Habitat fragmentation is one of the leading causes of species decline and extinction. The establishment of habitat corridors is a common conservation strategy because they increase landscape connectivity, but evidence supporting the efficacy of corridors has lagged behind their popularity. In particular, few studies have addressed the impact of corridors on plants and plant-animal interactions such as herbivory. Whether corridors will benefit plants can hinge upon these interactions.
During summer 2008 we studied herbivory of an early-succession annual (Solanum americanum) in experimentally fragmented habitat patches at the Savannah River Site (SRS) near Aiken, South Carolina. All patches have equivalent area (1.4 ha), but connectivity via corridors and edge-to-area ratios differ between patches. Our study system is designed to test the impact of connectivity, isolation and edge effects on plant fitness and community restoration. We planted 1,280 S. americanum seedlings in plots spaced at 2m, 15m, 28m, and 38m from habitat edges. We monitored levels of leaf herbivory and fruit production (our metric for plant fitness) throughout the growing season, and we conducted field surveys for grasshoppers, a common group of generalist herbivores in the habitat patches.
The presence or absence of corridors does not determine levels of herbivory or fitness for S. americanum. At the patch scale herbivory varies only with the amount of interior “core” habitat. Plants in isolated patches with low edge-to-area ratios and thus large amounts of “core” habitat experience greater levels of herbivory (ANOVA, p=0.05) and produce fewer fruits (ANOVA, p=0.007) than plants in patches with high edge-to-area ratios and thus low amounts of “core” habitat (whether connected or isolated).
These data may be explained with three considerations: (1) grasshoppers are the most common herbivores eating S. americanum in the experimental patches (based on personal observations and video monitoring); (2) field surveys indicate that grasshopper abundance is unaffected by the presence or absence of corridors—grasshoppers are equally abundant in all patch types; and (3) grasshoppers are least common near edges and most common in center “core” habitat (ANOVA, p<0.001).
Studies in 2009 will further investigate the makeup of grasshopper communities at SRS, the impact of grasshoppers on S. americanum herbivory and fitness, and the ways in which these herbivores respond to patch shape, edges and corridors.