Tuesday, August 16, 2016

Reef Sharks prefer bite-size Meals!

Epic pic by Tom, from The World's Best Safety Stop on The Best Shark Dive in the World!

This is starting to get embarrassing.
From the press release,
"Although black-tip, white-tip and grey reef sharks have long been thought of as top predators, we found that the chemical structure of the sharks' body tissue actually matched closely with that of large reef fishes such as groupers, snappers and emperors,".

"This result tells us that reef sharks and large fishes have a similar diet, but they don't eat each other. So rather than eating big fish, reef sharks are eating like big fish."

"We now know that reef sharks are an important link in the food chain, but they are not the last link in the food chain. In most cases, the top predators are tiger sharks, hammerhead sharks, or people,"
Or as the paper states,
Assignment of species into discrete trophic groups is standard protocol in community ecology and has facilitated unique insights into ecosystem function and alternative management scenarios, which are ultimately used to guide policy decisions.
At present, reef sharks are typically assigned to the apex of food webs, but our results indicate that this practice misrepresents trophic structure among high TP species.

Hence, we advocate a reassignment of reef sharks to an alternative trophic group (such as high-level mesopredators) that better reflects trophic similarities between reef sharks and large predatory fishes. This change is expected to refine our understanding of how reef communities function, and ultimately, improve management of reef sharks.

If indeed reef sharks are high-level mesopredators, who then are the apex predators on coral reefs?
Given their superior size and ability to eat reef sharks, we hypothesise that the role of apex predator is fulfilled by large, roving sharks such as G. cuvier, C. obscurus, C. albimarginatus, N. acutidens and S. mokarran. And surely Bull Sharks - or not?

Although large roving sharks are seldom seen during visual surveys of coral reefs and thus are typically considered rare, their actual abundances may be much higher than currently believed, since they accounted for approx. 9 % of all sharks captured by long-lining at our study sites (excludes N. ferrugineus) and they comprise a high proportion of sightings by baited remote underwater videos on the GBR. Therefore, it is plausible that large roving sharks are present in sufficient numbers to potentially exert top-down control of reef sharks and other high-level mesopredators on coral reefs.

Removal of apex predators such as wolves, lions and dingoes can invoke trophic cascades due to release of numerous prey species and subsequent flow-on effects to lower trophic levels. 
However, trophic cascades induced solely by removal of reef sharks are rare, subtle and/or equivocal, implying that reef sharks have relatively weak effects on community structure and function. 

A potential explanation is that functional redundancy exists among large piscivores, such that equivalent species compensate for any loss of reef sharks and thus buffer potential trophic cascades. This hypothesis is supported by our results, which indicate that (1) reef sharks and large predatory fishes are functionally similar (based on equivalent mean TPs and overlapping isotopic niches, and (2) these two groups of predators are dietary generalists and potentially consume prey in proportion to availability, thereby compensating for loss of species-level interactions. 
It is also noteworthy that large predatory reef fishes are highly diverse (more than 20 species on the GBR) and probably encompass a broader range of trophic niches than those of the four species considered here. 
In view of these results, we contend that functional redundancy exists among large piscivores and is sufficiently high on the GBR to stabilize community structure despite moderate to high fishing pressure and depletion of reef sharks in some areas.

We conclude that large conspicuous predators, be they elasmobranchs or any other taxon, should not axiomatically be regarded as apex predators without thorough analysis of their diet. In the case of reef sharks, our dietary analyses suggest they should be reassigned to an alternative trophic group such as high-level mesopredators.
There you have it.
So let's please stop proffering the same old tired nonsense - it is false in its generalization and as such, it is nothing but bad conservation.


Juerg said...

Bull sharks absolutely!

DaShark said...

Phew - they got me worried there for a min! :)