Idiosyncratic shifts in life-history traits at species' geographic range edges

Abstract

Anthropogenic changes drive shifts in species' geographic distributions and increase the occurrence of leading or trailing-edge marginal populations. Theoretical predictions and empirical observations indicate substantial changes in life-history traits in marginal populations, often involving dispersal and reproductive abilities. Using a common garden experiment, we studied the variation of life-history traits of populations sampled on spatial gradients extending from range-core to range-edge habitats for three expanding (miner's lettuce Claytonia perfoliata, Danish scurvygrass Cochlearia danica and rock samphire Crithmum maritimum) and one receding plant species (dune pansy Viola tricolor subs. curtisii). We monitored life-history traits related to dispersal, phenology, survival, reproductive output and selfing ability. Significant shifts in life-history traits between central and marginal populations strongly differed among species. Marginal populations of the three expanding species displayed modified seed weight in natura, suggesting increased dispersal abilities in leading-edge populations. Discarding unassessed maternal effects, this trait modification can be due to phenotypic plasticity or to genetic differentiation. In miner's lettuce, marginal expanding populations show advanced phenology and higher reproductive output, that may potentially influence their colonization ability. In rock samphire, life-history traits showed large intra- and inter-population variability that did not follow a core-to-edge geographic trend, except for seed size. Finally, the receding populations of the dune pansy displayed a shift towards a plant architecture maximizing survival but reducing individual reproductive success. Altogether, our results indicated a common trend for increased dispersal abilities in marginal populations of expanding species. However, shifts in species' distributions may drive idiosyncratic changes in other life-history traits, for which we observed no general evolutionary syndrome at range edges. These findings go along a stochastic view of trait evolution during range expansion, and question how to draw predictive projections of species' distribution shifts under current global change.