottom-up processes in the impact and recovery of an island plant community. The dominant plant community on both islands is maritime desert scrub, consisting of shrubs and suffrutescent perennials (Agave sebastiana, Atriplex barclayana, Euphorbia misera, Frankenia palmeri, Lycium brevipes, L. californicum, Malva pacifica, Suaeda moquinii), and two species of cacti, as well as winter annuals, Cryptantha spp. Eschscholzia ramosa, Hemizonia streetsii and Perityle emoryi. Using a paired approach, we removed herbivores from one island, while they remained temporarily on an adjacent, similar island. We combined this large-scale manipulation with smaller-scale mechanistic experiments: herbivore food-preference trials and herbivore exclosures on the control island. El Niño-related precipitation dominated vegetation dynamics early in the study. Differences in plant community structure due to selective herbivory between the experimental and control islands were detectable in the second year. Results from food-preference trials accurately predicted changes in the perennial plant community. When herbivores were removed from the experimental island, the abundance of their preferred plants increased while unpalatable species decreased. On the control island (herbivores present), we observed the opposite trend. However, we saw no recovery of vegetation inside the exclosures on the control island, constructed after the El Niño rains, probably due to the absence of rainfall. While the relationship between herbivore food preference and changes in plant cover is strong evidence of a top-down effect by exotic species, the influence of El Niño precipitation highlights the importance of bottom-up factors, such as water availability, in the recovery of arid plant communities from long-term disturbance.