Identifying Regional Shifts in Complex Species Interactions in the Gulf of Maine Mid-Intertidal

There is ample evidence that complex species interactions are important to the dynamics, structure, and function of ecological communities, and these can vary in space and time. Ecologists struggle to account for the vast majority of interactions during experiments because most quantification methods involve an intractable number of manipulations that scale with species richness. Multispecies Markov Chan Models (MMCMs) utilize probability matrices that describe how spatial units (e.g., rocky substrate) transition from one species to anther over a fixed period of time. Using individual matrix elements (transition probabilities) as estimates of species interactions, MMCMs provide a useful tool to capture multiple species interactions simultaneously, and test questions regarding the complex and indirect pathways via which individual species influence the dynamics and structure of communities. Here we generate MMCMs for Gulf of Maine (GOM) rocky mid-intertidal shores. We test the impact of spatial and temporal variation in community dynamics on MMCM predictions of community structure, and infer which ecological processes are important in structuring communities and causing community level shifts. Seasonal and annual variation are relatively unimportant to predict community structure, but regional variation is of critical importance. Of the pathways (transition probabilities) that differ regionally (northern, midcoast, and southern GOM), and potentially drive regional shifts in community structure and dynamics, different recruitment of a competitively dominant species (Mytilus eduis) seem to be important. Where M. edulis recruitment is high (southern GOM), it outcompetes others for space. Where M. edulis recruitment is low (mid and northern GOM), competitively subordinate species colonize available space, and regional differences in predation pressure determine the outcome of community dynamics. We test the role of a major predator (the dogwhelk - Nucella lapillus) in modulating species interactions and find that reducing dogwhelk density has minimal impact on community structure and M. edulis mortality, but decreases Semibalanus balanoides mortality. In summary, we find that Multispeices Markov Chain Models are useful tools in studying complex species interactions, and that a suite of direct and indirect effects are responsible for shifts in community structure and dynamics both regionally in the GOM mid-intertidal, and in response to a mobile predator