Self-Reinforcing Impacts of Plant Invasions Change Over Time

By Stephanie G. Yelenik and Carla M. D’Antonio

Santa Barbara, CA (November 20, 2013) – New research published in the journal Nature finds that long-term studies of invasives reveals that while ecosystem impacts and feedbacks shift over time, this may not benefit native species recovery.

While the impacts of plant invasions become less robust over time, invasive plants are more likely to be replaced by other invasives. That’s the finding of a two-decade study at field sties in Hawaii Volcanoes National Park. Researchers at UC Santa Barbara found that non-native plants can have large impacts on ecosystem functioning, including altering groundwater, soil salinity or pH, and pollination syndromes.

Controlling for mineralization and rainfall, the study demonstrates that ecosystem impacts and feedbacks shift over time, but it also indicates that this may not necessarily help native species’ recovery. Researchers conducted a large outplanting experiment to test how a suite of native and exotic woody species responded to shifting ecosystem impacts. They added nitrogen fertilizer to mimic earlier stages of Melinis invasion and reduced Melinis competition to mimic patches during late invasion.

Similar responses occurred in five of the seven outplanted species: Growth rates and survivorship increased due to reduced competition from the exotic grasses as well as nitrogen additions. This indicates that the changing impacts of the grass over time do not alter the seedlings’ ability to grow in the ecosystem.

Two nitrogen-fixing trees were exceptions: the native Hawaiian tree Acacia koa and the exotic tree Morella faya (from the Canary Islands but invading Hawaii today). These species did much better in later Melinis invasion conditions, and Morella faya did particularly well.

According to the researchers, an important lesson is that even if plant invasions can slow down on their own given enough time, native species may need further assistance in order to make a comeback. Other invaders may be poised to take advantage of reduced competition from the original invader.

Knowing the mechanisms of how and why invasions alter ecosystems can help predict what will happen, but without further management, native specifies many not come back. When non-native species are dying back and getting patchy, that may be the time to plant native species. It might turn out to be the most cost-effective way to get an ecosystem back to a more desirable state.

Sources:
Self-Reinforcing Impacts of Plant Invasions Change Over Time (Nature)
Impacts of Plant Invasions Become Less Robust Over Time: Invasive Plants Are More Likely to Be Replaced by Other ‘Invasives’