What were the trophic consequences of the extinction of the largest apex predator of all time?

我们知道，顶级捕食者的减少可能会对生态系统造成灾难性影响，对社会产生严重的连锁反应（O'Bryan et al. 2018）。这是因为顶端捕食者以优势物种为食，从而减少了食物网底部生物的消费压力（Bruno & O'Connor 2005）。因此，顶端捕食者可以通过级联效应影响群落结构。尽管已经在当前的时间尺度上观察和研究了顶端捕食者数量减少对食物网的影响（例如Estes等人，2011年），但目前尚不清楚它们的实际灭绝将如何影响世界海洋的群落结构。这种知识的缺乏是由于我们还没有经历过任何海洋顶级食肉动物的完全灭绝。因此，我们研究顶级捕食者灭绝的营养后果的唯一方法就是通过化石记录。例如，曾经在海洋中游动的最大的海洋掠食者巨齿鲨，在大约300万年前的上新世灭绝了。这次灭绝是如何影响海洋群落结构的？这些影响是如何在不同的营养水平上分散的？在上新世，巨齿鲨和36%的海洋“巨型动物”一起灭绝，其中包括鲨鱼、海洋哺乳动物和海龟（Pimiento et al. 2017）。由于地球陷入更新世冰期的冰川循环，导致沿海生产性栖息地的丧失，这些物种的灭绝与气候变化和海平面波动的增加有关。这种灭绝在大型恒温巨型动物中表现得最为明显。由于在广泛的生态行业内对资源的竞争加剧，对能量需求高的“巨齿鲨”（Megalodon）和古代鲸鱼（ancient whales）的栖息地空间缩小（Pimiento等人，2017），尽管浮游动物和浮游植物也出现了损失。该项目将使用宏观和微观化石数据以及生态建模技术的前沿组合来研究这一以前被忽视的全球灭绝事件对海洋食物网的影响。学生将问以下问题：1。上新世末期的巨型动物灭绝是由“自上而下”还是“自下而上”的营养级联引起的？通过观察物种灭绝率随时间变化的多样性模式，尚不清楚灭绝事件是“自上而下”的事件，即顶级捕食者是环境压力导致的初级灭绝的受害者，还是“自下而上”的事件，即食物网底部的初级灭绝或种群崩溃导致了营养网络中层叠的次级灭绝。通过根据化石数据重建古代食物网，我们可以模拟各种灭绝情景，然后与化石记录中的经验数据进行比较。顶端掠食者灭绝对上新世-更新世海洋的短期和长期影响是什么？通过比较灭绝前后海洋群落的营养结构，可以发现顶端捕食者灭绝对生态系统结构和稳定性的直接影响。然后，我们可以通过随后的更新世追踪海洋生态系统的生态恢复，并揭示生态系统结构的任何变化是否代表灭绝事件发生后的短暂不稳定时期，或者灭绝是否导致了海洋领域的永久政权转移。

We know the decline of apex predators can lead to catastrophic impacts on ecosystems which have severe knock-on effects for society (O'Bryan et al. 2018). That is because apex predators feed upon dominant species, thereby reducing consumer pressure on organisms at the base of the food web (Bruno & O'Connor 2005). As such, apex predators can influence community structure via cascading effects. Although the effects of the decline of apex predators on food webs have been observed and studied on current timescales (e.g. Estes et al. 2011), it is currently unknown how their actual extinctions will influence community structure in the world's oceans. This lack of knowledge is due to the fact that we have not yet experienced the complete extinction of any marine top predator. As such, the only way we can study the trophic consequences of the extinction of apex predators is by the the fossil record. For instance, the largest marine predator ever to swim the ocean, the Megalodon, became extinct in the Pliocene epoch, around 3 million years ago. How did this extinction affect marine community structure and how these effects were dispersed amongst different trophic levels?During the Pliocene, the giant shark Megalodon became extinct along with 36% of the marine "megafauna", which included species of sharks, marine mammals and sea turtles (Pimiento et al. 2017). These extinctions have been associated with increase climatic variability and sea-level oscillations as the Earth plunged into the glacial cyclity of the Pleistocene ice ages, leading to the loss of productive coastal habitats. The extinction was felt most keenly amongst large, homeothermic megafauna (e.g. "Megalodon" and ancient whales) with high energy requirements as competition for resources within broad ecological guilds increased has habitat space contracted (Pimiento et al. 2017) although losses have also been noted amongst the zooplankton and phytoplankton. This project will investigate the effects of this previously overlooked global extinction event on marine food webs using a cutting-edge combination of macro- and microfossil data and ecological modelling techniques. The student will ask the following questions:1. Was the megafaunal extinction at the end of the Pliocene driven by a "top-down" or "bottom up" trophic cascades?It is not evident from observing diversity patterns of extinction rates through time whether extinction events are "top down" events, where top predators are victims of primary extinctions caused by environmental stressors, or "bottom up" events where primary extinctions, or population crashes, at the bottom of food webs cause cascading secondary extinction up through the trophic network. By reconstructing ancient food webs from fossil data, we can model various extinction scenarios which we can then compare to empirical data in the fossil record.2. What were the short and long term effects of apex predator extinction in the Plio- Pleistocene oceans?By comparing the trophic structure of marine communities from pre- and post-extinction intervals we can discover what the immediate effects of apex predator extinction were on ecosystem structure and stability. We can then track the ecological recovery of marine ecosystems through the subsequent Pleistocene and uncover whether any changes in ecosystem structure represent brief periods of instability in the immediate aftermath of the extinction event or whether the extinction resulted in a permanent regime shift in the marine realm.