Department of Conservation Biology, Estación Biológica de Doñana, CSIC, Sevilla, Spain.
Oecologia. 2011 May;166(1):79-90. doi: 10.1007/s00442-010-1795-x. Epub 2010 Oct 17.
No environment is truly constant in time. As a result, animals have evolved multiple adjustments to cope with such fluctuations. However, the allocation of effort to costly activities that imply long-term commitments, such as breeding, may be extremely challenging when future resources change constantly and unpredictably, a context that has received little investigation. To fill this gap, we studied the breeding response by a wetland-dependent raptor, the black kite Milvus migrans, to within and between-years fluctuations in resource availability (inundation levels). The breeding performance of the population was decomposed into reproductive components expressed in a sequence of successive tasks along the breeding cycle (e.g. timing of laying, clutch size, hatching success, brood reduction). Variation in each component was related to resource levels observed at different key dates of the season in order to test whether and when population-level reproduction was adjusted to available resources. Along a 22-year time-series, inundation levels fluctuated unpredictably within and among years, and mostly affected the later components of kites' reproduction, such as hatching success and the incidence of brood reduction, which were the main determinants of the population yearly breeding output. Results were consistent with multiple adjustments to cope with uncertainty. As the season progressed and resources became easier to assess, a bet-hedging waiting strategy based on a conservatively small, invariant and asynchronous clutch gave way to real-time resource-tracking mechanisms mediated by progressive adjustments to current prey availability, so that population-level breeding rates were determined and tuned to resources rather late in the season. Such adjustments were the likely outcome of the interaction between parental tactics and environmental constraints. Behavioural flexibility, such as dietary opportunism, probably promoted further resistance to resource oscillations. Given that all ecosystems show some degree of unpredictability, resource-tracking adjustments, such as the ones depicted here, are likely to be commonplace in most communities.
没有任何环境在时间上是完全不变的。因此，动物已经进化出多种适应机制来应对这种波动。然而，当未来的资源不断变化且不可预测时，分配精力去从事需要长期承诺的高代价活动，如繁殖，可能会极具挑战性，而这种情况很少被研究。为了填补这一空白，我们研究了一种依赖湿地的猛禽——黑鸢（Milvus migrans）对资源可利用性（淹没水平）的年内和年际波动的繁殖反应。该种群的繁殖表现被分解为繁殖组成部分，这些组成部分以繁殖周期中连续任务的顺序表示（例如产卵时间、卵窝大小、孵化成功率、雏鸟减少）。每个组成部分的变化与季节不同关键日期观察到的资源水平相关，以测试种群水平的繁殖是否以及何时适应可用资源。在一个 22 年的时间序列中，淹没水平在年内和年际之间不可预测地波动，主要影响黑鸢繁殖的后期组成部分，如孵化成功率和雏鸟减少率，这是种群每年繁殖输出的主要决定因素。结果与多种适应不确定性的策略一致。随着季节的进展和资源变得更容易评估，基于保守的、不变的和异步卵窝的避险等待策略让位于实时资源跟踪机制，这种机制通过对当前猎物可利用性的渐进调整来介导，从而使种群水平的繁殖率在季节后期确定并适应资源。这种调整很可能是亲代策略和环境限制相互作用的结果。行为灵活性，如机会主义的食性，可能进一步促进了对资源波动的抵抗。鉴于所有生态系统都表现出一定程度的不可预测性，像这里描述的那样的资源跟踪调整可能在大多数群落中很常见。
