Long-term persistence of experimental populations beyond a species' natural range

Ecological experiments usually infer long-term processes from short-term data, and the analysis of geographic range limits is a good example. Species' geographic ranges may be limited by low fitness due to niche constraints, a hypothesis most directly tested by comparing the fitness of populations transplanted within and beyond the range. Such studies often fail to find beyond-range fitness declines strong enough to conclude that geographic range limits are solely imposed by niche limits. However, almost all studies only follow transplants for a single generation, which will underestimate the importance of niche limitation because critical but infrequent range-limiting events may be missed and methodological issues may artificially boost the fitness of beyond-range transplants. Here, we present the first multi-generation beyond-range transplant experiment that involves adequate replication and proper experimental controls. In 2005, experimental populations of the coastal dune plant Camissoniopsis cheiranthifolia were planted at four sites within and one site beyond the northern limit. Fitness of initial transplants was high beyond the limit, suggesting that the range was limited by dispersal and not niche constraints. To better address the niche-limitation hypothesis, we quantified density and fitness of descendant C. cheiranthifolia populations 12-14 yr (similar to 10 generations) after transplant. Average annual fruit production and density of reproductive individuals were as high beyond the range as at four comparable experimental populations and eight natural populations within the range, and the beyond-range population had more than tripled in size since it was planted. This provides unprecedented support for the conclusion that northern range limit of C. cheiranthifolia results from something other than niche limitation, likely involving constraints on local dispersal.