Invasive plant species are widely recognised to have severe ecological impacts in a wide range of ecosystems throughout the world, yet there are few experimental studies measuring community-level effects of invasive plant species. Thus most evidence is from correlative studies, and as such often cannot easily disentangle cause and effect. Through a combination of an addition and removal experiment and a correlative approach (multi-site comparisons), this study aimed to quantify the effects of a widespread invasive species, Mimulus guttatus, on species richness and soil properties of riparian plant communities.\ud The marked negative association between Mimulus cover and plant species richness identified through correlative multi-site comparisons was consistent with experimental removal studies which indicate Mimulus significantly alters the structure of riparian plant communities. Total C and N and soil moisture were marginally higher in invaded than in uninvaded disturbed sediment plots. Following Mimulus removal, there was an increase in the occurrence and abundance of another non-native species, Claytonia sibirica, as well as germination and establishment of Mimulus seedlings. This highlights that, although removal increased richness, bringing the plant community closer structurally to uninvaded vegetation, the application of removal as a management tool needs to be undertaken with caution, as it may create opportunities for other invaders.\ud \ud The impact of Mimulus appeared restricted to disturbed sediment communities, as addition experiments into herb-grass communities were relatively unsuccessful in establishing Mimulus. These patterns were consistent with the distribution of the species in riparian plant communities. The addition experiments highlight that, as well as competition from the resident vegetation community, mollusc herbivory Further hinders the establishment of Mimulus. Many manipulation studies have removed invasive plant species from heavily invaded communities, and it is often thought that invasive species usually affect plant community structure only where their cover is high. This study is unique in demonstrating impacts where cover of the invasive plant is relatively low
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