Biological invasions provide a unique opportunity to study the mechanisms that
regulate community composition and ecosystem function. Invasive species that are
also ecosystem engineers can substantially alter physical features in an environment,
and this can lead to cascading effects on the biological community. Aquatic-terrestrial
interface ecosystems are excellent systems to study the interactions among invasive
ecosystem engineers, physical features, and biological communities, because
interactions among vegetation, sediment, and fluids within biophysical feedbacks
create and modify distinct physical features. Further, these systems provide important
ecosystem services including coastal protection afforded by their natural features. In
this dissertation, I investigate the interactions and feedbacks among sand-binding
beach grass species (a native, Elymus mollis (Trin.), and two non-natives, Ammophila
arenaria (L.) Link and A. breviligulata Fernald), sediment supply, and dune shape
along the U.S. Pacific Northwest coast. Dunes dominated by A. arenaria tend to be
taller and narrower compared to the shorter, wider dunes dominated by A.
breviligulata. These patterns suggest an ecological control on dune shape, and thus,
coastal vulnerability to overtopping waves. I investigate the causes and consequences
of these patterns with experiments, field observations, and modeling. Specifically, I
investigate the relative roles of vegetation and sediment supply in shaping coastal
dunes over inter-annual and multi-decadal time scales (Chapter 2), characterize a
biophysical feedback between beach grass species growth habit and sediment supply
(Chapter 3), uncover the mechanisms leading to beach grass coexistence and whether
A. breviligulata can invade and dominate new sections of coastline (Chapter 4), and
examine the non-target effects resulting from management actions that remove
Ammophila for the recovery of the threatened Western Snowy plover (Charadrius
alexandrinus nivosus) (Chapter 5).
I found that vegetation and sediment supply play important roles in dune shape
changes across inter-annual and multi-decadal time scales (Chapter 2). I determined
that a biophysical feedback between the beach grass growth habits and sediment
supply results in species-specific differences in sand capture ability, and thus, is a
likely explanation for differences in dune shape (Chapter 3). I found that all three
beach grass species can coexist across different sediment deposition rates, and that this
coexistence is largely mediated by positive direct and indirect species interactions. I
further determined that A. breviligulata is capable of invading and dominating the
beach grass community in regions where it is currently absent (Chapter 4). Combined,
these findings indicate that A. breviligulata is an inferior dune building species as
compared to A. arenaria, and suggest that in combination with sediment supply
gradients, these species differences ultimately lead to differences in dune shape.
Potential further invasions of A. breviligulata into southern regions of the Pacific
Northwest may diminish the coastal protection ability of dunes currently dominated by
A. arenaria, but this effect could be moderated by the predicted near co-dominance of
A. arenaria in these lower sediment supply conditions. Finally, I found that the
techniques used to remove Ammophila for plover recovery have unintended
consequences for the native and endemic dune plant communities, and disrupt the
natural disturbance regime of shifting sand. A whole-ecosystem restoration focus
would be an improvement over the target-species approach, as it would promote the
return of the natural disturbance regime, which in turn, would help recover the native
biological community. The findings from this dissertation research provide a robust
knowledge base that can guide further investigations of biological and physical
changes to the coastal dunes, can help improve the management of dune ecosystem
services and the restoration of native communities, and can help anticipate the impacts
of future beach grass invasions and climate change induced changes to the coast.