The overall goal of my research is to understand the factors influencing the persistence of fish populations. In particular, I focus on diversity within and among populations as a mechanism for stability. Much of my work focuses on Pacific salmon but I am expanding into other study systems as well. Several themes have dominated recent research:
1. Ecology of natural selection
A consistent research thrust has been studies of natural selection in salmonid fishes to explore the ecological causes of selection, including abiotic (e.g., climate) and biotic (e.g., predation) factors. This has involved studies of juvenile salmonids in streams and studies of adult salmon on the breeding grounds. This research is important in showing how environmental heterogeneity contributes to the evolution of biological diversity. Current work in California is exploring the role of stream intermittency as a harsh abiotic filter shaping stream fish communities and traits.
2. Evolution (and loss) of biodiversity among salmon populations
Pacific salmon (Oncorhynchus spp.) are renowned for their extensive phenotypic and life history diversity. This biodiversity among salmon populations is thought to be important for long-term sustainability of population complexes due to shifts in production among the different life history components as a function of the prevailing environmental conditions. I am exploring stability-diversity relationships in California's recently collapsed fall-run Chinook salmon. Our initial results suggest that efforts to restore stability to this complex should consider how restoring different populations will affect the overall correlation structure and the synchrony in their abundance (Carlson & Satterthwaite 2011). Ongoing work funded by California Sea Grant is exploring the influence of hatchery release practices on the dynamics and stability of this stock complex. In the future, we will explore strategies for restoring the portfolio effect in this system thanks to a recently funded grant from California Dept. of Fish and Wildlife, including an explicit consideration of how management can incorporate trade-offs between long-term benefits of restoration and the potential opportunity costs for human systems in the short-term.
3. Eco-evolutionary dynamics
My expanding research interests include a focus on eco-evolutionary dynamics, that is, understanding how contemporary selection and evolution influence populations, communities, and ecosystems. As an example, I recently showed that selection on salmon body size influences the flux of salmon biomass across habitats (e.g., from the stream to the forest) and, importantly, that variation in the strength of selection among years alters these fluxes, which has consequences for the numerous organisms that consume salmon tissues.
As another new research thread, I ask the question, what are the conditions under which natural selection and adaptive evolution can rescue declining populations from impending extinction? To address this question, I have begun to conceptualize the process of extinction in a simple framework that explicitly considers natural selection as a demographic process. Natural selection, by definition, removes maladapted individuals from populations. When selection is very strong, it can move populations to a new (small) size such that demographic stochasticity can readily lead to extinction. If the population is able to weather this vulnerable period, however, selection can drive adaptive evolutionary change that might restore positive population growth, a phenomenon referred to as ‘evolutionary rescue’. Together with colleagues Peter Westley and Curry Cunningham, we have prepared a manuscript that distills the wealth of emergent literature on the theoretical foundations of evolutionary rescue, and identifies the laboratory examples of evolutionary rescue from model organisms, while also highlighting several heretofore unrecognized putative demonstrations of evolutionary rescue in the wild.
4. Harvest selection and evolution
I am very interested in understanding the role of “human predators” as agents of trait change in wild populations. Humans differ from other predators both in terms of the magnitude (number of individuals killed) and selectivity (traits of individuals killed) of predation. As a visiting scholar at the Centre for Ecological and Evolutionary Synthesis in Oslo, I quantified the form of natural and fishery selection acting on northern pike (Esox lucius) in Lake Windermere (U.K.) and found that the two selective forces often acted in opposition to one another. Moreover, my research demonstrated how natural selection and fishery selection interact to determine the total selection pressure experienced by this harvested population. We then investigated the relative role of harvest and natural selection in driving trait change in Windermere pike. I also worked with a group led by Chris Darimont to evaluate trait change in harvested populations compared to systems without harvest, which revealed that trait changes in exploited prey outpaced those in more natural contexts by 300%, on average. Ongoing work with Esben Olsen and Even Moland is exploring the possibility that fisheries remove dominant/bold individuals, which may have consequences for population growth inside and outside of marine reserves.
5. Evolutionary enlightened management
Recent research has demonstrated that adaptive evolution occurs on contemporary time scales (i.e., decades) and, moreover, that adaptive evolution is often associated with the same anthropogenic factors driving the current extinction crisis (e.g., overharvest, exotic species, habitat degradation). A long-term goal of mine is to apply evolutionary principles to guide management, conservation, and restoration efforts. Explicit consideration of the evolutionary challenges facing species and populations of concern should result in more effective management and conservation efforts. If this topic interests you, check out Evolutionary Applications, a journal devoted to these issues.