A review of the status of the New Zealand flatworm in the UK

The indigenous terrestrial planarian fauna (three species) of the UK is out numbered by introduced exotics, of which there are at least nine species. T he:New Zealand flatworm, Arthurdendyus triangulatus, is one of the most wid espread and apparent of these non-indigenous earthworm predators, particula rly in Northern Ireland and central Scotland. Despite its having been in th e UK for at least 35 yr, our knowledge of the biology and ecology of this s pecies remains somewhat limited. In Scotland, A. triangulatus occurs predominantly in botanical and domestic gardens, and is not generally considered to be a problem on agricultural l and. The situation in Northern Ireland is different; although predominantly found in domestic gardens, it appears to have colonised grass leys many lo calities although the impact on earthworm populations remains ambiguous. In England, records are increasing, predominantly from northern regions. Studies have indicated that earthworm species vary in terms of their vulner ability to predation by A. triangulatus; surface-active and anecic species are considered to be most at risk. However, A. triangulatus and earthworm p opulations are known to coexist, apparently in a state of dynamic equilibri um, in a number of localities. Naturally-occurring planarian populations are often severely constrained by food supply, but individuals survive lengthy periods of starvation. Such b ehaviour may preclude the natural recolonisation of habitats by prey specie s. Very much higher rates of population growth can be achieved where food i s not limited, and where favourable conditions result from horticultural pr actices. The mobility of the prey species may determine the dispersal strat egy of the flatworm predator, and a propensity not to move away from centre s of prey density might account for the relatively low rate of colonisation of agricultural land by A. triangulatus. Additionally, reliance on protect ed refuges, may explain the present, somewhat limited distribution in the U K, particularly in southern England. It is difficult to estimate the potential distribution of A. triangulatus, because of the lack of fundamental knowledge of its ecoclimatic requirement s. A better understanding is needed of the precise habitat requirements (an d constraints) of A. triangulatus, particularly where this species appears to have adapted to rural conditions.