The Differential Importance of Stream Units and Their Connectivity for Population Persistence

Local habitat quality and connectivity among habitats are key drivers of local population persistence in small streams. We evaluated how connectivity, stream order and habitat quality, and environmental stochasticity contribute to population dynamics using data from 10 cohorts of individually-tagged brook trout in a 2^nd and 3^rd order stream system in Massachusetts. With Bayesian hierarchical models, we identified spatial and temporal sources of (co)variation in body growth, survival, and movement. Connectivity was critical for the survival and body growth of larger fish as they outgrew smaller tributaries. After growing in the mainstem, these fish returned to spawn in the small tributaries in fall, but simulations suggested very rapid local extinction (2-5 generations) in the tributaries if access was blocked. Fish in a naturally-isolated population survived despite isolation because they did not emigrate, reproduced younger and appeared to experience lower environmental variation compared to fish in the connected tributaries.  Although there was considerable variation in seasonal survival across years, population growth was predominately constrained by reduced body growth in hot/dry summers. Combining multiple-year sampling of a stream system with advanced statistical analysis, we were able to identify the key pathways driving population dynamics.