Predator-mediated apparent competition between an introduced grass, Agrostis capillaris, and a native fern, Botrychium australe (Ophioglossaceae), in New Zealand

This study provides an example of how invasive plant species may indirectly affect native species through apparent competition by altering the local invertebrate community. The native New Zealand fern Botrychium australe (Ophioglossaceae) is thought to benefit from disturbances such as fire, because this species is generally found in disturbed habitats with low shade and increasing competition. However, in this study we show that a mapped population of B. australe experienced a marked decrease in survival and reproduction after an accidental fire in May 1995. Mortality was not due to the direct effects of fire; in the year following the fire, survival was normal and reproduction was higher than in previous years. however, after 1996 an introduced grass (Agrostis capillaris) spread across the area and B. australe survival in the second to fourth years after the fire (1996 1999, 59.63%) was significantly lower than survival before the fire, and plants did not produce spores at all during this 3-yr period. B. australe survival was not lower in plots with dense. A. capillaris growth. From 1997 onwards, B. australe plants suffered very extensive defoliation by a herbivore, and insecticide and molluscicide experiments in 1999 and 2000 showed that the introduced slug Deroceras reticulation was responsible for the damage. The slug increased after the fire in association with the spread of Agrostis capillaris, probably because this plant created a suitable habitat for slugs.