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ABSTRACT It is unclear how environmental change influences standing genetic variation in wild populations. Here, we characterised environmental conditions that protect versus erode polymorphic chemical defences inBoechera stricta(Brassicaceae), a short‐lived perennial wildflower. By manipulating drought and herbivory in a 4‐year field experiment, we measured the effects of driver variation on vital rates of genotypes varying in defence chemistry and then assessed interacting driver effects on total fitness (estimated as each genotype's lineage growth rate,λ) using demographic models. Drought and herbivory interacted to shape vital rates, but contrasting defence genotypes had equivalent total fitness in many environments. Defence polymorphism thus may persist under a range of conditions; however, ambient field conditions fall close to the boundary of putatively polymorphic environment space, and increasing aridity may drive populations to monomorphism. Consequently, elevated intensity and/or frequency of drought under climate change may erode genetic variation for defence chemistry inB. stricta. 

ABSTRACT Theory suggests that the drivers of demographic variation and local adaptation are shared and may feedback on one other. Despite some evidence for these links in controlled settings, the relationship between local adaptation and demography remains largely unexplored in natural conditions. Using 10 years of demographic data and two reciprocal transplant experiments, we tested predictions about the relationship between the magnitude of local adaptation and demographic variation (population growth rates and their elasticities to vital rates) across 10 populations of a well‐studied annual plant. In both years, we found a strong unimodal relationship between mean home‐away local adaptation and stochastic population growth rates. Other predicted links were either weakly or not supported by our data. Our results suggest that declining and rapidly growing populations exhibit reduced local adaptation, potentially due to maladaptation and relaxed selection, respectively. 

Abstract All populations are affected by multiple environmental drivers, including climatic drivers such as temperature or precipitation and biotic drivers such as herbivory or mutualisms. The relative response of a population to each driver is critical to prioritizing threat mitigation for conservation and to understanding whether climatic or biotic drivers most strongly affect fitness. However, the importance of different drivers can vary dramatically across species and across populations of the same species. Theory suggests that the response to climatic versus biotic drivers can be affected by both the species' fundamental niche breadth and the latitude of the population at which the response is measured. However, we have few tests of how these two factors affect relative response to drivers separately, let alone tests of how niche breadth and latitude together influence responses. Here, we use a meta‐analysis of published studies on population response to climatic and biotic drivers in terrestrial plants, combined with estimates of climatic niche breadth and position within climatic niche derived from herbarium records, to show that species' niche breadth is the primary determinant of response to climatic versus biotic drivers. Namely, we find that response to climatic drivers changes only minimally with increasing niche breadth, while response to biotic drivers increases with niche breadth. We see similar relationships when considering range size instead of niche breadth. Surprisingly, we find no effects of latitude on the relative effect of climatic versus biotic drivers. Our work suggests that populations of species with small and large ranges experience similar extirpation risks due to the negative impacts of climate change. By contrast, populations of species with large (but not small) ranges may be highly susceptible to changes in densities or distributions of interacting species. 

Abstract Life table response experiments (LTREs) decompose differences in population growth rate between environments into separate contributions from each underlying demographic rate. However, most LTRE analyses make the unrealistic assumption that the relationships between demographic rates and environmental drivers are linear and independent, which may result in diminished accuracy when these assumptions are violated. We extend regression LTREs to incorporate nonlinear (second‐order) terms and compare the accuracy of both approaches for three previously published demographic datasets. We show that the second‐order approach equals or outperforms the linear approach for all three case studies, even when all of the underlying vital rate functions are linear. Nonlinear vital rate responses to driver changes contributed most to population growth rate responses, but life history changes also made substantial contributions. Our results suggest that moving from linear to second‐order LTRE analyses could improve our understanding of population responses to changing environments. 

Multiple, simultaneous environmental changes, in climatic/abiotic factors, interacting species, and direct human influences, are impacting natural populations and thus biodiversity, ecosystem services, and evolutionary trajectories. Determining whether the magnitudes of the population impacts of abiotic, biotic, and anthropogenic drivers differ, accounting for their direct effects and effects mediated through other drivers, would allow us to better predict population fates and design mitigation strategies. We compiled 644 paired values of the population growth rate ( λ ) from high and low levels of an identified driver from demographic studies of terrestrial plants. Among abiotic drivers, natural disturbance (not climate), and among biotic drivers, interactions with neighboring plants had the strongest effects on λ . However, when drivers were combined into the 3 main types, their average effects on λ did not differ. For the subset of studies that measured both the average and variability of the driver, λ was marginally more sensitive to 1 SD of change in abiotic drivers relative to biotic drivers, but sensitivity to biotic drivers was still substantial. Similar impact magnitudes for abiotic/biotic/anthropogenic drivers hold for plants of different growth forms, for different latitudinal zones, and for biomes characterized by harsher or milder abiotic conditions, suggesting that all 3 drivers have equivalent impacts across a variety of contexts. Thus, the best available information about the integrated effects of drivers on all demographic rates provides no justification for ignoring drivers of any of these 3 types when projecting ecological and evolutionary responses of populations and of biodiversity to environmental changes.