Sunday, August 07, 2016

Reef Shark mediated Trophic Cascades? Not so fast!

Source - click for detail.

Bingo, and I cite,
Resident reef sharks do not sit atop the ecosystem like guardian angels; they dwell among the masses feeding on anything and everything they can.
They’re raccoons, not wolves.

On land, top predators like wolves prey upon large herbivores such as deer and elk, thereby protecting the grass and low-hanging trees from overgrazing. Such knock-on effects through an ecosystem are known as trophic cascades. In this case, those at the very bottom of the food chain—plants—are dependent on those at the top.

But of the 26 key species of sharks on coral reefs, only a few infrequent visitors—namely tiger sharks, bull sharks, and hammerheads—can be placed in the top tier of the food chain. “Shark” isn’t a blanket term for a huge voracious hunter, but a family of fish that encompasses a diversity of diets and lifestyles. The vast majority of species, such as whitetips and grey reef sharks, for example, are more akin to large-mouthed groupers and giant trevally—they are all mesopredators.
Remember this?
There it is suggested that only the adults of a few large-bodied Sharks with vast home ranges truly qualify as apex predators. The reef-associated species among them, like Tigers, Bulls and Great Hammerheads are however mere transients, which is why the evidence for (Reef-) shark-induced trophic cascades that benefit herbivorous fishes is weak or equivocal, or as this paper illustrates
In the absence of suitable long-term empirical studies, comparisons of the trophic structure between protected and fished sites can provide insight into the effects of shark removal on coral reefs. Theoretically, the loss of sharks would result in an increase in mesopredators, with cascading effects towards multiple prey at lower trophic levels. However, a critical evaluation of the available empirical studies finds weak evidence for shark-driven cascades.

Nearly all studies reported simultaneous declines across all trophic levels driven primarily by high levels of fishing pressure in populated and heavily fished locations (bingo again!).

Most studies focused on links between sharks and fish species at the base of the food chain: herbivores. This interest stems from the key ecological role herbivores provide on coral reefs, enhancing coral resilience by consuming fleshy macroalgae that outcompete corals for space. Theoretically, high shark abundance might lead to reduced mesopredator abundance and allow herbivorous fish to escape predation and become more abundant. However, empirical demonstration of trophic cascades involving sharks, mesopredators, and herbivores has proved elusive and nearly all studies find that reductions in shark densities occur in conjunction with reductions in mesopredators and herbivores .
Further, in regions where herbivores are not targeted for fisheries, higher densities of sharks inside marine reserves had no effect on the density or biomass of herbivorous fish.

While a recent study purports to show evidence of a shark-driven trophic cascade following catastrophic reef disturbance, no differences were found in the abundance of lower trophic groups (herbivores, corallivores, or planktivores) among fished and unfished atolls before disturbance, despite mesopredators being more abundant in areas where sharks were depleted by fishing
Increases in herbivore abundance in the area protected from shark fishing were observed only following cyclone disturbance, but the consequences of habitat damage confound interpretation. Because the cyclone caused loss of coral and a concomitant increase in algal resources, food limitation provides an alternative, and perhaps parsimonious, explanation for the positive response of herbivores rather than a shark-driven trophic cascade.
Alas, the last paragraph concerns this paper.
And despite Mark et al having mounted a vigorous rebuttal, I fear that upon reading the riposte by Roff, I remain equally unconvinced.
Alas - because I really liked that paper!

Long story short?
Nah I'm not gonna repeat myself like an old broken record - re-read it here!

1 comment:

Ian Campbell said...

But sharks save reefs.

And they only eat disease riddled fish keeping all reefs healthy.

And they keep out immigrants.

And they provide jobs....