Evolutionary rescue and the limits to phenotypic plasticity: testing theory in the field

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Rapid climate change and habitat loss will cause many species to become extinct this century unless they can cope with changing and more extreme ecological conditions. Understanding what limits species' ecological tolerances is therefore an issue of critical scientific importance because it allows us to predict the consequences of ongoing rates of environmental change to populations and therefore to ecological communities. A common way that organisms deal with environmental variation is to be 'plastic', i.e. to change their morphological, physiological or behavioural traits (their phenotypes) directly in response to their local environment, without requiring rapid evolutionary change. Such 'phenotypic plasticity' buffers changes in the environment, and can maintain fitness across the range of environments typically experienced by a species. Currently, most of the global responses of biodiversity to climate change have been ascribed to such phenotypic plasticity, rather than to actual evolutionary change, underlining its importance in maintaining ecological outputs.However, the ability of phenotypic plasticity to cope with environmental change has limits. Not only is maintaining variation in gene expression likely to be energetically expensive, it also evolves to maintain fitness only within the range of environments a species experiences in its recent past. In novel or extreme conditions, there is therefore no reason that a species' plastic responses will still improve their ability to survive and produce offspring. Instead, plastic responses that were adaptive in former environments may actually reduce their fitness in new environments. This idea is especially worrying because it predicts that plasticity will be unable to cope as ecological change continues, leading to sudden population declines as critical environmental limits are exceeded. By contrast, other theoretical models predict that plastic responses will be able to evolve more quickly in novel environments, generating faster evolutionary responses than predicted by laboratory experiments under common garden conditions. We will test these theoretical predictions by measuring the plastic responses of two ecologically divergent species of ragwort (genus Senecio) to changes in their altitudinal position, both within and outside their prevailing distributions on the slopes of Mount Etna, Sicily. These species, Senecio aethnensis and S. chrysanthemifolius differ in a number of phenotypic traits, as well as in the expression of key genes that are associated with adaptation to different altitudes. We will transplant genotypes of both species into a range of field conditions and monitor their performance and plasticity over a two-year period in order to determine each genotype's response to conditions outside its normal ('home') environment. We will measure growth and development parameters, and reproductive parameters as a measure of each genotype's local fitness, and test the degree to which the declines in fitness expected with changes in altitude are offset by plastic changes in their phenotype and in gene expression. We predict that although observed plastic responses will keep individuals healthy and productive under their species' usual range of altitudinal conditions, phenotypic responses will no longer be appropriate with altitudinal changes beyond these limits. Such an empirical finding will have important implications for predicting the continued ability of species to respond plastically to climate change. In particular, it will suggest, the rapid evolution will be necessary to prevent population and species' extinction where rates of environmental change exceed prevailing conditions within their geographical range.