Effects of habitat restoration on top-down and bottom-up forces regulating vulnerable populations
Habitat restoration is a commonly employed strategy in endangered species conservation, especially for species found in rare or fragmented habitats. Whether such management actions promote populations by creating sources, or inadvertently harm populations by creating sinks or ecological traps has rarely been explored. Here, we assessed the top-down and bottom-up forces impacting local population densities of Appalachian Brown butterflies (Satyrodes appalachia), a surrogate species of the endangered St. Francis’ satyr butterfly (Neonympha mitchellii francisci), in experimentally restored wetlands at Ft. Bragg military base in North Carolina. Habitat restoration within existing forested riparian corridors was created as a 2x2 factorial design where we either removed all trees, installed temporary dams, or both in 30m x 30m plots. In each plot, we evaluated plant quality of the butterflies’ host plant, the sedge Carex mitchelliana. We also placed pairs potted sedges with sentinel eggs, one open to predators and another with predators excluded, in each plot to quantify predation pressure.
We found that plant quality differed between restoration treatments; leaves from sites with trees removed had lower percent nitrogen and lower percent silica. These differences were mirrored in the allometry of adults emerging from each treatment, suggesting the importance of bottom-up effects. We also found strong top-down effects through increased predation pressure on butterfly eggs in restoration treatments relative to a control. Habitat restoration appears to alter the forces regulating populations, and the relative importance of top-down vs. bottom-up forces may depend on the type of habitat manipulation.