Constraints on coastal dune invasion for a notorious plant invader

Although most biological invasions are not successful, relatively few studies have examined otherwise notorious invaders in systems where they are not highly problematic. The annual grass Bromus tectorum is a dominant invader in western North America, but is usually confined to human-dominated and disturbed systems (e.g. roadsides and parking lots) in the East where it remains virtually unstudied. This study aims to address fundamental ecological questions regarding B. tectorum in a Cape Cod dune ecosystem. (i) What is the range of variation in population dynamics and the potential for population growth? (ii) Which factors influence its local abundance and distribution? We observed substantial variation in population dynamics over 3 years, with the number of adult B. tectorum individuals increasing substantially between the first 2 years (λ = 9.24) and then decreasing (λ = 0.43). Population growth in terms of total seeds was similarly variable, but to a lesser extent (λ = 2.32 followed by λ = 0.32). Experimental soil disturbance led to a more than 10-fold increase in mean seedling emergence, and high sensitivity to differences in emergence carried this effect through the life cycle. In contrast, barriers to seed dispersal had no effect on population dynamics, suggesting limited dispersal in this system. Across the landscape, the presence of B. tectorum was associated with areas of higher plant diversity as opposed to those with a strong dominant (e.g. the foredune, dominated by Ammophila breviligulata, or low heathlands, characterized by Hudsonia tomentosa and Arctostaphylos uva-ursi). Overall, we find that B. tectorum is capable of both substantial population growth and decline in a dune ecosystem, but is likely limited without disturbance and dispersal agents. Thus, management actions that restrict dune access (e.g. for nesting habitat) likely have the co-benefit of limiting the invasive potential of B. tectorum.