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

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.