Traits mediate the influence of landscape connectivity on plant species’ abundance and persistence

Background/Question/Methods

Habitat connectivity plays a key role in community assembly by promoting movement of organisms between habitat patches. The combined effect of increased movement by individuals and subsequent population-level processes may explain why connectivity leads to higher species richness. However, seed arrival does not guarantee recruitment or persistence, and linking movement potential with the dynamics of subpopulations—a key element for demonstrating “functional connectivity” —has historically proven difficult to demonstrate.

We explored patterns of species’ abundances in a replicated landscape experiment, where connectivity between patches of longleaf pine savanna was manipulated using habitat corridors, controlling for area and patch shape. Using patches connected or unconnected by corridors we asked whether connectivity influenced the direction or magnitude of plant species’ abundances over successional time. We also determined whether seed dispersal mode mediates species responses to connectivity. Last, we identified whether corridors function by increasing local colonization rates or reducing local extinction rates.  To address our questions, we collected plant richness and percent cover data in 1x1 m plots in six patches of each type (connected, unconnected) in 2001, 2003, and 2009. We used Rank Occupancy-Abundance Profiles (ROAPs) and permutation tests to compare overall abundances between connected and unconnected patches over successional time.

Results/Conclusions

Effects of connectivity on overall abundances were evident as early as 2001 (two growing seasons after the experiment was created) with 16 species being significantly more abundant on one patch type than another. In all years, we observed more species present in higher abundances in unconnected relative to connected patches, although this doesn’t emerge as a significant difference until 2009. Animal-dispersed species were more abundant on connected patches relative to winged, while species dispersed by wind or with unassisted dispersal were more abundant in unconnected patches. For species more abundant in unconnected patches, local extinction occurred less frequently and colonization occurred more frequently than on connected patches.  In contrast, species more abundant on connected patches did not experience more local colonization events over time, but suffered fewer local extinctions than in unconnected patches, invoking rescue effects. Our results demonstrate that plant species abundances respond to landscape connectivity quickly, and responses reflect dispersal mode. Notably, animal-dispersed species—more abundant on connected patches—are also woody, long-lived species. Dispersal mode and longevity may have additive benefits in connected landscapes: getting to a site (via vertebrates), and longevity (lifespan, competitive ability) together allow for persistence, ultimately determining the structure of the plant community.