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The relative contributions of recruitment and post-recruitment processes are pivotal to understanding the mechanisms influencing the population dynamics of organisms. We estimated recruitment by oviposition to populations of the mayfly Baetis bicaudatus in multiple streams of one drainage basin in western Colorado, USA, and subsequent changes in larval population densities of one cohort over one generation. Our goal was to compare the potential for recruitment limitation and post-recruitment processes to explain variation in population dynamics of one cohort in streams with predatory fish (brook trout) and fishless streams. We censused Baetis egg masses oviposited in 14 streams during summer 2001, and estimated larval densities of this cohort during September 2001, April and June 2002. There was substantial variability among streams in densities of eggs and subsequent larval densities, with qualitatively different larval dynamics in fish and fishless streams. In fishless streams egg densities were generally lower than in fish streams due to hydrogeomorphic habitat constraints; and increasing egg densities resulted in higher densities of early stage larvae. This pattern was consistent with recruitment limitation of population size of early stages, which persisted until April. In contrast, increasing egg densities in trout streams resulted in lower densities of early stage larvae, consistent with strong density-dependent losses due to post-recruitment processes. In both fish and fishless streams the loss rates of early stage larvae during winter were density-independent; but from April to June the pattern suggests strong density-dependent losses of larvae in fish streams, and weaker, although significant density-dependent losses in fishless streams as larvae developed to late instars from April to June. We conclude that recruitment can limit the population size of early larval stages of Baetis related to the availability of preferred oviposition sites for females; but post-recruitment processes eventually (in lower risk environments) or almost immediately swamp the effects of recruitment on population dynamics especially in fish streams. This study illustrates that the relative influences of recruitment limitation and post-recruitment processes on mayfly population dynamics varies across life stages and among environments with different habitat constraints and predation risks.
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