Landscape-scale eco-evolutionary dynamics: selection by seed predators and fire determine a major reproductive strategy.

Recent work in model systems has demonstrated significant effects of rapid evolutionary change on ecological processes (eco-evolutionary dynamics). Fewer studies have addressed whether eco-evolutionary dynamics structure natural ecosystems. We investigated variation in the frequency of serotiny in lodgepole pine (Pinus contorta), a widespread species in which postfire seedling density and ecosystem structure are largely determined by serotiny. Serotiny, the retention of mature seeds in cones in a canopy seed bank, is thought to be an adaptation for stand-replacing fire, but less attention has been paid to the potential selective effects of seed predation on serotiny. We hypothesized that spatial variation in percentage serotiny in lodgepole pine forests results from an eco-evolutionary dynamic where the local level of serotiny depends on the relative strengths of conflicting directional selection from fire (favoring serotiny) and seed predation (favoring cones that open at maturity). We measured percentage serotiny, the abundance of American red squirrels (Tamiasciurus hudsonicus; the primary pre-dispersal seed predator of lodgepole pine), and several measures of forest structure in Yellowstone National Park, USA. Fire frequency strongly predicted the frequency of serotiny, a pattern that is well-supported in the literature. At sites with high fire frequency (return intervals of -135-185 years) where fire favors increased serotiny, squirrel abundance was negatively associated with serotiny, suggesting that selection from predation can overwhelm selection from fire when squirrels are abundant. At sites with low fire frequency (return intervals of -280-310 years), serotiny was nearly universally uncommon (< 10%). Finally, forest structure strongly predicted squirrel density independently of serotiny, and serotiny provided no additional explanatory power, suggesting that the correlation is caused by selection against serotiny exerted by squirrels, rather than squirrels responding to variation in percentage serotiny.