Reciprocal selection causes a coevolutionary arms race between crossbills and lodgepole pine

Few studies have shown both reciprocal selection and reciprocal adaptations for a coevolving system in the wild. The goal of our study was to determine whether the patterns of selection on Rocky Mountain lodgepole pine (Pinus contorta spp. latifolia) and red crossbills (Loxia curvirostra complex) were concordant with earlier published evidence of reciprocal adaptations in lodgepole pine and crossbills on isolated mountain ranges in the absence of red squirrels (Tamiasciurus hudsonicus). We found that selection (directional) by crossbills on lodgepole pine where Tamiasciurus are absent was divergent from the selection (directional) exerted by Tamiasciurus on lodgepole pine. This resulted in divergent selection between areas with and without Tamiasciurus that was congruent with the geographic patterns of cone variation. In the South Hills, Idaho, where Tamiasciurus are absent and red crossbills are thought to be coevolving with lodgepole pine, crossbills experienced stabilizing selection on bill size, with cone structure as the agent of selection. These results show that crossbills and lodgepole pine exhibit reciprocal adaptations in response to reciprocal selection, and they provide insight into the traits mediating and responding to selection in a coevolutionary arms race.     

Diversifying coevolution between crossbills and black spruce on Newfoundland

Coevolution is increasingly recognized as an important process structuring geographic variation in the form of selection for many populations. Here we consider the importance of a geographic mosaic of coevolution to patterns of crossbill (Loxia) diversity in the northern boreal forests of North America. We examine the relationships between geographic variation in cone morphology, bill morphology, and feeding performance to test the hypothesis that, in the absence of red squirrels (Tamiasciurus hudsonicus), black spruce (Picea mariana) has lost seed defenses directed at Tamiasciurus and that red crossbills (L curvirostra) and black spruce have coevolved in an evolutionary arms race. Comparisons of cone morphology and several indirect lines of evidence suggest that black spruce has evolved defenses in response to Tamiasciurus on mainland North America but has lost these defenses on Newfoundland. Cone traits that deter crossbills, including thicker scales that require larger forces to separate, are elevated in black spruce on Newfoundland, and larger billed crossbills have higher feeding performances than smaller billed crossbills on black spruce cones from Newfoundland. These results imply that the large bill of the Newfoundland crossbill (L. c. percna) evolved as an adaptation to the elevated cone defenses on Newfoundland and that crossbills and black spruce coevolved in an evolutionary arms race on Newfoundland during the last 9000 years since glaciers retreated. On the mainland where black spruce is not as well defended against crossbills, the small-billed white-winged crossbill (L leucoptera leucoptera) is more efficient and specializes on seeds in the partially closed cones. Finally, reciprocal adaptations between crossbills and conifers are replicated in black spruce and Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia), with coevolution most pronounced in isolated populations where Tamiasciurus are absent as a competitor. This study further supports the role of Tamiasciurus in determining the selection mosaic for crossbills and suggests that a geographic mosaic of coevolution has been a prominent factor underlying the diversification of North American crossbills.     

Interactions among moths, crossbills, squirrels, and lodgepole pine in a geographic selection mosaic

Repeated patterns among biological communities suggest similar evolutionary and ecological forces are acting on the communities. Conversely, the lack of such patterns suggests that similar forces are absent or additional ones are present. Coevolution between a seed predator, the red crossbill (Loxia curvirostra complex), and lodgepole pine (Pinus contorta var. latifolia) exemplifies the ecological and evolutionary predictions for coevolving systems. In the absence of another seed predator and preemptive competitor (pine squirrels Tamiasciurus hudsonicus), natural selection by crossbills results in the evolution of larger cones with thicker distal scales, while relaxation of selection by squirrels results in the evolution of cones with more seeds and a greater ratio of seed mass to cone mass. However, in one range, the Little Rocky Mountains, distal scale thickness has diverged as expected but cone size has not. In these mountains seed predation by lodgepole pine cone borer moths (Eucosma recissoriana) was about 10 times greater than in other ranges lacking squirrels. We quantified moth predation and cone traits and found that moths select for smaller cones with fewer seeds. Thus, selection by moths in the Little Rocky Mountains counters both selection by crossbills for large cone size and relaxation of selection by squirrels favoring more seeds per cone and accounts for the relatively small and few-seeded cones in these mountains. It is also apparent that selection by crossbills changes seed defenses in a manner that favors seed predation by moths, whereas selection by squirrels likely reduces such predation. These results demonstrate the importance of considering the evolutionary consequences of community context in locally evolved (coevolved) traits and interactions.     

Replicated population divergence caused by localized coevolution? A test of three hypotheses in the red crossbill‐lodgepole pine system

Several lines of evidence support the hypothesis that local populations of red crossbills (Loxia curvirostra complex) enter into a predator-prey arms race with lodgepole pine (Pinus contorta latifolia) in the absence of competing pine squirrels (Tamiasciurus hudsonicus). Nevertheless, the alternative hypotheses that neutral evolution or factors other than squirrels have caused crossbill population differentiation have not been thoroughly tested. We compared crossbill and pine cone morphology between island populations where squirrels are absent or present, and mainland sites where squirrels are present, in order to distinguish among these hypotheses. All comparisons supported an effect of squirrel absence, not island status, on crossbill and cone morphology. Hence our results provide further evidence that strong localized coevolutionary interactions in a geographic mosaic have driven adaptive population differentiation. In addition, vocal differentiation of crossbills was related to the absence of squirrels, but not to island status. As morphological and vocal differentiation is correlated with reproductive isolation in crossbills, the geographic mosaic of coevolution also seems to promote ecological speciation.     

Diversifying Coevolution between Crossbills and Conifers

Coevolution between granivorous crossbills (Loxia spp.) and conifers has been a prominent process in the diversification of crossbills. A striking example occurs in western North America where coevolution between crossbills and Rocky Mountain lodgepole pine (Pinus contorta latifolia) is ongoing in isolated ranges without the crossbill’s dominant competitor for seeds, the red squirrel (Tamiasciurus hudsonicus). Preferential foraging by crossbills on lodgepole pine cones in the South Hills and Albion Mountains, two small mountain ranges in southern Idaho where red squirrels are absent, has led to the evolution of larger, thicker-scaled cones than in nearby ranges where red squirrels are present. This in turn has favored the evolution of larger-billed crossbills that have diverged from other crossbills in the region. However, such diversifying coevolution, resulting from geographic variation in the distribution of strongly interacting species, is vulnerable to species introductions. For example, the introduction of red squirrels caused the precipitous decline and perhaps extinction of the Newfoundland crossbill and perhaps a crossbill endemic to the Cypress Hills, Canada. In general, species introductions act to reduce the geographic variation in species interactions, which may be critical for the diversification of many taxa.     

A role for habitat area in the geographic mosaic of coevolution between red crossbills and lodgepole pine

Understanding how resource abundance limits adaptive evolution and influences species interactions is an important step towards developing insight into the role of microevolutionary processes in establishing macroevolutionary patterns. We examined how variation in resource abundance (forest area of lodgepole pine Pinus contorta ssp. latifolia) influenced patterns of co-adaptation and coevolution between red crossbill (Loxia curvirostra complex) and lodgepole pine populations. First, we found that crossbill abundance increased logarithmically as forest area increased in mountain ranges lacking a preemptive competitor (pine squirrels Tamiasciurus hudsonicus). Second, seed defences against predation by crossbills increased with increases in crossbill density, suggesting that seed defences have likely evolved in proportion to the intensity of selection that crossbills exert. Third, the average bill size of crossbill populations increased with increasing seed defences, which implies that crossbill offenses increased with increases in seed defences. The large bill size on the largest range is the result of coevolution with lodgepole pine with this crossbill population perhaps speciating. Local adaptation of crossbill populations on smaller ranges, however, is more likely the result of resident crossbills representing a subset of the potential colonists (phenotypic sorting) than of local evolution. In the smallest range, migration and possibly more frequent extinction likely impede local adaptation and may result in maladaptation.     

Enhanced seed defenses potentially relax selection by seed predators against serotiny in lodgepole pine

Serotiny, the retention of seeds in a canopy seed bank until high temperatures cause seeds to be released, is an important life history trait for many woody plants in fire‐prone habitats. Serotiny provides a competitive advantage after fire but increases vulnerability to predispersal seed predation, due to the seeds being retained in clusters in predictable locations for extended periods. This creates opposing selection pressures. Serotiny is favored in areas of high fire frequency, but is selected against by predispersal seed predators. However, predation also selects for cone traits associated with seed defense that could reduce predation on serotinous cones and thereby relax selection against serotiny. This helps explain the elevated defenses in highly serotinous species. However, whether such interactions drive variation in seed defenses within variably serotinous populations has been studied rarely. We investigated the effects of phenotypic selection exerted by red squirrel (Tamiasciurus hudsonicus) predation on Rocky Mountain lodgepole pine (Pinus contorta latifolia) seeds. Squirrels preferentially harvested cones with more and larger seeds, indicating a preference for a higher food reward. We found evidence for stronger selection on trees with serotinous cones, which presumably accounts for the elevated defenses of and lower predation on serotinous compared to non‐serotinous cones. Lower levels of predation on serotinous cones in turn lessen selection against serotiny by squirrels. This has important implications because the frequency of serotiny in lodgepole pine has profound consequences for post‐fire communities and ecosystems widespread in the Rocky Mountains.     

CONSISTENCY AND VARIATION IN PHENOTYPIC SELECTION EXERTED BY A COMMUNITY OF SEED PREDATORS

Phenotypic selection that is sustained over time underlies both anagenesis and cladogenesis, but the conditions that lead to such selection and what causes variation in selection are not well known. We measured the selection exerted by three species of predispersal seed predators of lodgepole pine (Pinus contorta latifolia) in the South Hills, Idaho, and found that net selection on different cone and seed traits exerted by red crossbills (Loxia curvirostra) and cone borer moths (Eucosma recissoriana) over 10 years of seed crops was similar to that measured in another mountain range. We also found that the strength of selection increased as seed predation increased, which provides a mechanism for the correlation between the escalation of seed defenses and the density of seed predators. Red crossbills consume the most seeds and selection they exert accounts for much of the selection experienced by lodgepole pine, providing additional support for a coevolutionary arms race between crossbills and lodgepole pine in the South Hills. The third seed predator, hairy woodpeckers (Picoides villosus), consumed less than one‐sixth as many seeds as crossbills. Across the northern Rocky Mountains, woodpecker abundance and therefore selective impact appears limited by the elevated seed defenses of lodgepole pine.     

COEVOLUTION BETWEEN HISPANIOLAN CROSSBILLS AND PINE: DOES MORE TIME ALLOW FOR GREATER PHENOTYPIC ESCALATION AT LOWER LATITUDE?

Crossbills (Aves: Loxia) and several conifers have coevolved in predator-prey arms races over the last 10,000 years. However, the extent to which coevolutionary arms races have contributed to the adaptive radiation of crossbills or to any other adaptive radiation is largely unknown. Here we extend our previous studies of geographically structured coevolution by considering a crossbill-conifer interaction that has persisted for a much longer time period and involves a conifer with more variable annual seed production. We examined geographic variation in the cone and seed traits of two sister species of pines, Pinus occidentalis and P. cubensis, on the islands of Hispaniola and Cuba, respectively. We also compared the Hispaniolan crossbill (Loxia megaplaga) to its sister taxa the North American white-winged crossbill (Loxia leucoptera leucoptera). The Hispaniolan crossbill is endemic to Hispaniola whereas Cuba lacks crossbills. In addition and in contrast to previous studies, the variation in selection experienced by these pines due to crossbills is not confounded by the occurrence of selection by tree squirrels (Tamiasciurus and Sciurus). As predicted if P. occidentalis has evolved defenses in response to selection exerted by crossbills, cones of P. occidentalis have scales that are 53% thicker than those of P. cubensis. Cones of P. occidentalis, but not P. cubensis, also have well-developed spines, a known defense against vertebrate seed predators. Consistent with patterns of divergence seen in crossbills coevolving locally with other conifers, the Hispaniolan crossbill has evolved a bill that is 25% deeper than the white-winged crossbill. Together with phylogenetic analyses, our results suggest that predator-prey coevolution between Hispaniolan crossbills and P. occidentalis over approximately 600,000 years has caused substantial morphological evolution in both the crossbill and pine. This also indicates that cone crop fluctuations do not prevent crossbills and conifers from coevolving. Furthermore, because the traits at the phenotypic interface of the interaction apparently remain the same over at least several hundred thousand years, divergence as a result of coevolution is greater at lower latitude where crossbill-conifer interactions have been less interrupted by Pleistocene events.     

The geographic selection mosaic for ponderosa pine and crossbills: a tale of two squirrels

Recent research demonstrates how the occurrence of a preemptive competitor (Tamiasciurus) gives rise to a geographic mosaic of coevolution for crossbills (Loxia) and conifers. We extend these studies by examining ponderosa pine (Pinus ponderosa), which produces more variable annual seed crops than the conifers in previous studies and often cooccurs with tree squirrels in the genus Sciurus that are less specialized than Tamiasciurus on conifer seed. We found no evidence of seed defenses evolving in response to selection exerted by S. aberti, which was apparently overwhelmed by selection resulting from inner bark feeding that caused many developing cones to be destroyed. In the absence of S. aberti, defenses directed at crossbills increased, favoring larger-billed crossbills and causing stronger reciprocal selection between crossbills and ponderosa pine. However, crossbill nomadism in response to cone crop fluctuations prevents localized reciprocal adaptation by crossbills. In contrast, evolution in response to S. griseus has incidentally defended cones against crossbills, limiting the geographic range of the interaction between crossbills and ponderosa pine. Our results suggest that annual resource variation does not prevent competitors from shaping selection mosaics, although such fluctuations likely prevent fine-scale geographic differentiation in predators that are nomadic in response to resource variability.     

A coevolutionary arms race causes ecological speciation in crossbills

We examined three ecological factors potentially causing premating reproductive isolation to determine whether divergent selection as a result of coevolution between South Hills crossbills (Loxia curvirostra complex) and Rocky Mountain lodgepole pine (Pinus contorta latifolia) promotes ecological speciation. One factor was habitat isolation arising because of enhanced seed defenses of lodgepole pine in the South Hills. This caused the crossbill call types (morphologically and vocally differentiated forms) adapted to alternative resources to be rare. Another occurred when crossbills of other call types moved into the South Hills late in the breeding season and feeding conditions were deteriorating so that relatively few non-South Hills crossbills bred ("immigrant infecundity"). Finally, among those crossbills that bred, pairing was strongly assortative by call type (behavioral isolation). Total reproductive isolation between South Hills crossbills and the two other crossbills most common in the South Hills (call types 2 and 5) summed to .9975 and .9998, respectively, on a scale of 0 (no reproductive isolation) to 1 (complete reproductive isolation). These extremely high levels of reproductive isolation indicate that the divergent selection resulting from the coevolutionary arms race between crossbills and lodgepole pine is causing the South Hills crossbill to speciate.     

Patterns of exploitation of annually varying Pinus sylvestris cone crops by seed‐eaters of differing dispersal ability

Food consumption by animals depends on functional and numerical responses, and particularly the latter for food specialists. Some birds and mammals are conifer seed specialists and are likely to exploit the varying seed production in different ways according to differences in dispersal capabilities. This in turn may determine which species is more likely to affect the evolution of cone morphology. This study examined the inter‐annual pattern of foraging on Scots pine Pinus sylvestris cones by crossbills Loxia spp. (highly dispersive) and red squirrels Sciurus vulgaris (weakly dispersive) over 16 yr in three stands of ancient native pines at Abernethy Forest in Highland Scotland. There were synchronous annual variations in cone production across the stands, and an indication of a three‐year cycle. The number of cones taken by crossbills was correlated with cone production, indicating a numerical response by the birds, up to a certain limit of cone production. By contrast, the number of cones taken by red squirrels was not correlated with cone production. Rather, the percentage of cones taken by squirrels was high when cone production was low, and low when production was high. There was also a long‐term decline in the number of cones taken by squirrels, suggesting a decline in squirrel numbers. Although a high percentage of cones was removed from some cohorts on some trees, either by crossbills (maximum of 94.8%) or red squirrels (100%), the mean percentage of cones taken by crossbills from trees was small, ranging from 3.7 to 17.1% across all cone cohorts. For red squirrels, mean values ranged from 0.1 to 46.1% across all cohorts. However, given that crossbills can track changes in cone production by rapid numerical responses (i.e. through migration), and take a larger percentage of cones from cohorts of high production (10.7%), compared with red squirrels (3.6%), crossbills may be more important in driving the evolution of cone morphology because future trees are more likely to come from cohorts of high cone production.     

Home range,territoriality, caching behaviour and food supply of the red squirrel (Tamiasciurus hudsonicus fremonti) in a subalpine lodgepole pine forest

Caching behaviour, the size of cone caches, home range and territoriality in red squirrels (Tamiasciurus hudsonicus fremonti) were studied during the autumn of 1982 in a pure lodgepole pine (Pinus contorta) stand on the east slope of the Colorado Front Range of the Rocky Mountains, U.S.A. The studies were based on a new technique of tracking the movement of squirrels within their habitat using marked pine cones. This method indicated a mean home range size of 0.56 ha (range 0.29–0.80) and a squirrel density of 1.3 animals ha^?1. The squirrels were territorial, defending part or all of their home range. The number of cones on the trees within each home range appeared to be adequate to last the resident squirrel for a year or much longer. Middens varied in size and construction and there was considerable individual variation in caching behaviour and the number of cones cached in each midden. The supplies of food within the middens were small and would provide sufficient energy for no more than 1–4 weeks. Nevertheless, small caches of food may be critical to the overwinter survival of the squirrels in their harsh winter environment. It is suggested that pure lodgepole pine with its constancy of food supply is a good habitat for studying established theories about red squirrel territoriality and caching behaviour.     

CAN SELECTION BY AN ECTOPARASITE DRIVE A POPULATION OF RED CROSSBILLS FROM ITS ADAPTIVE PEAK?

The bill structures of different call types of red crossbills (Loxia curvirostra complex) in western North America usually approximate the predicted optima for foraging on single species of conifers. One clear exception is the call type in the South Hills, Idaho, that is coevolving in an evolutionary arms race with Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia). Although South Hills crossbills forage only on the cones of these lodgepole pines, their average bill depth is smaller than that predicted to be optimal. Because preliminary data showed that large-billed males were more likely to exhibit symptoms of ectoparasitic mite (Knemidokoptes jamaicensis) infestation, the goal of our study was to further quantify the incidence of mite infestation and determine whether selection by mites may have favored smaller-billed crossbills and thus driven crossbills away from the foraging optimum. We estimated annual survival of both infected and uninfected South Hills crossbills using program MARK, which allows for auxiliary variables such as bill size and sex to be included in survival analyses. Mite infestation depressed crossbill survival and, especially for males, caused directional selection against larger-billed individuals. Such selection may explain why South Hills crossbills have smaller bills than the optimum and why average bill size for males has decreased from 1998 to 2003. This selection may also explain why the degree of sexual size dimorphism has decreased by nearly 50% since 1998.     

The geographic selection mosaic for squirrels, crossbills and Aleppo pine

The interactions between many species are structured in a geographic mosaic of populations among which selection is divergent. Here we tested the hypothesis that such a geographic selection mosaic arises for common crossbills (Loxia curvirostra) feeding on seeds in the cones of Aleppo pine (Pinus halepensis) because of geographic variation in the occurrence of European red squirrels (Sciurus vulgaris). On the Iberian Peninsula, Sciurus exerted directional selection favouring larger cones with larger scales, which has caused cones there to be larger than in the Balearic Islands where Sciurus are absent. Moreover, cones on the Iberian Peninsula are so large that they are apparently little used by the relatively small-billed crossbills on the Peninsula; selection by Sciurus seems to have made the cones so difficult to feed on that crossbills rely mostly on the seeds of other conifers. Where crossbills are present but Sciurus are absent (Mallorca Island), cones were smaller as a result of relaxation of selection by Sciurus. However, cones on Mallorca had proportionally thicker scales in comparison to where both Sciurus and crossbills are absent (Ibiza Island), presumably as an adaptation against crossbill predation. Here crossbills specialize on Aleppo pine, have relatively large bills and have apparently coevolved in an arms race with Aleppo pine. These results suggest that Sciurus has influenced both the geographic selection mosaics for crossbills and conifers and the adaptive radiation of crossbills in Eurasia much like Tamiasciurus has done in the North America.     

THE IMPACT OF TREE SQUIRRELS (TAMIASCIURUS) ON LIMBER PINE SEED DISPERSAL ADAPTATIONS

Tree squirrels (Tamiasciurus) are important selective agents on conifer reproductive strategies (Smith 1970, 1975). Although this is well established for wind-dispersed pines, the impact of tree squirrels on bird-dispersed pines has been largely ignored. I assessed the impact of tree squirrels on the allocation of reproductive energy in the bird-dispersed limber pine (Pinus flexilis) by comparing its cone and seed traits from three sites in the Rocky Mountains where tree squirrels (Tamiasciurus) are present to those from three mountain ranges in the Great Basin where tree squirrels are absent. As predicted, differences between the two regions in individual cone and seed traits are consistent with the hypothesis that tree squirrels are important selective agents on these traits. In the absence of tree squirrels, limber pine allocates more than twice as much energy to kernel compared with that invested in putative seed defenses (cone, resin, and seed coat) as does limber pine where tree squirrels are present. Such a large difference is particularly striking, because tree squirrels may have become extinct in the Great Basin in only the last 12,000 yr. Although many factors influence the allocation of energy to cones and seeds, no single factor other than the presence of tree squirrels is compatible with the large and consistent differences between limber pine in the Rocky Mountains and Great Basin. These results show that tree squirrels are an important constraint on the evolution of cone and seed traits that promote the dispersal of seeds by birds.     

Within-season synchrony of a masting conifer enhances seed escape

Predator satiation resulting from interannual reproductive synchrony has been widely documented in masting plants, but how reproductive synchrony within a year influences seed escape is poorly understood. We evaluated whether the intra-annual reproductive synchrony of individual white spruce trees (Picea glauca) increased seed escape from their primary predispersal seed predator, North American red squirrels (Tamiasciurus hudsonicus). Trees with cones that matured synchronously relative to those of other trees within red squirrel territories were significantly more likely to escape squirrel predation in years with both low and superabundant levels of cone production, generating a significantly positive linear selection differential for increasing intra-annual reproductive synchrony. Thus, this masting plant escapes seed predation in numbers through interannual synchrony in seed production and in time through intra-annual synchrony of seed availability.     

Cones structure and seed traits of four species of large‐seeded pines: Adaptation to animal‐mediated dispersal

Seed dispersal selection pressures may cause morphological differences in cone structure and seed traits of large‐seeded pine trees. We investigated the cone, seed, and scale traits of four species of animal‐dispersed pine trees to explore the adaptations of morphological structures to different dispersers. The four focal pines analyzed in this study were Chinese white pine (Pinus armandi), Korean pine (P. koraiensis), Siberian dwarf pine (P. pumila), and Dabieshan white pine (P. dabeshanensis). There are significant differences in the traits of the cones and seeds of these four animal‐dispersed pines. The scales of Korean pine and Siberian dwarf pine are somewhat opened after cone maturity, the seeds are closely combined with scales, and the seed coat and scales are thick. The cones of Chinese white pine and Dabieshan white pine are open after ripening, the seeds fall easily from the cones, and the seed coat and seed scales are relatively thin. The results showed that the cone structure of Chinese white pine is similar to that of Dabieshan white pine, whereas Korean pine and Siberian dwarf pine are significantly different from the other two pines and vary significantly from each other. This suggests that species with similar seed dispersal strategies exhibit similar morphological adaptions. Accordingly, we predicted three possible seed dispersal paradigms for animal‐dispersed pines: the first, as represented by Chinese white pine and Dabieshan white pine, relies upon small forest rodents for seed dispersal; the second, represented by Korean pine, relies primarily on birds and squirrels to disperse the seeds; and the third, represented by Siberian dwarf pine, relies primarily on birds for seed dispersal. Our study highlights the significance of animal seed dispersal in shaping cone morphology, and our predictions provide a theoretical framework for research investigating the coevolution of large‐seeded pines and their seed dispersers.     

Coevolution between crossbills and black pine: the importance of competitors,forest area and resource stability

Studies of predator‐prey interactions have found that geographically structured coevolution has played an important role in the adaptive diversification of crossbills (Loxia spp.). We extend those studies by considering common crossbills (L. curvirostra) in the Mediterranean where they rely on seeds in the cones of black pine (Pinus nigra). On the continent, where tree squirrels (Sciurus vulgaris) are present, enhanced defenses against crossbills were most evident in larger areas of pine forest. On islands in the absence of tree squirrels, crossbills and black pine have coevolved in a predator‐prey arms race on Cyprus but not Corsica. In contrast to other conifers that island endemic crossbills rely upon, black pine does not hold seeds in its cones year round. Consequently, key to the strong crossbill–pine interaction on Cyprus is likely the presence of an alternative conifer that provides seeds during early summer when black pine seeds are scarce.     

From the ground up: biotic and abiotic features that set the course from genes to ecosystems

Spatial variation in cone serotiny in Rocky Mountain lodgepole pine (Pinus contorta ssp. latifolia) across Yellowstone National Park influences initial pine recruitment after stand‐replacing fire with tremendous population, community, and ecosystem consequences. A previous study showed that much of the spatial variation in serotiny results from the balance of selection arising from high frequencies of fire favoring serotiny countered by opposing selection exerted by American red squirrels (Tamiasciurus hudsonicus) as seed predators. This earlier study, however, assumed stable local red squirrel densities over multiple generations of pines. Here, we examine environmental properties that might contribute to long‐term stability in the densities of red squirrels among sites. We found that the amount of clay in the soil, an indicator of plant and fungal growth—the latter an important food resource for red squirrels—and the coefficient of variation (CV) in diameter at breast height (DBH) of forest trees together account for a substantial amount of variation in red squirrel density. Soil development occurs over very long time scales, and thus, intersite variation in the amount of clay is unlikely to shift across pine generations. However, CV of DBH and squirrel density increase with stand age, which acts to amplify selection against serotiny with increasing interfire interval. Regardless, much of the variation in the CV of DBH is accounted for by soil bulk density, mean annual temperature, and surface curvature, which are unlikely to vary in their relative differences among sites over time. Consequently, these soil and abiotic attributes could contribute to consistent spatial patterns of red squirrel densities from one pine generation to the next, resulting in consistent local and spatial variation in selection exerted by red squirrels against serotiny.