Managing and suppressing invasive plants has become an important part of land management across a range of ecosystems. After becoming dominant, some invaders appear to stall ecosystems in a persistent exotic state. Identifying the mechanisms locking systems in a state of non-native dominance can help managers identify actions that will lead to directional change. Here we present relevant background theory on alternative stable states as it applies to the management of terrestrial ecosystems, and explore examples from alien-grass dominated ecosystems in Hawaii where we have investigated feedbacks and missing links that contribute to the persistence of undesirable states using field experiments and detailed quantitative observations. We ask: (1) What biotic interactions and feedbacks may be missing that are essential for change in the desired successional trajectory? And (2) What current novel feedbacks involving alien species might contribute to the lack of change in these novel ecosystem states?
Results/Conclusions
Dry and mesic Hawaiian ecosystems have been dramatically altered by the introduction of perennial pasture grasses from Africa and the Americas. Attempts to counteract grass dominance have included the planting of native nitrogen-fixing trees including Acacia koa (koa) and Sophora chrysophylla (mÄmane ) that can compete with, and coexist with grasses and create a native canopy attractive to birds. We demonstrate that these trees consistently elevate soil N availability and perpetuate grass dominance rather than suppressing it. This dominance in return reduces desired native woody understory recruitment. Thus, although these trees add to native diversity and structure, they do not facilitate succession to diverse native ecosystems. Using experiments and observational data we identify, (1) missing elements such as bird-mediated seed dispersal, and canopy-promoted safe site development, and (2) new invader feedbacks (grass dominance due to tree derived N) that must be changed to encourage succession towards native dominance. We demonstrate that at a local scale these challenges may be overcome by management actions such as high density planting of native understory. Yet ultimately the planting of native N-fixing trees presents a management tradeoff between rapid canopy development with some benefits to habitat structure, and long term negative impacts to understory biodiversity.