The influence of a competitor on the geographic mosaic of coevolution between crossbills and lodgepole pine

The geographic mosaic theory of coevolution posits that the form of selecti on between interacting species varies across a landscape with coevolution i mportant and active in some locations (i.e., coevolutionary hotspots) but n ot in others (i.e., coevolutionary coldspots). We tested the hypothesis tha t the presence of red squirrels (Tamiasciurus hudsonicus) affects the occur rence of coevolution between red crossbills (Loxia curvirostra complex) and Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia) and thereby provides a mechanism giving rise to a geographic mosaic of selection. Red s quirrels are the predominant predispersal seed predator and selective agent on lodgepole pine cones. However, in four isolated mountain ranges east an d west of the Rocky Mountains, red squirrels are absent and red crossbills are the main predispersal seed predator. These isolated populations of pine have apparently evolved without Tamiasciurus for about 10,000 to 12,000 ye ars. Based on published morphological, genetic, and paleobotanical studies, we infer that cone traits in these isolated populations that show parallel differences from cones in the Rocky Mountains have changed in parallel. We used data on crossbill and conifer cone morphology and feeding preferences and efficiency to detect whether red crossbills and lodgepole pine exhibit reciprocal adaptations, which would imply coevolution. Cone traits that ac t to deter Tamiasciurus and result in high ratios of cone mass to seed mass were less developed in the isolated populations. Cone traits that act to d eter crossbills include larger and thicker scales and perhaps increased ove rlap between successive scales and were enhanced in the isolated population s. In the larger, isolated mountain ranges crossbills have evolved deeper, shorter, and therefore more decurved bills to exploit these cones. This pro vides crossbills with higher feeding rates, and the change in bill shape ha s improved efficiency by reducing the concomitant increases in body mass an d daily energy expenditures that would have resulted if only bill size had increased. These parallel adaptations and counter adaptations in red crossb ills and lodgepole pine are interpreted as reciprocal adaptations and imply that these crossbills and pine are in coevolutionary arms races where red squirrels are absent (i.e., coevolutionary hotspots) but not where red squi rrels are present (i.e., coevolutionary coldspots).