Connecting process with pattern: Towards a mechanistic understanding of invasions' impacts on coastal dune ecology and geomorphology

Background/Question/Methods

Invasive ecosystem engineers can strongly influence a system’s ecology and geomorphology. Conversely, physical processes can alter both geomorphology and ecological composition, including the influence of invasive species. Interactions among these biological and physical components result in biophysical feedbacks. These feedbacks are especially apparent on the US Pacific Northwest coast where invasive sand-binding grasses (Ammophila arenaria and A. breviligulata), sand, and wind interact to form dunes. Ammophila species engineered large and continuous foredune ridges, replaced much of the endemic and sparsely vegetated dynamic dunes, and reduced native diversity of fauna and flora. Over the last two decades, field data show co-varying gradients in sediment supply, dominant grass invader, and foredune geomorphology – tall, steep dunes with low sediment supply are dominated by A. arenaria, and shorter, wider dunes with high sediment supply are dominated by A. breviligulata. These patterns suggest both ecological and physical controls on foredune geomorphology. Motivated by these patterns, we used experiments to isolate both sides of this biophysical feedback: (1) how species-specific structure and density influence dune formation in a moveable bed wind tunnel experiment, and (2) how sand deposition influences species’ growth form, species interactions, community composition, and invasion success in a mesocosm experiment. Additionally, we used a variety of models (mixed-effects, Lotka-Volterra, and structural equation) to assess the relative influence of ecological and physical factors shaping coastal dune geomorphology. We then combined this mechanistic process understanding with field data patterns to ask when and where ecological versus physical forces influence dune geomorphology.   

Results/Conclusions

Our experiments and field observations reveal that over decadal time scales, foredune geomorphology is primarily explained by differences in species-specific growth form and structure. At high sediment supply, A. breviligulata appears to out-compete A. arenaria; the foredunes in this region are lower and wider, reflecting the combination of A. breviligulata’s dominance and horizontal, low density growth. At low sediment supply, both invaders can co-exist, but A. arenaria is the dominant species on these dunes (A. breviligulata has not invaded yet). This suggests that tall foredunes in this region reflect A. arenaria’s superior dune building capacity, due to its high density and vertical growth. Without this mechanistic understanding, we would not have been able to tease apart the relative influence of ecological and physical factors. Understanding these biophysical feedbacks is necessary for improving predictions of coastal vulnerability, biodiversity, and invasion scenarios, especially given climate change projections of sea level rise and increasing wave heights.