Do invasive species provide functional resilience for ecosystems affected by extinction? Evidence from exotic-dominated forests in Hawai’i in the context of the Biodiversity-Ecosystem Function Paradigm

Composition and diversity changes may pose considerable risk to important ecosystem properties such as biomass stocks and productivity that are critical to human welfare.  Invasive species are widely recognized as a major agent of extinction, but they also paradoxically cause regional and local diversity to increase, making ecosystem-level effects difficult to predict.  We examined this dynamic on Hawai’i Island, where a 100+% increase in regional plant species richness has created hundreds of hectares of new forest communities dominated entirely by exotic trees.  We compared 10 exotic- and 10 native-dominated forests along a 1500-year substrate age and nutrient gradient to determine how regional diversity and species identity affect forest biomass stocks.  Using analysis of covariance, we found a significant effect of forest type on biomass (P < 0.02) that was also influenced by an interaction between forest type and substrate age (P < 0.03).  Biomass stocks on older nitrogen-rich sites differed little between exotic- and native-dominated forests (~240 v. 300 Mg/ha; respectively).  On young nitrogen-limited sites, however, we found that an increase in regional diversity—namely the addition of N₂-fixing species—had allowed exotic forests to reach biomass stocks more than seven-times higher than native forests (~260 v. just 35 Mg/ha).  Substrate age and mean species richness were each strongly correlated with biomass of native forests (R² = 0.78, 0.54; respectively), but not exotic forests (R² = 0.01, 0.00).  These findings suggest that native forest biomass under certain nutrient conditions has been limited by the regional species pool, and that those limitations have been significantly lessoned by invasion.  While invasion poses a clear and urgent threat to native species diversity on Hawai’i, the increase in regional diversity may also provide resilience to basic properties like standing biomass as native species are excluded or driven extinct.