Extreme environmental variation sharpens selection that drives the evolution of a mutualism

Paired acoustic recordings and point count surveys reveal Clark's nutcracker and whitebark pine associations across Glacier National Park

Global declines in tree populations have led to dramatic shifts in forest ecosystem composition, biodiversity, and functioning. These changes have consequences for both forest plant and wildlife communities, particularly when declining species are involved in coevolved mutualisms. Whitebark pine (Pinus albicaulis) is a declining keystone species in western North American high-elevation ecosystems and an obligate mutualist of Clark's nutcracker (Nucifraga columbiana), an avian seed predator and disperser. By leveraging traditional point count surveys and passive acoustic monitoring, we investigated how stand characteristics of whitebark pine in a protected area (Glacier National Park, Montana, USA) influenced occupancy and vocal activity patterns in Clark's nutcracker. Using Bayesian spatial occupancy models and generalized linear mixed models, we found that habitat use of Clark's nutcracker was primarily supported by greater cone density and increasing diameter of live whitebark pine. Additionally, we demonstrated the value of performing parallel analyses with traditional point count surveys and passive acoustic monitoring to provide multiple lines of evidence for relationships between Clark's nutcracker and whitebark pine forest characteristics. Our findings allow managers to gauge the whitebark pine conditions important for retaining high nutcracker visitation and prioritize management efforts in whitebark pine ecosystems with low nutcracker visitation.

Urbanization shapes phenotypic selection of fruit traits in a seed-dispersal mutualism

Urbanization is currently one of the trademarks of the Anthropocene, accelerating evolutionary processes and reshaping ecological interactions over short time scales. Species interactions represent a fundamental pillar of diversity that is being altered globally by anthropogenic change. Urban environments, despite their potential impact, have seldom been studied in relation to how they shape natural selection of phenotypic traits in multispecies interactions. Using a seed-dispersal mutualism as a study system, we estimated the regime and magnitude of phenotypic selection exerted by frugivores on fruit and seed traits across three plant populations with different degrees of urbanization (urban, semiurban, and rural). Urbanization weakened phenotypic selection via an indirect positive impact on fruit production and fitness and, to a lesser extent, through a direct positive effect on species visitation rates. Our results show that urban ecosystems may affect multifarious selection of traits in the short term and highlight the role of humans in shaping eco-evolutionary dynamics of multispecies interactions.

Seed Masting Causes Fluctuations in Optimum Litter Size and Lag Load in a Seed Predator

The episodic production of large seed crops by some perennial plants (masting) is known to increase seed escape by alternately starving and swamping seed predators. These pulses of resources might also act as an agent of selection on the life histories of seed predators, which could indirectly enhance seed escape by inducing an evolutionary load on seed predator populations. We measured natural selection on litter size of female North American red squirrels (Tamiasciurus hudsonicus) across 28 years and five white spruce (Picea glauca) masting events. Observed litter sizes were similar to optimum litter sizes during nonmast years but were well below optimum litter sizes during mast years. Mast events therefore caused selection for larger litters ( β'=0.25 ) and a lag load ( L=0.25 ) on red squirrels during mast years. Reduced juvenile recruitment associated with this lag load increased the number of spruce cones escaping squirrel predation. Although offspring and parents often experienced opposite environments with respect to the mast, we found no effect of environmental mismatches across generations on either offspring survival or population growth. Instead, squirrels plastically increased litter sizes in anticipation of mast events, which partially, although not completely, reduced the lag load resulting from this change in food availability. These results therefore suggest that in addition to ecological and behavioral effects on seed predators, mast seed production can further enhance seed escape by inducing maladaptation in seed predators through fluctuations in optimal trait values.

Climate extremes are associated with invertebrate taxonomic and functional composition in mountain lakes

Climate change is expected to increase climate variability and the occurrence of extreme climatic events, with potentially devastating effects on aquatic ecosystems. However, little is known about the role of climate extremes in structuring aquatic communities or the interplay between climate and local abiotic and biotic factors. Here, we examine the relative influence of climate and local abiotic and biotic conditions on biodiversity and community structure in lake invertebrates. We sampled aquatic invertebrates and measured environmental variables in 19 lakes throughout California, USA, to test hypotheses of the relationship between climate, local biotic and environmental conditions, and the taxonomic and functional structure of aquatic invertebrate communities. We found that, while local biotic and abiotic factors such as habitat availability and conductivity were the most consistent predictors of alpha diversity, extreme climate conditions such as maximum summer temperature and dry-season precipitation were most often associated with multivariate taxonomic and functional composition. Specifically, sites with high maximum temperatures and low dry-season precipitation housed communities containing high abundances of large predatory taxa. Furthermore, both climate dissimilarity and abiotic dissimilarity determined taxonomic turnover among sites (beta diversity). These findings suggest that while local-scale environmental variables may predict alpha diversity, climatic variability is important to consider when projecting broad-scale aquatic community responses to the extreme temperature and precipitation events that are expected for much of the world during the next century.

Biotic interaction strength and the intensity of selection

Although the ecological and evolutionary impacts of species interactions have been the foci of much research, the relationship between the strength of species interactions and the intensity of selection has been investigated only rarely. I develop a simple model demonstrating how the opportunity for selection varies with interaction strength, and then use the relationship between the maximum value of the selection differential and the opportunity for selection (Arnold & Wade 1984) to evaluate how selection differentials vary in relation to species interaction strength. This model predicts an initial deceleration and then an accelerating increase in the intensity of selection with increasing strength of antagonistic interactions and with decreasing strength of mutualistic interactions. Empirical data from several studies provide support for this model. These results further support an evolutionary mechanism for some striking patterns of evolutionary diversification including the latitudinal species gradient, and should be relevant to studies of eco-evolutionary dynamics.

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.

Species interactions alter evolutionary responses to a novel environment

Studies of evolutionary responses to novel environments typically consider single species or perhaps pairs of interacting species. However, all organisms co-occur with many other species, resulting in evolutionary dynamics that might not match those predicted using single species approaches. Recent theories predict that species interactions in diverse systems can influence how component species evolve in response to environmental change. In turn, evolution might have consequences for ecosystem functioning. We used experimental communities of five bacterial species to show that species interactions have a major impact on adaptation to a novel environment in the laboratory. Species in communities diverged in their use of resources compared with the same species in monocultures and evolved to use waste products generated by other species. This generally led to a trade-off between adaptation to the abiotic and biotic components of the environment, such that species evolving in communities had lower growth rates when assayed in the absence of other species. Based on growth assays and on nuclear magnetic resonance (NMR) spectroscopy of resource use, all species evolved more in communities than they did in monocultures. The evolutionary changes had significant repercussions for the functioning of these experimental ecosystems: communities reassembled from isolates that had evolved in polyculture were more productive than those reassembled from isolates that had evolved in monoculture. Our results show that the way in which species adapt to new environments depends critically on the biotic environment of co-occurring species. Moreover, predicting how functioning of complex ecosystems will respond to an environmental change requires knowing how species interactions will evolve.

Conflicting selection from an antagonist and a mutualist enhances phenotypic variation in a plant

The raw material for evolution is variation. Consequently, identifying the factors that generate, maintain, and erode phenotypic and genetic variation in ecologically important traits within and among populations is important. Although persistent directional or stabilizing selection can deplete variation, spatial variation in conflicting directional selection can enhance variation. Here, we present evidence that phenotypic variation in limber pine (Pinus flexilis) cone structure is enhanced by conflicting selection pressures exerted by its mutualistic seed disperser (Clark's nutcracker Nucifraga columbiana) and an antagonistic seed predator (pine squirrel Tamiasciurus spp.). Phenotypic variation in cone structure was bimodal and about two times greater where both agents of selection co-occurred than where one (the seed predator) was absent. Within the region where both agents of selection co-occurred, bimodality in cone structure was pronounced where there appears to be a mosaic of habitats with some persistent habitats supporting only the seed disperser. These results indicate that conflicting selection stemming from spatial variation in community diversity can enhance phenotypic variation in ecologically important traits.

The roles of tolerance in the evolution, maintenance and breakdown of mutualism

Tolerance strategies are cost-reduction mechanisms that enable organisms to recover some of the fitness lost to damage, but impose limited or no cost on antagonists. They are frequently invoked in studies of plant-herbivore and of host-parasite interactions, but the possible roles of tolerance in mutualism (interspecific cooperation) have yet to be thoroughly examined. This review identifies candidate roles for tolerance in the evolution, maintenance and breakdown of mutualism. Firstly, by reducing the cost of damage, tolerance provides a key pathway by which pre-mutualistic hosts can reduce the cost of association with their parasites, promoting cooperation. This holds for the evolution of 'evolved dependency' type mutualism, where a host requires an antagonist that does not direct any reward to their partner for some resource, and of 'outright mutualism', where participants directly trade benefits. Secondly, in outright mutualism, tolerance might maintain cooperation by reducing the cost of a persisting negative trait in a symbiotic partner. Finally, the evolution of tolerance might also provide a pathway out of mutualism because the host could evolve a cheaper alternative to continued cooperation with its mutualistic partner, permitting autonomy. A key consequence of tolerance is that it contrasts with partner choice mechanisms that impose large costs on cheats, and I highlight understanding any trade-off between tolerance and partner choice as an important research topic in the evolution of cooperation. I conclude by identifying tolerance as part of a more general phenomenon of co-adaptation in mutualism and parasitism that drives the evolution of the cost/benefit ratio from the interaction.

A seed predator drives the evolution of a seed dispersal mutualism

Although antagonists are hypothesized to impede the evolution of mutualisms, they may simultaneously exert selection favouring the evolution of alternative mutualistic interactions. We found that increases in limber pine (Pinus flexilis) seed defences arising from selection exerted by a pre-dispersal seed predator (red squirrel Tamiasciurus hudsonicus) reduced the efficacy of limber pine's primary seed disperser (Clark's nutcracker Nucifraga columbiana) while enhancing seed dispersal by ground-foraging scatter-hoarding rodents (Peromyscus). Thus, there is a shift from relying on primary seed dispersal by birds in areas without red squirrels, to an increasing reliance on secondary seed dispersal by scatter-hoarding rodents in areas with red squirrels. Seed predators can therefore drive the evolution of seed defences, which in turn favour alternative seed dispersal mutualisms that lead to major changes in the mode of seed dispersal. Given that adaptive evolution in response to antagonists frequently impedes one kind of mutualistic interaction, the evolution of alternative mutualistic interactions may be a common by-product.

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.