The Coevolution of Pine Squirrels (Tamiasciurus) and Conifers

Abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecological Monographs.

Conflicting selection from fire and seed predation drives fine-scaled phenotypic variation in a widespread North American conifer

“…Although this is certainly unrealistic, the available data demonstrate that squirrel populations are stable over short to medium timescales (i.e., years to decades). In part, this is driven by stable cone production of lodgepole pine, which is the primary food source for red squirrels in this system (21,(30)(31)(32). The strictly territorial habit of red squirrels also promotes population stability, as vacant territories are rapidly reoccupied and overwinter survival of squirrels lacking territories is extremely low (28,31).…”

“…Cones that were not identifiable in this manner were treated as missing data and ignored for analysis. The rate of disappearance between intervals is an appropriate measure of the predation rate from red squirrels because no other organism removes cones from lodgepole pine (due to extreme defense requiring specialized predators), and cones do not generally fall from branches (even after they have opened) (21,25). Because survival of serotinous cones is likely to vary as the cone ages, we analyzed survival as a function of the degree of weathering, with separate estimates for green (first-year), brown (likely 1-to 5-y-old), and gray (>5-y-old) cones.…”

“…To address how squirrel density affected cone survival, we used the distance of each tree to the center of the nearest squirrel territory as a proxy for density. Territory centers were located by an exhaustive ground search by two observers and were identified by the presence of a large, active midden containing burrows and discarded cone remnants (21). Because red squirrels are strictly territorial, the average distance to a midden decreases with increasing density; for four sites where mean squirrel density for the entire site was known (from ref.…”

“…Squirrels rely on large caches of lodgepole pine cones to survive the winter, and because entire cones are removed and stored intact, seed dispersal as a result of squirrel foraging is negligible (28). Previous work has shown that energy intake rates on nonserotinous cones are slightly higher than on serotinous cones, implying that preferential feeding on nonserotinous cones is a more profitable strategy (21). However, red squirrels cache nonserotinous cones almost exclusively during several weeks in the fall between seed maturation and when cones open and seeds are dispersed.…”

“…Postdispersal seed predation may be another mechanism favoring the evolution of serotiny if delayed and synchronous seed release limits the abundance of postdispersal predators (7,20). However, escape from postdispersal predation may come at the expense of extreme losses to predispersal predators that can easily access the canopy seed bank (21,22), potentially exerting selection favoring increased allocation to seed defense (21,(23)(24)(25).…”