Dispersal through fitness landscapes in a social bird: from individuals to populations

环境变化对自然种群的影响可能是自然科学家面临的最紧迫的问题。种群的分布受到其环境的空间变化的影响，在许多情况下，已经确定了驱动种群变化的因素。然而，是个人而不是群体对他们在生活中遇到的环境变化做出反应，正是这些反应对个人的生存和繁殖能力(即他们的‘适应性’)的后果决定了他们的群体在更大的空间尺度上发生了什么。因此，为了了解时空变化影响种群的过程，迫切需要了解这种细微变化对行为、生活史以及最终对个体适应性的影响。这个项目的总体目标是通过调查环境中细微的异质性对群居鸟类种群中个体的扩散和进化适应性的影响来填补这一知识空白，并利用对个体反应的理解来解释种群水平的影响。通过研究一个社会物种，其中有限的扩散在亲属选择的帮助的表达中起着至关重要的作用，我们也可以测试环境异质性在推动合作方面的作用。长尾山雀是一种小鸟，它的社会制度在英国鸟类中是独一无二的，在这种制度下，失败的繁殖者经常帮助近亲抚养后代。自1994年以来，一直密切监测生活在不同栖息地的一群单独标记的鸟类的行为、生态和遗传组成，为这一项目提供了独特的资源。使用终生生殖成功数据对大量个体的直接适合度(来自繁殖)和间接适合度(来自帮助)进行了量化。关于适合度的详细信息对极少数物种是可用的，在任何其他系统中，直接和间接适合度都没有以这种方式被量化，这为我们提供了一个独特的机会来解决这些基本的生态问题。该项目的第一个目标是调查动物如何在它们的环境中沿梯度移动。当动物决定从一个地方分散到另一个地方时，会对哪些因素做出反应？基本上有两种分散的决定。出生扩散是从出生地到第一次繁殖地的迁移，而繁殖扩散是在同一繁殖季节或不同季节之间观察到的连续繁殖尝试之间的移动。使用长期数据和建模方法，我们将检验关于在可变环境中影响个体运动的因素的假设。第二个目标是对社会环境和繁殖成功进行实验操作，以测试亲缘关系和繁殖力梯度与个体的出生和繁殖分散决定之间是否存在因果关系。第三个目标是检验帮助行为是一种减少个体间适应差异的策略的假设。换句话说，帮助是一种风险规避或押注对冲策略，当直接(即通过繁殖)获得健康的可能性较低时，个人会在某些时间或地点采用这种策略。此外，我们还将调查个体对种群水平现象的适合度差异的后果，检验社会物种由于其社会相互作用而经历正密度依赖的假设。满足这些目标将为个体水平对空间和时间环境异质性的反应影响适合度并转化为种群水平影响的机制提供新的见解，从而解决我们对生态过程理解中的关键差距。

The effect of environmental variability on natural populations is perhaps the most pressing concern facing natural scientists. The distribution of populations is affected by spatial variation in their environment and the factors driving population change have been identified in many cases. However, it is individuals rather than populations that respond to variation in the environment that they encounter in their lives, and it is the consequences of these responses for individuals' capacity to survive and reproduce (i.e. their 'fitness') that determines what happens to their population at larger spatial scales. Therefore, to understand the processes through which spatial and temporal variation affects populations, there is an urgent need to understand the impact of that fine-scale variation on the behaviour, life history and ultimately the fitness of individuals. The overall objective of this project is to fill this gap in knowledge by investigating the effect of fine-scale heterogeneity in the environment on the dispersal and evolutionary fitness of individuals in a population of social birds, and to use that understanding of individual responses to explain population-level effects. By studying a social species in which limited dispersal plays a vital role in the expression of kin-selected helping, we can also test the role of environmental heterogeneity in driving cooperation. The long-tailed tit is a small bird that exhibits a social system unique among UK bird species in which failed breeders often help close kin to raise their offspring. The behaviour, ecology and genetic composition of a population of individually marked birds living in a heterogeneous habitat has been closely monitored since 1994, providing a unique resource for this project. Direct fitness (from breeding) and indirect fitness (from helping) have been quantified for a large number of individuals using lifetime reproductive success data. Such detailed information on fitness is available for very few species, and in no other system has both direct and indirect fitness been quantified in this manner, offering us a unique opportunity to address these fundamental ecological issues.The first objective of the project is to investigate how animals move along gradients in their environment. What factors do animals respond to when deciding to disperse from one place to another? There are essentially two kind of dispersal decisions. Natal dispersal is the movement from birthplace to the place of first breeding, while breeding dispersal is the movement observed between successive breeding attempts that may occur within the same breeding season or between seasons. Using long-term data and a modeling approach we will test hypotheses concerning the factors influencing individual movements within a variable environment. The second objective is to conduct experimental manipulations of the social environment and of reproductive success to test whether there is a causal relationship between kinship and fecundity gradients and the natal and breeding dispersal decisions of individuals.The third objective is to test the hypothesis that helping behaviour is a strategy that reduces the variation in fitness among individuals. In other words, helping is a risk-averse or bet-hedging strategy that individuals adopt at times or in places when the probability of gaining fitness directly (i.e. by reproduction) is low. In addition we will investigate the consequences of variation in individual fitness for population-level phenomena, testing the hypothesis that social species experience positive density dependence due to their social interactions.Meeting these objectives will provide novel insights into the mechanisms through which individual-level responses to spatial and temporal environmental heterogeneity affect fitness and translate into population-level impacts, thereby addressing key gaps in our understanding of ecological processes.