Monday, May 01, 2017

Australia: Reef Shark mediated Trophic Cascade?

Competitors: Grey Reefies and Snappers. Source.

Well well.

Read this.
If I understand it correctly, it shows that several species of mesopredatory Snappers were doing great on reefs where (higher-ranking but equally mesopredatory) Reef Sharks were rare; but on reefs were Reef Sharks were abundant, the Sharks appear to have out-competed, or otherwise influenced the Snappers who had to shift their diet to less nutritious benthic invertebrates and were doing very poorly indeed as a consequence - synopsis here.
The conclusion being that
our findings suggest that overfishing of sharks has the potential to trigger trophic cascades on coral reefs and that further declines in shark populations globally should be prevented to protect ecosystem health.
Yes to the possibility of top-down effects - but if the Snappers' fitness was being adversely affected by the presence of Sharks, who, exactly, did benefit from it? Obviously not the Snappers - but also not the prey Fishes and Cephalopods that were still being killed; and also not the benthic invertebrates that were being killed on top of that.
So where is the evidence that Sharks help preserve ecosystem health = instead, is this not rather the negation of Healthy Reefs need Sharks?

Or am I missing something here?

PS: great comment by Michael in the comments section - thanks!


Rebecca Bateman-John said...

I'm confused!

DaShark said...

Me, too! :)

Michael Sealey said...

I understand the opposite, it was the mesopredators who were able to shift their diet over the years thanks to the removal of sharks and not vice versa. The consequences of these shifts can cause problems such as less control over the population of invertebrates, and an over predation on herbivores and other prey of the mesopredators, and I quote "However, herbivores and other potential prey of the mesopredatory fishes were significantly more abundant at the Rowley Shoals compared to the Scott Reefs...".

In my opinion the paper is a good study of how the removal of sharks influences the trophic dynamics on a reef, but maybe doesn't give the evidence to prove a trophic cascade.

DaShark said...

Thanks Michael - excellent point! :)

Re the trophic cascades, quite a few have been recently disproven so as always, caveat emptor!