Spatial pattern in ecosystems may be driven by competition. I uses a combination of observational field surveys across spatial and temporal scales, manipulative field experiments, experimental mesocosms and probabilistic and mechanistic models to study the dynamics of competition and coexistence between Zostera japonica and Dendraster excentricus and the resulting landscape spatial patterns between them. By studying the interactions between individuals and small patches of these two species, I have found insights about landscape-scale patchy spatial patterns. In the first chapter I tested the hypothesis that sand dollars and seagrass behave as alternative stable states of the same ecosystem. Using field experiments I demonstrated that either species could invade the other and persist in the habitat. Markov models and eigenvector analysis extended these results to a timespan in which persistence of states could be inferred, and found that the identity of surrounding areas is important to patch state changes. In the second chapter I examined spatial pattern of seagrass and sand dollars in the interaction zone. Through a paired experiment examining transition rates at boundaries and a cellular automaton model, I found that the ratio of neighbors of different identities strongly determines the probability of transition. These rates generate temporary spatial structure on small scales. Finally, in my third chapter, I examine aggregation as a mechanism for sand dollar resistance to seagrass spread. By examining the movement patterns and aggregating behavior of sand dollars in the field, mesocosms, and simulations, I built a framework for the study of sand dollar spatial pattern generation. I found that sand dollars alter their behavior when near conspecifics, slowing their velocity and reducing the frequency of movement, simple behavior changes that result in congregations. These results inform our understanding of alternative stable states and spatial patterns in nature, tell us about the interactions and competition between these two species in a specific place and may inform management of the two species and for seagrass conservation writ broadly.