Population cycling is a widespread phenomenon, observed across a multitude of taxa in both laboratory and natural conditions. Historically, the theory associated with population cycles was tightly linked to pairwise consumer-resource interactions and studied via deterministic models, but current empirical and theoretical research reveals a much richer basis for ecological cycles. Stochasticity and seasonality can modulate or create cyclic behaviour in non-intuitive ways, the high-dimensionality in ecological systems can profoundly influence cycling, and so can demographic structure and eco-evolutionary dynamics. An inclusive theory for population cycles, ranging from ecosystem-level to demographic modelling, grounded in observational or experimental data, is therefore necessary to better understand observed cyclical patterns. In turn, by gaining better insight into the drivers of population cycles, we can begin to understand the causes of cycle gain and loss, how biodiversity interacts with population cycling, and how to effectively manage wildly fluctuating populations, all of which are growing domains of ecological research.

Rare phenotypes and behaviors adopted by only few individuals in a population are often overlooked, yet they may serve a heightened role for many organisms coping with warming climates. In threatened spring-run Chinook salmon spawning at the edge of the species range (Central Valley, CA USA), latemigrating juveniles were critical to cohort success in years characterized by multi-year droughts and ocean heatwaves. Late migrants rely on cool over-summer river temperatures, and are thus increasingly rare due to the combined effects of warming and dam construction. Yet our results suggest, the further loss of this within-population diversity could have critical impacts to their persistence in a warming climate. Our modeling predicts that thermally appropriate river conditions to support this phenotype will shrink rapidly in the future, and will primarily occur above impassable dams. Importantly, while late migrants dominated returns in some years, interannual variability in individual growth rates and migratory strategies suggests the importance of portfolio effects for these at-risk populations.Reconnecting and maintaining diverse habitat mosaics to support phenotypic and phenological diversity will be integral to the long term persistence of this species.