Ecosystem structure changes following a marine megafaunal Pliocene extinction and the role of continental shelf habitat loss

The end of the Pliocene (~2.5-3 Mya) saw a period of biotic turnover in marine ecosystems with significant losses in marine megafauna (36% of genera globally) including the giant apex predator, Otodus megalodon. This recently identified extinction event coincided with a period of notable climatic change, with glaciation of the Northern Hemisphere causing a drop in global sea level. A positive correlation has been found between extinction probability and those genera that had high energy requirements and were associated with coastal, shelf habitats. As such, it has been hypothesised that these extinctions may have been driven by sea level drop resulting in loss of shelf area and a reduction in the neritic zone, a highly productive feeding habitat.

Megafauna, and particularly apex predators, are known to play important trophic roles in extant communities yet the ecological consequences of losing these large taxa from marine ecosystems in currently not well understood. Using a trait-based inference model, we reconstructed food webs to assess the impact of this Pliocene extinction event on North Atlantic trophic community structure and dynamics. The model distributes trophic links using rules based on optimal foraging theory and functional traits assigned to every trophic species (e.g., body size, depth distribution and feeding habit). Through analysing the differences in food web topology pre- and post- megafaunal extinction event, we identify consistency in structure and connectedness between taxa within the webs. However, diversity changes within trophic levels and a decrease in competition in the Pleistocene indicates this turnover event did result in changes to the marine ecosystem makeup. Furthermore, our results show that the trophic role held by O. megalodon in the Pliocene appears to have been lost in the Pleistocene, with no other taxa taking its place.

In addition to these findings, we also consider alternate hypotheses that may have impacted megafaunal extinction other than shelf loss. As the North Atlantic possesses areas of long continental shelf, neritic zone habitats may have only been shifted, not lost. Consequently, other factors may have played a more significant role.