One of the key factors in an alien species becoming invasive in an area is that it survives better in the new habitat than the native species, right? Well it’s not always quite that simple. Researchers from Oregon State University have documented a case where stronger native grasses were out-competed and replaced by inferior challengers. During the 17th and 18th centuries, the species rich grasslands of California were rapidly overwhelmed by grasses introduced by Spanish settlers. According to Eric Seabloom (OSU), "The complete change in grassland ecosystems brought with it more damaging fire regimes, shorter grazing seasons, and altered water and carbon cycles". The surprising thing about this changeover was that the invaders shouldn’t have been able to compete strongly enough to win out. The scientists claim it was an inside job.

Barley yellow dwarf virus (BYDV) had long been present in the native grassland population, a constant stress on those species. The addition of the alien grasses appears to have altered the dynamics of the situation and increased the BYDV infection rates in the natives. With the added pressure of BYDV, they were unable to compete effectively and were rapidly replaced. The authors of the study suggest that while it’s too late in the day to restore the native grasslands, understanding of how an existing pathogen could have allowed exotic species to invade where they otherwise couldn’t, might be the key to understanding and controlling future invasions.
The contact for the team of OSU researchers is Elizabeth Borer. CAB Abstracts features many abstracts for further reading and research on grasslands of many kinds, invasive weeds and plant pathology.

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