Climate mal-adaptation and biotic interactions at species' range limits

Why species have restricted geographic distributions or why species do not occur everywhere is still an open question in ecology and evolutionary biology. It is assumed that the species range limits normally reflect the ecological conditions where the species stop occurring because of a lack of habitat suitability. Moreover, these populations at the margins are known to suffer from a history of small population size and the accumulation of genetic drift. Additionally, biotic interactions have been recently proposed to act negatively at the range edges. Among them, pollination services are particularly important as reproduction and population dynamics of the majority of the flowering plant species rely upon them. However, pollinator services are not constant, varying across different temporal and spatial scales.
Here I tested whether a history of small population size, enhanced genetic drift, and the accumulation of deleterious mutations in range-edge populations was linked with reduced adaptation in the North American Arabidopsis lyrata. I performed a transplant experiment with sites across and beyond the species distribution with source plant populations from the centre and the periphery, these last ones with a history of range expansion or long-term isolation. Additionally, I monitored pollination interactions in natural populations over a transect spanning from the southern to the northern range limit and over different temporal and spatial scales using time-lapse cameras.
The results from the transplant experiment shown that plant multiplicative performance declined toward the southern range limit and beyond, but not in the northern range. Furthermore, populations shown evidence of climate adaptation to two suggested niche variables, temperature in spring, and precipitation of the wettest quarter. However, the signature of adaptation was reduced in populations with a history of small population size, and additionally, the heterosis effect was increased in populations with heightened genomic estimates of load, longer expansion distance or long-term isolation, and a selfing mating system. Genetic drift and mutation accumulation due to past range expansion and long-term isolation of small populations at the range margins is therefore a strong determinant of population-mean performance.
In the pollinators project, I found that the plant-pollinator network for A. lyrata is a generalist system, and southern populations had lower pollination services compared to center and northern populations. The diurnal activity of the pollinators was mostly explained by air temperature conditions, occurring the majority of the visits during the mid-day. The density of flowers in a patch explained partially the spatial variation, but the signature was specific for each taxonomic group. Even though no evidence of niche partitioning was found, the different taxonomic groups of pollinators differed in their activity window where some taxa were more tolerant under certain temperatures or intervals of the day.