Background information and definitions
Non-native plant species can be introduced into or naturally invade terrestrial habitat or water bodies, where they can out-compete native species altering the habitats. For example, in the UK swamp stonecrop Crassula helmsii can out-compete native plant species and form thick mats covering whole ponds. In parts of the USA, invasive reed canarygrass Phalaris arundinacea is widespread and develops dense, tall stands in shallow wetland habitats. Invasive water fern Azolla filiculoides has been found to cause declines in amphibian populations (Gratwicke & Marshall 2003) and Japanease knotweed Fallopia japonica to reduce foraging success of green frogs Rana clamitans (Maerz et al. 2005).
Gratwicke B. & Marshall B.E. (2001) The impact of Azolla filiculoides Lam. on animal biodiversity in streams in Zimbabwe. African Journal of Ecology, 39, 216–218.
Maerz, J. C., Blossey, B. & Nuzzo, V. (2005) Green frogs show reduced foraging success in habitats invaded by Japanese knotweed. Biodiversity & Conservation, 14, 2901–2911.
Supporting evidence from individual studies
A before-and-after study in 1972–1991 of ponds on heathland in Hampshire, England, UK (Banks, Beebee & Denton 1993) found that pond restoration and creation with swamp stonecrop Crassula helmsii control, vegetation clearance, liming and captive-rearing and releasing toadlets resulted in a three-fold increase in natterjack toad Bufo calamita populations. Spawn string counts, which relate to the female breeding population, increased from 15 to 43. Swamp stonecrop was eliminated from two of six new ponds it invaded and controlled in two others. Nine small ponds (< 1,000 m2) were created and four restored by excavation. Swamp stonecrop was pulled up and treated with herbicide. In addition, one pond was treated with limestone (1983–1989), scrub was cleared by cutting and uprooting (40 ha) and bracken was treated with herbicide (12 ha). Captive-reared toadlets were released in 1975 (8,800), 1979, 1980 and 1981 (1,000 each). Each year, toads were monitored every 10 days in March and August.Study and other actions tested
A replicated, controlled study in 2000–2001 of a wetland in Washington, USA (Kapust, Mcallister & Hayes 2012) found that Oregon spotted frogs Rana pretiosa laid eggs in more plots than expected by chance following mowing of invasive reed canarygrass Phalaris arundinacea. No eggs were laid in unmown plots. Egg mass clusters (1–18 egg masses) were recorded in two of 32 mown plots. Three egg mass clusters (5–20 masses) were also recorded outside study plots in habitat that appeared structurally similar to mown plots. Breeding sites were located using systematic searches within the reed canarygrass dominated wetland. Four of seven sites found were selected and used as the centre of a 30 m diameter circle. Within each circle, eight pairs of randomly located 3 m diameter plots were created. One of each pair was mown close to the ground in August 2000. Breeding was monitored in February–March 2001 using visual encounter surveys.Study and other actions tested