A keystone selective agent? Pine squirrels and the frequency of serotiny in lodgepole pine

Serotiny is a key life history trait in fire-prone habitats that is favored in plants that experience stand-replacing fires within their average life span. Although variation in fire frequency has been the focus of most studies attempting to understand variation in serotiny among populations and species of plants, other factors can select against serotiny. One agent in particular that can select against serotiny is an efficient pre-dispersal seed predator that eats a large fraction of the seeds held in the canopy. To test whether selection by such a predator selects against scrutiny, we compared levels of serotiny in fire-adapted Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia) in five ranges lacking its dominant pre-dispersal seed predator, the pine squirrel (Tamiasciurus hudsonicus) to 344 stands with pine squirrels. Where pine squirrels were absent, the frequency of serotiny was consistently near 100%, whereas where squirrels were present, the frequency was variable, rarely approached the high levels found in areas without squirrels, and averaged much less than 50%. This indicates that squirrels select against serotiny and that, in the absence of squirrels, the frequency of serotiny would probably be uniformly higher throughout much of the Rocky Mountains. Because serotiny levels also influence the density of seedlings following a fire, squirrels, by selecting strongly against scrutiny, have the potential to alter the early stages and perhaps the course of succession and various community attributes.