Propagule size and sex ratio influence colonisation dynamics after introduction of a non-native lizard

1. The composition of founding populations plays an important role in colonisation dynamics and can influence population growth during early stages of biological invasion. Specifically, founding populations with small propagules (i.e., low number of founders) are vulnerable to the Allee effect and have reduced likelihood of establishment compared to those with large propagules. The founding sex ratio can also impact establishment via its influence on mating success and offspring production. 2. Our goal was to test the effects of propagule size and sex ratio on offspring production and annual population growth following introductions of a non-native lizard species (Anolis sagrei). We manipulated propagule composition on nine small islands, then examined offspring production, population growth, and survival rate of founders and their descendants encompassing three generations. 3. By the third reproductive season, per capita offspring production was higher on islands seeded with a relatively large propagule size, but population growth was not associated with propagule size. Propagule sex ratio did not affect offspring production, but populations with a female-biased propagule had positive growth, whereas those with a male-biased propagule had negative growth in the first year. Populations were not affected by propagule sex ratio in subsequent years, possibly due to rapid shifts towards balanced (or slightly female-biased) population sex ratios. 4. Overall, we show that different components of population fitness have different responses to propagule size and sex ratio in ways that could affect early stages of biological invasion. Despite these effects, the short lifespan and high fecundity of A. sagrei likely helped small populations to overcome Allee effects and enabled all populations to successfully establish. 5. Our rare experimental manipulation of propagule size and sex ratio can inform predictions of colonisation dynamics in response to different compositions of founding populations, which is critical in the context of population ecology and invasion dynamics.

1. 奠基种群（founding population）的组成在定殖动态中发挥着关键作用，且可影响生物入侵早期阶段的种群增长。具体而言，与拥有大型繁殖体（propagule）的奠基种群相比，携带小型繁殖体（即奠基个体数量较少）的奠基种群更易受到阿利效应（Allee effect）的影响，其成功定殖的概率也更低。奠基种群的性比同样可通过影响交配成功率与后代产量，对定殖过程产生作用。
2. 本研究旨在探究引入外来蜥蜴物种棕安乐蜥（Anolis sagrei）后，繁殖体大小与性比对后代产量及年种群增长的影响。我们在9座小型岛屿上对繁殖体组成进行了操控实验，随后对奠基个体及其涵盖3个世代的后代的后代产量、种群增长情况与存活率展开了调查。
3. 至第三个繁殖季，投放了相对较大繁殖体的岛屿上的人均后代产量更高，但种群增长与繁殖体大小并无关联。繁殖体性比并未对后代产量产生影响，但在第一年中，携带雌性偏倚（female-biased）繁殖体的种群呈现正增长，而携带雄性偏倚（male-biased）繁殖体的种群则出现负增长。在后续年份中，种群未受繁殖体性比的影响，这可能是因为种群性比快速向平衡（或轻微雌性偏倚）的方向转变。
4. 综合来看，本研究表明，种群适合度（population fitness）的不同组分对繁殖体大小与性比的响应存在差异，这种差异可能会对生物入侵的早期阶段产生影响。尽管存在上述效应，但棕安乐蜥较短的寿命与较高的繁殖力（fecundity）或许帮助小型种群克服了阿利效应，使得所有种群均成功定殖。
5. 本研究针对繁殖体大小与性比开展的稀有操控实验，可为基于不同奠基种群组成的定殖动态预测提供参考，这在种群生态学与入侵动态研究语境中具有重要意义。