Disentangling fragmentation effects on herbivory in understory plants of longleaf pine savanna

Habitat fragmentation affects species and their interactions through
intertwined mechanisms that include changes to fragment area, shape,
connectivity and distance to edge. Disentangling these pathways is a
fundamental challenge of landscape ecology and will help identify
ecological processes important for management of rare species or
restoration of fragmented habitats. In a landscape experiment that
manipulated connectivity, fragment shape, and distance to edge while
holding fragment area constant, we examined how fragmentation impacts
herbivory and growth of nine plant species in longleaf pine savanna.
Probability of herbivory in open habitat was strongly dependent on
proximity to forest edge for every species, increasing with distance
to edge in six species (primarily grasses and annual forbs) and
decreasing in three species (perennial forbs and a shrub). In the two
species of perennial forbs, these edge effects were dependent on
fragment shape; herbivory strongly decreased with distance to edge in
fragments of two shapes, but not in a third shape. For most species,
however, probability of herbivory was unrelated to connectivity or
fragment shape. Growth was generally determined more strongly by leaf
herbivory than by distance to edge, fragment shape, or connectivity.
Taken together, these results demonstrate consistently strong edge
effects on herbivory, one of the most important biotic factors
determining plant growth and demography. Our results contrast with the
generally inconsistent results of observational studies, likely
because our experimental approach enabled us to tease apart landscape
processes that are typically confounded.