Eco-evolutionary dynamics in Pacific salmon

Abstract

Increasing acceptance of the idea that evolution can proceed rapidly has generated considerable interest in understanding the consequences of ongoing evolutionary change for populations, communities and ecosystems. The nascent field of 'eco-evolutionary dynamics' considers these interactions, including reciprocal feedbacks between evolution and ecology. Empirical support for eco-evolutionary dynamics has emerged from several model systems, and we here present some possibilities for diverse and strong effects in Pacific salmon (Oncorhynchus spp.). We specifically focus on the consequences that natural selection on body size can have for salmon population dynamics, community (bear-salmon) interactions and ecosystem process (fluxes of salmon biomass between habitats). For example, we find that shifts in body size because of selection can alter fluxes across habitats by up to 11% compared with ecological (that is, numerical) effects. More generally, we show that selection within a generation can have large effects on ecological dynamics and so should be included within a complete eco-evolutionary framework.

Figure 1

Conceptual model for understanding eco-evolutionary dynamics—a similar version appears in Bailey et al. 2009. The field of ecology has traditionally focused on the dynamics of populations, communities and ecosystems as well as feedbacks across these levels: that is, black solid lines in the figure. The field of evolutionary biology has traditionally focused on changes at the genetic or phenotypic levels, as well as feedbacks between them: that is, blue dotted lines in the figure. The field of eco-evolutionary dynamics considers both how changes in population, community and ecosystem dynamics affect phenotyes (green dashed lines) but also how phenotypic change influences populations, communities, and ecosystems (red dash-dotted lines). A full color version of this figure is available at the Heredity journal online.

Figure 2

Total number of sockeye salmon spawning in Hansen Creek (1986–2009).

Figure 3

Conceptual illustration of the Hansen creek study system highlighting downstream Lake Aleknagik, the shallow creek mouth, the upper portion of the creek and the creek riparian zone. Fluxes were calculated across habitats, with the flux change into the upper reaches of the creek reduced by mortality at the creek mouth and the flux change into the riparian zone increased by the transportation of salmon carcasses from the creek to the riparian zone by bears. We estimated the relative influence of ecology (size-independent mortality) and selection (size-dependent mortality) on these flux changes across years, and have included two plots to illustrate the influence of selection on the flux change into the creek because of mortality at the creek mouth (a) and the flux change into the riparian zone because of predation by bears (b). Artwork by J Kiyoko Shiosaki.

Figure 4

Length-frequency histograms highlighting the size selection occurring at the mouth of Hansen Creek for both female (top) and male (bottom) sockeye salmon in 2004, a year of strong selection at the creek mouth (Carlson and Quinn, 2007). The two bars represent those fish that stranded and died at the mouth of creek (black bars) versus those that successfully ascended the creek mouth and died in the creek itself (gray bars). Note that only a subset of fish dying at the mouth versus instream were measured for length and so these data emphasize size selection only, and not the overall mortality that occurred at the mouth.

Figure 5

Length-frequency histograms for a single year (2004) highlighting size-selective predation by bears on Hansen Creek sockeye salmon plotted separately for females (top) and males (bottom). The two bars represent those fish that died of senescence (gray bars) versus those that were killed by bears (black bars). Note that only a subset of fish dying of senescence versus because of bears were measured for length and so these data emphasize size-selective predation only, and not the overall mortality because of bear predation.

Figure 6

Length-at-ocean age for Hansen Creek female (top) and male (bottom) sockeye salmon based on data collected across the years 1998–2005. The bars correspond to ocean age 1 (black diagonal lines, males only), 2 (gray bars) and 3 (black bars).

Figure 7

Selection on salmon body size leads to indirect selection on age composition, which has implications for population dynamics. In panel (a), we have plotted the length-frequency histogram (left y axis) for the subset of male salmon that were measured for body length during the 2004 spawning season, representing males that died at the mouth or in the stream. The right y axis shows the percentage of known-aged males in each length bin that spent 2 years at sea (1 minus this proportion represents the percentage of known-aged males that spent 3 years at sea). We then estimated the age of all fish measured for length based on the percentage of known-aged two-ocean and three-ocean fish in each length bin, which we have plotted in panel (b). Note that this represents the pre-selection distribution of length-at-age for the subset of fish measured for body length. Finally, we estimated the post-selection distribution of length-at-age by estimating the age of all fish that successfully ascended the creek mouth, which we have plotted in panel (c). A comparison of panels b and c reveals that selection against large-bodied salmon indirectly favors younger (two-ocean) fish.