Current Extent and Potential Distribution of Yellow Flag Iris in

3.3 Ecological impactsThe ability of yellow flag iris to tolerate a wide range of conditions makes it a threatto many natural habitats that are periodically wet. These include riparian floodplains,lake shorelines, swamps, marshes, fens, and brackish wetlands.In North America, yellow flag iris has invaded and dominated a range of vegetationtypes (Tu 2003). Along the Mercer River in California, it completely displaced Typhaand other indigenous marsh plants within 20 years of establishment (Raven andThomas 1970), although it is possible that other factors also contributed (Thomas1980). Along the riparian floodplains associated with the Lower Waikato River,yellow flag iris appears to displace some of the few indigenous plant communitiesthat still exist, including raupo (Typha orientalis) and Carex sedgeland (author’s pers.obs.).Once established, yellow flag iris develops a thick rhizome mat that can suppressgermination of other plant seedlings and also elevate local topography by trappingsediment and thereby creating a drier habitat (Thomas 1980). This can allow it tospread into previously unsuitable habitat but also enable other species to invade,altering successional trajectories. Along the Potomac River near Washington D.C.,yellow flag iris has contributed to the conversion of riparian marshes to less wetforests dominated by ashes (Fraxinus spp.) rather than willows (Salix spp.) (Thomas1980).While a reasonable range of invertebrates feed on the stems, roots, leaves, fruits andseeds of the plant it has not been recorded in the diet of any bird species (Sutherland1980). In North America wildlife has declined in areas where yellow flag iris hasdisplaced key food species such as Peltandra virginica (Thomas 1980; Cox 1999). Noresearch has been conducted in New Zealand to determine whether it provides anyhabitat value for indigenous fauna.4. CURRENT EXTENT AND POTENTIAL DISTRIBUTION4.1 MethodsThe current extent and potential distribution of yellow flag iris along the LowerWaikato River was mapped during a 2.5 hour helicopter survey from Ngaruawahia toTuakau on 6 April 2011. Aerial photographs at a scale of 1:2,500 were used duringthe low-flying survey, to delineate the extent and density of yellow flag iris and toidentify potential habitat. Density of yellow flag iris was assigned to one of fourcategories, as listed in Table 1.Approximately 500 photographs were taken during the helicopter survey. These wereused, along with Google Map and a report from a ground-based survey conductedbetween Huntly and Te Onetea Stream (Department of Conservation 2010), to refinemaps of yellow flag iris extent and potential distribution.© 20115Contract Report No. 2714