Mate-finding Allee effects can be exacerbated or relieved by sexual cannibalism

1. Allee effects occur when individual or population survival decreases due to populations being small or sparse. A key mechanism underlying Allee effects is difficulty in finding mates at low densities. Species may be particularly vulnerable to mate-finding Allee effects if females rely on an abundance of males to reproduce successfully. In sexually cannibalistic species, females may consume males before or after copulation, potentially reducing the supply of males to the point where a mate-finding Allee effect occurs.
2. In this study, we investigate the extent to which sexual cannibalism can modulate mate-finding Allee effects, and the conditions under which sexual cannibalism is likely to be particularly detrimental to population viability.
3. We created an individual-based model that tracked specific females throughout the breeding season and used extinction risk and per capita growth rate to measure the strength of the Allee effects. We varied both founder population size and mate encounter rate independently of each other to expose the mechanism driving the Allee effects. We also analysed how cannibalism-derived female fecundity benefits affected extinction risk.
4. We found that sexual cannibalism could lead to high extinction risk, particularly when cannibalism occurred before copulation, founder population size was small, and mate encounter rates were low. However, post-copulatory cannibalism reduced extinction risk, if cannibalism increased female fecundity enough. We found that there were strong threshold effects, in which small changes in encounter rate could strongly alter population extinction risk.
5. We find that sexual cannibalism is likely to negatively impact population survival as population size and mate encounter rate decrease. This may be exacerbated if male quality declines and female hunger increases in declining populations. As many top invertebrate predators, such as spiders and mantids are sexually cannibalistic, this may have ecosystem-wide impacts. We also suggest that other reproductive behaviours, such as rejecting all but high quality mates or requiring multiple mates to ensure fertility, are also likely to cause mate-finding Allee effects when habitat quality degrades.
03-Mar-2020

1. Allee效应（Allee effects）是指当种群规模过小或种群密度过低时，个体或种群的存活率出现下降的现象。该效应的核心机制之一是种群低密度下个体难以寻找到配偶。若雌性个体依赖充足的雄性个体才能成功完成繁殖，这类物种便极易受到配偶寻找型Allee效应的影响。在存在性食同类（sexual cannibalism）行为的物种中，雌性可能在交配前或交配后捕食雄性，这可能会削减雄性种群的供给量，直至触发配偶寻找型Allee效应。
2. 本研究旨在探究性食同类行为对配偶寻找型Allee效应的调控程度，以及性食同类行为可能对种群存续造成显著危害的具体条件。
3. 我们构建了一个基于个体的模型（individual-based model），在整个繁殖季内追踪特定雌性个体的动态，并以种群灭绝风险和人均增长率来衡量Allee效应的强度。我们独立调整奠基种群规模与配偶相遇率，以揭示驱动Allee效应的内在机制。此外，我们还分析了性食同类行为带来的雌性繁殖力收益对种群灭绝风险的影响。
4. 研究结果显示，性食同类行为会提升种群灭绝风险，尤其当交配前发生性食同类、奠基种群规模较小且配偶相遇率较低时。但若交配后的性食同类行为能够显著提升雌性的繁殖力，则可降低种群灭绝风险。我们还发现存在显著的阈值效应：配偶相遇率的小幅变化即可显著改变种群的灭绝风险。
5. 我们的研究表明，随着种群规模与配偶相遇率下降，性食同类行为可能对种群存续产生负面影响。若在种群衰退过程中雄性质量下降、雌性饥饿程度加剧，这一负面影响可能会进一步加重。包括蜘蛛、螳螂在内的诸多顶级无脊椎动物捕食者均存在性食同类行为，这一现象可能会产生生态系统级别的影响。我们还提出，其他生殖行为——例如仅接受高质量雄性交配，或需要多次交配以确保受精——在生境质量下降时，也可能引发配偶寻找型Allee效应。
2020年3月3日