Concern about colonization of marshes by plant species such as Phragmites australis and Lythrum salicaria has highlighted the need for management strategies. However, there is a lack of information in the literature on which to base these decisions. This study compares the alpha diversity of marshes to assess the impact of invasion by Phragmites and Lythrum. Species occurrence and stem density were measured in marshes dominated by Phragmites, Lythrum, Typha spp., or other herbaceous perennials in the Charles River watershed in eastern Massachusetts, USA, and spices richness, Shannon's H', Simpson's reciprocal (1/D), and Pielou's J were compared among six community types. The three diversity indices had significantly higher values for Typha-Lythrum marshes than for any of the other marsh types (Tukey test, p < 0.05), with mean values ( s.d.) of H' = 2.00 0.74, 1/D = 3.51 1.68 and J = 0.69 0.1. Marshes dominated by Phragmites had the lowest diversity, with H' = 0 and D = 1, i.e. they were monospecific. Typha dominated marshes had the second lowest values, with H' = 0.17 0.05, 1/D = 1.05 0.01, and J = 0.11 0.03. These results support the idea that a reduction in diversity can be expected in marshes colonized by Phragmites. However, the high diversity found in the Typha-Lythrum marshes contradicts the expectation of lower diversity after invasion by Lythrum. This information may alter marsh management decisions.
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