Moderate

high

FIGURE 7.9 Average height of Carex pansa along a 30 m transect in a noninvaded, moderately invaded, and highly invaded backdune. Post hoc analysis shows differences in C. pansa height across the invasion gradient (ANOVA, F = 12.44, P < 0.01, N(none) = 18, N(moderate) = 31, N(high) = 0). Values are means and 1 SE.

abundance of Dudleya caespitosa along the partially invaded transect compared with either of the other dune transects. We cannot rule out a pre-existing difference between the sites but the sites are in very close proximity and have the same slope and aspect. Also D'Antonio (unpublished) found that heavy herbivory by deer occurred on Dudleya when C. edulis was removed from around it, suggesting a protective function offered by C. edulis. Despite the lack of rigorous data relevant to this issue, we feel that invasive species entering a range of abiotic environments offer the opportunity to explore when ecosystem engineers might act as facilitators via their effects on the harshness of the environment. In this case, the modulating effect of the invader on the soil surface was overridden by either the volume or chemistry of the accumulated litter material.

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