Floristic homogeneity underlies environmental diversification of northern New Zealand urban areas

Urban areas are an important conduit for alien invaders. A floristic study was made of the spontaneously occurring plant species of 11 towns in the lower North Island. Four hundred and eighty-three species were found, from 93 families. Viola sepincula and Sarcococca ruscifolia are recorded as naturalised for the first time. Only Poa annua occurred in > 50% of plots. The majority of species originated from Eurasia and the Mediterranean, having arrived accidentally. A third are annuals and 89% are exotics. Basically homogenous, small floristic differences between towns appear to be related to either chance invasion events or to the environment. Influential environmental factors include the distance to the coast and the size of the central business district. The inland tourist town of Taupo is the most extreme, separated from warmer, coastal, more isolated towns, and from larger, more developed centres. The underlying homogeneity of New Zealand urban floras appears to be due to their recent and relatively uniform history of colonisation and urbanisation.