Life-history optimisation in response to climate change

预测气候变化将如何影响人口是我们今天面临的主要问题之一。在许多生态系统中，一个物种的物候会影响更高营养水平物种的生活史事件的最佳时间。在季节性环境中，消费者被选择去安排一些需要消耗能量的活动，比如繁殖，以配合短时间的有利条件，比如猎物可用性的高峰1,2。然而，对环境变化的不同反应可能导致物种之间物候不匹配，并可能影响个体适应度（如后代数量）和种群人口学（如种群大小）3,4,5。因此，在环境变化的背景下，确定关键生活史事件的最佳年度时间，并了解个体对这一事件的跟踪程度，对于预测气候变化下的人口趋势至关重要。事件的正确时机取决于在此之前的行为决策的最佳顺序，同时考虑每个步骤的相关成本和收益。不同决策之间的权衡受到个人状态、他们之前的行为以及他们所面临的环境条件的影响。例如，林地雀形目动物，如天雀科的山雀，当幼虫的供应与毛虫丰度的高峰相吻合时，繁殖成功率就会最大化7,8。在确定雏鸟的最佳时间之前，首先要决定繁殖的时间和努力，包括：(1)产卵日期，(2)窝数，(3)孵化期。这些特征通过权衡而错综复杂地联系在一起，并受到环境因素的影响。例如，在春季温度较高的年份，雌性可能会做出以下反应：(1)提前产卵日期和/或(2)减少窝卵数量和/或(3)在最后一颗卵下之前开始孵化，以预期更早的毛虫高峰11,12,13。在这个例子中，后代的数量（更小的一窝）可以用来交换它们的质量（增加与资源的同步）。个体之间在调节繁殖周期的不同方面的程度上存在很大差异，这被认为与个体条件有关。大量的研究已经探索了生殖行为中与免疫功能相关的成本作为一种调节生命史权衡的机制，尽管这些生理特征往往难以测量，并且有复杂的途径作为支撑。另一方面，羽毛质量可能是一种更容易量化的货币，例如，在繁殖时间和换毛率之间进行权衡。具体来说，羽毛的绝缘能力被认为会导致延迟繁殖对越冬生存和未来繁殖成功的负面影响。然而，我们需要更好地了解(1)羽毛质量与其绝缘性能之间的关系，以及(2)在繁殖周期的每个阶段，羽毛状况如何调节生殖时间（如产卵日期）和生殖投资（如窝数）对适合度（即未来生存和繁殖）的影响。

Predicting how climate change will affect populations is one of the major issues facing us today. In many ecosystems, the phenology of one species will influence the optimum timing of life-history events for species at higher trophic levels. In seasonal environments, consumers are selected to time energetically demanding activities, for instance reproduction, to coincide with a short period of favorable conditions, such as the peak in prey availability1,2. However, differential responses to environmental change may lead to phenological mismatches between species and potentially impact on individual fitness (e.g. offspring number) and population demography (e.g. population size)3,4,5. Therefore, determining the optimal annual timing of key life-history events in the context of environmental variation and understanding how closely individuals can track this are crucial for projecting population trends under climate change.The correct timing of an event relies on the optimal sequence of behavioral decisions preceding it, while considering the associated costs and benefits at each step. The trade-offs between different decisions are influenced by the individual's state, their previous actions as well as the environmental conditions they are faced with6. For instance, woodland passerines, such as tits in the Paridae family, maximize their reproductive success when the provisioning of young coincides with the peak in caterpillar abundance7,8. The optimal timing of the nestling stage is preceded by decisions about reproductive timing and effort including (1) the laying date, (2) the clutch size and (3) the incubation period. These traits are intricately linked through trade-offs and are cued by environmental factors9.10. For instance, in years with higher spring temperatures, females may respond plastically to (1) advance lay date and/or (2) reduce clutch size and/or (3) begin incubation before the last egg is laid in anticipation of an earlier caterpillar peak11,12,13. In this example, the quantity of offspring (smaller clutch size) may be traded for their quality (increased synchrony with resource).There is substantial variation between individuals in the extent to which they can adjust different aspects of their breeding cycle, thought to be linked to individual condition14,15. A large body of research has explored immune function-related costs of reproductive behavior as a mechanism mediating life-history trade-offs16, though these physiological traits are often difficult to measure and underpinned by complex pathways. On the other hand, plumage quality may be a more easily quantifiable currency underlying the trade-offs between, for instance, reproductive timing and moult rate17,18. Specifically, the insulating capacity of feathers has been suggested to drive the negative impacts of delayed breeding on overwinter survival and future reproductive success19. However, we need to better understand (1) the relationship between feather quality and its insulating properties, and (2) how plumage condition may mediate the effects of both reproductive timing (e.g. lay date) and reproductive investment (e.g. clutch size) on fitness (i.e. future survival and reproduction) during each stage of the breeding cycle.