Competitive dynamics at patch boundaries in a two-state ecological mosaic: An intertidal tug-of-war
On flat sandy beaches in Washington state, non-native seagrass (Zostera japonica) competes for space with sand dollars (Dendraster excentricus). Because of the clonal nature of seagrass and benthic, motile lifestyle of sand dollars, shifts between these two ecological states likely depend on invasion dynamics at territory edges. How does this spatial component affect state shifts? Can we predict coexistence? I examined spatial dynamics at boundaries between sand dollars and seagrass on Orcas Island, Washington with varying disturbance. Experimental plots were 1x4m areas spanning a pre-existing boundary between sand dollars and seagrass at the 2m midline. Fences on long sides prevented movement between treatments; open short ends allowed invasion. Treatments included: control; sand dollar removal in the 1m adjacent to the midline boundary on the sand dollar side; full seagrass removal (blades and rhizomes) within 1m of the midline on the seagrass side; dual species removal, with both species fully removed from 0.5m on each side of the midline; and “mowed” seagrass, with blades removed within 1m of the midline. Eight parallel presence/absence transects were taken monthly (May-August 2014) within each plot, and the distribution of first contacts from the midline of each plot was compared across treatments.
Plots were analyzed as one-dimensional strips, like many individual tug-of-wars, to describe the invasion front of the plots. Control treatments showed slow, steady encroachment of seagrass into sand dollar territory, with gains up to 30 cm in two months. Sand dollars recolonized sand dollar removal treatments immediately, with comparable long-term results to the control. Seagrass removal treatments reinvaded removal areas slowly, but “mowed” treatments grew back more quickly. Seagrass removal and “mowed” treatments were invaded by sand dollars at modest rates, gaining as much as 50 cm, compared to controls where sand dollars lost ground. These results suggest that seagrass can outcompete sand dollars under low disturbance conditions, but sand dollars are quicker to colonize following disturbances like clam harvest and grazing by migratory waterfowl.