Aim We propose a trait-based approach for assessing and mapping potential niche overlap between native and exotic species at large spatial scales.Location The Mediterranean Sea.Methods We developed two complementary indices based on ecological, biological and ecomorphological similarities between native and exotic species. The first index (FNN r ) allows identifying areas where native species are the most at risk in terms of potential ecological interactions with exotic species. The second index (FGO) is species-specific and allows identifying native species that display the broadest functional niche overlap and range overlap with exotic species. We illustrated our approach using the Mediterranean coastal fish fauna as a case study. Results We found that endemic and exotic fishes had a similar global functional niche at the Mediterranean scale, that is, they filled the same functional space defined by trait axes. FNN r hotspots were found to be moderately congruent with exotic species richness hotspots. Higher values of FNN r were observed along the coasts of the Levantine Sea. The computation of the FGO index showed that the geographical range of a given endemic species overlapped in average with 52 exotic species. Species showing the highest FGO values displayed localized and/or fragmented distributions in the eastern Mediterranean basin.Main conclusions Our findings suggest that the number of exotic species alone cannot be used as a broad-scale indicator of potential impact because this metric does not account for functional relatedness between native and exotic species. The trait-based indices developed in this study can be used for other taxa in both aquatic and terrestrial ecosystems and should help environmental managers to set up local-scale studies on areas where the potential impact of exotic species on native biodiversity is the highest.
Tree species in the northern hemisphere have advanced and retreated with interglacial and glacial periods, and are currently subject to rapid anthropogenic climate change. These observations prompt questions about the mechanisms allowing tree populations to respond quickly to selection pressures when establishing into new areas. Focusing on the northern expanding range edge of Picea sitchensis, a widespread conifer of western North America, we ask how genetic structure and diversity develop during colonization, and assess the role of demographic history in shaping the evolutionary trajectory of a colonizing population. By combining nearly 500 years of tree-ring and genetic data at the expansion front on the Kodiak Archipelago, we show that allelic richness - but not expected heterozygosity - increased rapidly during early stages of establishment in the 1600s, while genetic differentiation from populations further from the front decreased. This trend ended in the 1700s, after an increase in population growth rate. These findings highlight the major role of long-distance pollen dispersal in the recovery of genetic diversity during initial stages of colonization, and suggest that demographic dynamics including an initial lag in population growth are likely limiting factors in the adaptation of tree populations tracking their niche in a changing climate.
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