Effects of sex-ratio selection on the demography of an annual plant: challenging the seed-centric view of population dynamics

生物的分布和丰度既取决于物种在当地的持久性，也取决于它们在可用栖息地的殖民化和当地灭绝之间的平衡所带来的区域持久性。地方和区域进程都取决于人口增长率及其组成个体的繁殖力的密度依赖性调节。种群增长随着种群密度的降低而降低，这是由于个体存活率和繁殖力的降低，这是由于对资源和发芽微生境的竞争（负密度依赖）；如果在低密度下交配机会有限（正密度依赖，或“Allee效应”），当密度低于某个临界阈值时，人口增长就会增加。负密度依赖的相互作用预计将调节种群，因为它们增长到局部平衡；正密度依赖繁殖阻止了太小的种群的生存，并可能限制殖民化的成功，从而限制了一个物种的区域持久性。在植物中，正密度依赖性最有可能发生在雌雄异株物种中，那里的种子产量可能有限；此外，雄性和雌性功能之间存在权衡，这将严重限制种子生产。在这里，我们首次提出测试这些因素在决定种群动态中的作用。我们的研究的新贡献将是它对植物人口统计学的“种子中心观点”提出的挑战。我们在局部和区域尺度的植物种群动态之间建立了联系，这主要取决于种子生产，而植物性别分配的自然选择，在异交物种中，将多达一半的种群生殖资源从种子生产转向雄性功能。由于性别分配的选择极大地影响了种子的生产，我们预计它会影响物种的局部和区域动态。令人惊讶的是，这种联系以前似乎没有出现过。我们的研究将解决性别分配选择如何影响植物种群的负密度依赖和正密度依赖调节的问题。在控制条件下，通过一系列的操纵实验，我们将评估雌雄同体种群中雄性的存在对种群种子产量的影响程度，从而降低种群在不同密度下的内在生长率。这个物种对我们的研究特别有用，因为它在生殖系统中表现出惊人的多样性，种群中要么只有雌雄同体，要么只有雌雄同体，要么只有雌雄同体。通过研究具有不同性别系统的种群，我们将确定雄性功能的资源分配如何通过竞争以及通过种子传播对新斑块或种群的殖民化影响植物种群的调节。我们还将建立一个大型的田间实验，在该实验中，我们将验证雌雄异株种群（仅由雄性和雌性组成）的建立在低密度下（由于花粉对种子集的限制）比自交雌雄同体种群受到更严重的损害的假设。我们将通过实验来评估种子库的动态对当地和区域种群动态的贡献程度。最后，我们将在控制条件下使用实验交配阵列来确定雌雄异株和雌雄同体群体的密度（即株间距离）与结实率之间的关系。这些实验研究的结果将用于参数化模型，该模型通过正和负密度依赖关系将性别分配与植物种群的局部和区域调节联系起来。

The distribution and the abundance of organisms depend on both the persistence of species locally as well as on their regional persistence through a balance between colonisation of available habitat and local extinctions. Both local and regional processes depend on the density-dependent regulation of population growth rates and the fecundity of their constituent individuals. Population growth decreases with population density as a result of reduced individual survivorship and fecundity through competition for resources and microsites for germination (negative density dependence); and population growth increases with density below some critical threshold if, for example, mating opportunities are limited at low density (positive density dependence, or the 'Allee effect'). Negative density-dependent interactions are expected to regulate populations as they grow towards their local equilibrium; positive density-dependent reproduction prohibits the survival of populations that are too small and may limit colonisation success, thus limiting the regional persistence of a species. In plants positive density dependence can most likely occur in dioecious species, where seed production may be limited; in addition, there is a trade-off between male and female function that will be expected to severely constrain seed production. Here we propose for the first time to test the roles of these in determining population dynamics. The novel contribution of our study will be in the challenge it poses to the 'seed-centred view' of plant demography. We draw a link between plant population dynamics at local and regional scales, which depends fundamentally on seed production, and natural selection on a plant's sex allocation, which diverts up to half a population's reproductive resources away from seed production towards male function in outcrossing species. Because selection on sex allocation compromises seed production so greatly, we expect it to affect a species local and regional dynamics. Surprisingly, this link does not appear to have been made before. Our study will address questions about the way in which selection on sex allocation affects both the negative and positive density-dependent regulations of plant populations. Using a series of manipulative experiments under controlled conditions, we will assess the extent to which the presence of males in hermaphroditic populations of the species Mercurialis annua compromises population seed production, thus reducing a population's intrinsic rate of growth at different densities. This species is particularly useful for our purposes because it displays striking variation in its reproductive system, with populations comprising either only hermaphrodites, males with females, or males with hermaphrodites. By studying populations with different sexual systems, we will determine how the allocation of resources to male function affects the regulation of plant populations through competition, and through the colonisation of new patches or populations by seed dispersal. We will also establish a large field experiment in which we will test the hypothesis that the establishment of dioecious populations (comprising only males and females) will be more severely compromised at low densities (because of pollen limitation on seed set) than self-fertile hermaphroditic populations. We will experimentally assess the extent to which seed-bank dynamics may contribute to the local and regional dynamics of populations. Finally, we will use experimental mating arrays under controlled conditions to determine the relationship between density (i.e., interplant distance) and seed set for dioecious and hermaphroditic populations. The results of these experimental studies will be used to parameterise models that link sex allocation with the local and regional regulation of plant populations through both positive and negative density dependence.