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。人口分布受其环境空间变化的影响，在许多情况下，驱动人口变化的因素已经查明。然而，是个体而不是种群对他们在生活中遇到的环境变化做出反应，并且这些反应对个体生存和繁殖能力（即他们的“健康”）的影响决定了在更大的空间尺度上他们的种群会发生什么。因此，为了了解空间和时间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健康的影响。本项目的总体目标是填补这一知识空白，通过调查环境中的细尺度异质性对社会鸟类种群中个体的扩散和进化适应性的影响，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驱动合作中的作用。长尾山雀是一种小型鸟类，在英国鸟类中表现出独特的社会体系，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异质生境中的单独标记的鸟类种群的行为、生态和遗传组成，为该项目提供了独特的资源。直接健身（从育种）和间接健身（从帮助）已经量化了大量的个人使用终身生殖成功的数据。这种详细的适合度信息是非常少的物种，并在没有其他系统都直接和间接适合度以这种方式进行量化，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移动沿着梯度在他们的环境。当动物决定从一个地方分散到另一个地方时，它们会对哪些因素做出反应？基本上有两种分散决策。纳塔尔扩散是指从出生地到第一次繁殖地的迁移，而繁殖扩散是指在同一繁殖季节或不同季节之间连续繁殖尝试之间观察到的迁移。使用长期的数据和建模方法，我们将测试假设的因素影响个人的运动在一个可变的环境。第二个目标是对社会环境和繁殖成功率进行实验操作，以检验亲属关系和繁殖力梯度与个体的纳塔尔和繁殖扩散决策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适应性差异的策略这一假设。换句话说，帮助是一种风险规避或赌注对冲策略，个体在直接获得适应性（即通过繁殖）的概率较低的时候或地方采取这种策略。此外，我们还将研究个体适应度变化对种群水平现象的影响，检验社会性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时空环境异质性的反应影响适应度并转化为种群水平影响的机制提供新的见解，从而填补我们对生态过程理解的关键空白。

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.