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Abstract
The effect of the invasive macrophyte Myriophyllum spicatum L. on native macrophyte diversity was studied across 21 lakes at small (1 quadrat) and large (18 quadrats) scales and in mesocosms. No relationship was observed between M. spicatum and native richness at either spatial scale, or native evenness at small scales; however, at large scales native evenness was negatively correlated with the cover of M. spicatum in lakes. This suggests that M. spicatum can grow in lakes with other dominant species, contrary to predictions that invasive species will outcompete other dominant species. While macrophyte communities were mainly distinguished by environmental and spatial variables, M. spicatum was a significant predictor of the remaining variation in community composition. Within lakes M. spicatum occurred in deep water apart from other species; however, competition studies in mesocosms did not find sufficient evidence that this isolation was due to superior competitiveness.
