Alternative states in space and time: An examination of spatially dependent competitive interactions between ecosystem engineers

Background/Question/Methods

Ecosystem engineers alter their environments in ways that facilitate their own persistence. When two species engineer the same environment in opposing ways, competition results and a system of alternative stable states may emerge. We hypothesize that sand dollars and seagrasses in the Salish Sea of Washington state are such a system of alternative stable states. The sand dollar Dendraster excentricus lives in high density on shallow beaches and destroys sediment structure through burrowing, reducing infaunal biodiversity.  It co-occurs with the invasive seagrass Zostera japonica, whose rhizomes anchor sediment and provide structured habitat for epiphytes and invertebrates. Where the two co-occur, they form a patchy mosaic with little mingling between species. We asked: What factors mediate switches between states? Is either species a competitive dominant? We conducted field surveys of spatial patterns of D. excentricus and Z. japonica in the San Juan Archipelago, estimating percent cover over transects and through aerial photographs. We also conducted reciprocal transplants and removals of seagrass and sand dollars and monitored over one year to estimate rates of transition between states. Finally, we used these rates in a Markov chain model to estimate stable stage distributions of Z. japonica under different conditions.

Results/Conclusions

1) Field surveys of naturally occurring spatial distributions show a mutually exclusive condition between D. excentricus and Z. japonica.  Comparisons between aerial photographs at multiple time points showed slow changes in space. 2) D. excentricus rapidly returns to areas where individuals have been removed, but is unable to invade areas where seagrass is present. Z. japonica recovery from disturbance is slow and dependent on edge growth. However, the presence of D. excentricus did not negatively affect Z. japonica growth or persistence in any treatment. 3) Markov chain modeling of short-term dynamics shows that the surrounding matrix affects rates of species replacement. In addition, Z. japonica achieves dominance in all treatments except undisturbed sand dollars and reaches stability in less than two years in all treatments. These results suggest that when competing for space, Z. japonica may have the advantage. Over longer monitoring periods, however, seasonal trends may emerge. These dynamics merit further study into the role of spatial patchiness to maintaining coexistence.