PS 12-169 - Effects of invasive plant removal on frugivory network structure and robustness

Abstract: Removing invasive species is a key tool for conservation, however, this removal disrupts important species interactions including frugivory (animals eating fruits). To identify how invasive species removal changes interactions, we manipulated frugivory networks in experimental plots on Oʻahu, Hawaiʻi. We removed the fruits of key invasive plants (Miconia crenata and Rubus rosifolius) in impact plots to test interaction changes (rewiring) and functional resilience. We predicted that rewiring would increase but functional resilience would decrease after invasive fruit removal. We used a Before/After and Control/Impact design on eight 20x20m plots for 6–8 weeks before and after fruit removal. Birds and rodents eating fruit were captured using motion-triggered game cameras and observations and compiled to build frugivory interaction networks. We measured functional resilience through network structure and robustness to subsequent random species loss. Across all plots, we detected 16 animal species (15 introduced) and 15 plant species (6 introduced) in 1,211 frugivory interactions (79% with introduced plants). Higher levels of rewiring occurred on impact plots compared to controls suggesting that invasive fruit removal forces animals to feed on other fruits. However, in one plot frugivory dramatically decreased in the after period, suggesting that birds may leave the area when fruit abundance declines. Network structure was highly variable across impact and control plots and before and after removal treatments. While networks were never nested, some networks were modular and specialized but no clear directional changes in network structure or robustness were observed. In conclusion, removal of key invasive fruits increased interaction rewiring but did not affect functional resilience in a clear direction. In sum, species removal tends to increase rewiring while the idiosyncratic effects on network structure and robustness require further investigation.