Competition for space among algae and sessile invertebrates is prevalent in benthic marine communities. Basic models of space competition predict that the species with a higher ratio of growth to mortality always excludes an inferior species, but a variety of mechanisms can produce more complex dynamics. Positive feedbacks such as recruitment facilitation produce alternate stable states where competitive outcomes depend on initial conditions and shifts may be difficult to reverse. Size dependence in competitive abilities may allow a weaker competitor to persist with an overgrowing competitor. As the strength of these interactions, both positive and negative, depends upon the spatial relationships between neighbors, models that explicitly consider space and local interactions may produce results quite different from well-mixed models. If habitat is heterogeneous, the outcome of competition can depend upon species' survival and dispersal in different habitat types. The amount and arrangements of habitat can further affect neighborhood interactions. I use spatially explicit simulations to ask how alternate stable states produced by recruitment facilitation in space competition between benthic invertebrates and algae are affected by habitat heterogeneity and dispersal distance. Using simulations of space competition with overgrowth, I ask whether size dependence in space competition improves the ability of coral to recover after disturbance when competing with crustose coralline algae (CCA). I then explore potential mechanisms for improving coral persistence, from facilitation and settlement cues to settlement habitats for coral larvae, competitive reversals, and patchy disturbance. I find that heterogeneous space does not eliminate alternative stable states, but does reduce the range in which they occur. Size refuges from overgrowth only allow coral to compete with CCA in homogeneous space when coral preempts space more rapidly, and then coral persistence is limited by the inability to invade established CCA. Positive interactions with other CCA species have only minor effects on coral persistence. Protected habitat for coral settlement provides a means for long-term coexistence with CCA, but additional mechanisms such as competitive reversal or patchy CCA mortality are necessary to allow coral to persist outside of protected habitat.
Thesis or Dissertation
Department of Ecology, Evolution and Marine Biology, UC Santa Barbara