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CORVALLIS, Ore. - A new study on Santa Cruz Island in California is demonstrating that once invasive plant species have taken over an ecosystem for an extended period of time, it can be difficult, complicated and possibly expensive to get native vegetation to recover - even when the causes of the invasive takeover are removed.

In this case, exotic grasses and other plants largely displaced native shrubs because of new seed introductions and grazing by cattle, sheep and feral pigs during the past century. Even when those animals were all removed and native shrubs allowed to grow back, at least one important shrub called Artemisia californica made almost no progress over a six-year period.

According to findings from researchers at Oregon State University and the University of California at Santa Barbara, the Artemisia caused elevated nitrogen levels in the soil, which was needed by grasses that had come in to displace it. In other words, the native plant helped provide its invasive competitors with the very nutrient they needed to thrive, working against its own recovery.

"Because of their weedy nature and fast growth, it is sometimes assumed that invasive plants increase nitrogen cycling," said Stephanie Yelenik, a research associate in the Department of Forest Ecosystems and Society at OSU. "In this case, we found that a native shrub was increasing soil nitrogen levels, which aided the invasive grasses and worked against its own best interests. But in the same ecosystem, we found a native shrub that did the opposite, and decreased nitrogen levels, and was recovering on its own."

This doesn't mean that it will be impossible for soil chemistry and native ecosystems to recover, Yelenik said - just that it's not always obvious what forces might be at work, and understanding the various mechanisms at work and their effects will be essential to success.

In some cases, if it's considered important that a full range of native species be brought back, more aggressive and costly manual treatments may be necessary, she said.

The research was published in Ecological Applications, and supported by the U.S. Department of Agriculture.