Scientists have identified two ecological mechanisms that help prevent takeovers by invasive species.

Researchers at the University of California, Santa Barbara, wanted to understand why and how some alien species execute highly successful -- and highly damaging -- ecosystem takeovers, while others are eradicated or thwarted from the outset.

The "killer algae" species Caulerpa taxifolia has a reputation for quickly outcompeting native species and colonizing marine habitats. But in the wake of its earliest appearance in Southern California waters in 2000, eradication efforts prevented a large-scale invasion.

Another problematic algae, Sargassum horneri, didn't meet the same resistance, and has spread throughout Southern California.

Over the past two decades, researchers have monitored the encroachment of the Asian brown algae species into new marine ecosystems. As part of their latest research effort, scientists analyzed the algae's invasion patterns in various marine protected areas, or MPAs, surrounding the Northern Channel Islands off the coast of Santa Barbara.

"We found evidence for two distinct ecological mechanisms -- competition and herbivory -- providing resilience, but the community states really aren't equivalent," Jennifer Caselle, a research biologist at UCSB's Marine Science Institute, said in a news release. "On one hand, in the old MPA, we found a healthy kelp forest outcompeting the invasive and in the fished areas, an urchin barren affording herbivory."

Researchers found older marine preserves tended to host more mature kelp forests, with native species resilient enough to keep invaders at bay. Meanwhile, ecosystems where fishing was allowed tended to feature larger populations of smaller herbivore species, like urchins.

The lack of top predators allowed for a high numbers of grazers. The appetite of the grazers helped prevent the invasive algae from taking over. Scientists have discovered similar patterns in the Caribbean.

Newer protected areas with neither a mature and diverse kelp forest nor an abundance of herbivores were less successful at defending against the invasion of alien algae.

Researchers published their findings this week in the journal Ecology Letters.

"Our study shows that resilience does not rely on a single trophic level," Caselle said. "We looked at the entire community, finding top-down pressure from those protected urchin predators. That reverberated all the way through the food web, ultimately providing resistance to an invasive species. In areas outside the MPAs, those top predators are in low abundance and urchins are in high abundance."

While sustainable management of marine ecosystems can help prevent or curb alien invasions, researchers say the best strategy is prevention.

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