Data from: 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 years (~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.

生态学实验通常需从短期数据推断长期过程，对地理分布范围极限的分析便是典型范例。物种种群的地理分布范围可能受生态位限制导致的适合度降低所制约，验证这一假说最直接的方式，便是对比移植至分布范围内部与外部的种群适合度。此类研究往往未能发现足够显著的分布范围外适合度下降，无法得出地理分布范围极限仅由生态位限制所决定的结论。然而，几乎所有相关研究仅对移植种群进行单一代次的追踪，这会低估生态位限制的重要性——因为关键但偶发的分布范围限制事件可能被遗漏，且实验方法问题可能人为提升了分布范围外移植种群的适合度。本研究开展了首个具备充分实验重复与规范对照的多代次分布范围外移植实验。2005年，研究人员将海岸沙丘植物Camissoniopsis cheiranthifolia的实验种群，种植于其北部分布极限内的4个站点，以及1个分布范围外的站点。初始移植种群在分布范围外的适合度较高，这表明该物种的分布极限受扩散限制而非生态位限制。为更严谨地验证生态位限制假说，我们在移植后的12至14年（约10个世代），对后代种群的密度与适合度进行了量化。结果显示，分布范围外种群的年均结实量与繁殖个体密度，均与4个对照实验种群以及8个分布范围内的自然种群相当，且该范围外种群的种群规模自种植以来已增长至最初的三倍以上。本研究为"Camissoniopsis cheiranthifolia的北部分布极限并非由生态位限制所导致，而是可能受局地扩散限制制约"这一结论提供了前所未有的证据支持。