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

性别比选择对一年生植物种群统计的影响：挑战以种子为中心的种群动态观点

• 批准号: NE/G00420X/1
• 负责人: Robert Freckleton
• 金额: $ 2.88万
• 依托单位: University of Sheffield
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2009
• 资助国家: 英国
• 起止时间: 2009 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FG00420X%2F1
• 关键词: Effects; sex; ratio; selection; demography

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.

生物的分布和丰度既取决于当地物种的持久性，也取决于它们通过在现有栖息地的殖民化和当地灭绝之间取得平衡而在区域内的持久性。地方和区域进程都取决于对人口增长率的密度调节及其组成个体的繁殖力。种群增长随着种群密度的下降而下降，这是由于通过争夺资源和萌发微站点而降低了个体存活率和繁殖力(负密度依赖)；例如，如果交配机会在低密度下受到限制，则种群增长随着密度低于某个临界阈值而增加(正密度依赖，或‘Allee效应’)。负密度依赖的相互作用有望在种群增长接近其局部平衡时对其进行调控；正密度依赖繁殖阻止太小的种群的生存，并可能限制殖民成功，从而限制物种的区域持久性。在植物中，正密度依赖很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，在雄性和雌性功能之间存在权衡，预计将严重限制种子产量。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。我们这项研究的新贡献将在于它对植物人口统计学的“以种子为中心的观点”提出的挑战。我们将植物种群在局部和区域尺度上的动态与自然选择联系起来，前者在根本上取决于种子生产，后者取决于植物的性别分配，后者将多达一半的种群生殖资源从种子生产转移到异交物种的雄性功能上。由于性别分配的选择对种子产量的影响如此之大，我们预计它将影响一个物种的局部和区域动态。令人惊讶的是，这种联系似乎以前没有出现过。我们的研究将解决关于性别分配的选择如何影响植物种群的负向和正向密度依赖调节的问题。通过在受控条件下的一系列操纵实验，我们将评估在多大程度上，两性种群中雄性的存在会损害种群的种子生产，从而降低种群在不同密度下的内在增长率。本种对我们的目的特别有用，因为它在生殖系统中表现出惊人的变异，种群要么只包含两性的，要么雄性带雌性，要么雄性带两性。通过研究具有不同性别系统的种群，我们将确定分配给雄性功能的资源如何通过竞争和通过种子传播对新斑块或种群的定居来影响植物种群的调节。我们还将建立一个大型的田间试验，在这个试验中，我们将检验这样的假设，即在低密度下(由于种子上的花粉限制)，雌雄异株种群(只包括雄性和雌性)的建立将比自育的两性种群更严重。我们将通过实验评估种子库动态对当地和区域种群动态的贡献程度。最后，我们将使用受控条件下的实验交配阵列来确定雌雄异株和两性种群的密度(即株间距离)和结实率之间的关系。这些实验研究的结果将被用于将性别分配与植物种群的局部和区域调控联系起来的模型，这些模型既有正向密度依赖，也有负密度依赖。