Transplant populations of a coastal dune endemic plant can self-replace outside the natural range at multiple sites and in multiple years

Background/Question/Methods

Why species have stable limits to their geographic ranges is an unresolved but important area of study from evolutionary and ecological standpoints. The most direct test of the factors limiting a species’ range is a beyond-range transplant experiment. If populations planted beyond the range can replace themselves, we can infer that poor dispersal has stalled further range expansion. Beyond-range transplant experiments have been done with diverse species, and eight of ten well-executed and controlled experiments beyond geographic limits display estimated population growth rates above one, indicating apparently stable population persistence. However, experiments rarely include multiple planting sites (especially beyond the range), and are seldom repeated across years or monitored for more than one generation. If environments beyond the range are heterogeneous and experience inter-annual changes in conditions, then persistence, or lack thereof, at one site does not necessitate the same fate in other years or at other sites beyond the range. Here, we repeat a beyond-range transplant experiment using the Pacific coastal dune endemic plant Camissoniopsis cheiranthifolia 13 years after the first experiment, and we use an additional beyond-range planting site.

Results/Conclusions

In 2018, we planted ~ 550 individuals into each of five sites: two within the range, one at the northern range limit, and two beyond the range. As in the original experiment, lifetime fitness of transplants was higher at the site 60 km beyond the northern range limit than at the within-range planting sites. Moreover, when planted even further beyond the limit (220 km), individual lifetime fitness was higher than or on par with transplants at within-range sites. Top-performing source populations at the site 60 km beyond the range differed from the top-performers at the same site in 2005, and from those at the site 220 km beyond-range in 2018, suggesting environmental differences among sites and years. In 2021, four years after transplanting, there were 1624 and 1421 descendants at the sites 60 and 220 km beyond-range, respectively, which represents population size increases of ~ 2.6 and 2.9x since the initial generation. This provides strong evidence that the northern range limit of this species is not limited by low fitness but likely by poor dispersal capacity, as it can establish self-replacing populations beyond the range in at least two sites and in two different years.