trophic cascades -- 8/3/22
Today's selection -- from Why Sharks Matter by David Shiffman. The ripple effects that come from the removal or diminished size of a predator's population:
"Sometimes the ecological effects resulting from changes in predator populations ripple through the food chain. This ripple effect is called a trophic cascade. The classic example of a trophic cascade comes from the Pacific Northwest. When orca whales began to consume more and more sea otters in the kelp forests of the North Pacific, it wasn't surprising that sea otter populations declined. But the plot thickened! One of sea otters' favorite foods is the sea urchin, which they consume by adorably crushing them with rocks on their bellies. The population declines of sea otters then resulted in sea urchin predation release. The increasing sea urchin population ate more and more of their preferred food, seaweeds called kelp, resulting in kelp declines. All of this was caused by a change at the top of the food web. Even though area whales and otters don't eat kelp, changes in how orcas interact with otters significantly affected kelp. And that was bad for everything that lived in the kelp forest.
"The most famous example of a trophic cascade in a terrestrial ecosystem occurred in Yellowstone National Park as a result of wolf declines. Fewer wolves meant an increase in the wolf prey population, including giant herbivores like elk. More elk meant more grazing, and perhaps most impactfully, grazing in areas where elk were previously afraid to graze, such as riverbanks that restricted their ability to run away from a predator. This led to major disruptions in a unique Yellowstone ecosystem called an aspen forest. The Yellowstone case study also remains one of the best examples of predator restoration: when wolves were eventually restored, they ate more elk, bringing the population back under control and pushing elk back to their normal feeding grounds. As a result, the aspen forest is growing back.
"What about sharks, which are sometimes called the 'wolves of the sea'? There are two commonly cited examples of shark-driven trophic cascades. Both are considered fairly controversial in the marine biology world, but I'll explain them here because you're likely to come across them in the conservation discourse. The first, documented in a 2007 paper led by Ram Myers, took place near North Carolina's Outer Banks, where seven species of apex predatory sharks have declined significantly since the 1970s. Sandbar sharks experienced the least decline: 87% since 1972. Declines exceeding 99% since 1972 have been documented among several other species. These declines were believed to result in predation release of small sharks and rays, including the cownose ray. The authors claim that this increase in cownose rays was partially responsible for a collapse in populations of bay scallops, once a commercially important fish in the region, resulting in a shark ----> cownose ray -----> scallop trophic cascade.
"To me, the key message of this study was 'Sharks are important and bad things can happen when we overfish them, so let's not do that.' Others got a different (and unfortunate) message from this study: 'Oh my god, cownose ray populations are exploding. We need to kill them all to save our scallop fishery!' This led to the birth of the 'Save the Bay, Eat a Ray' movement. There were even fishing tournaments for cownose rays where anglers used explosive-tipped arrows to shoot at the surface-swimming rays, which is hardly sporting in my opinion. It is unlikely that ray populations could survive this kind of pressure for any extended time, given their very low reproductive rates. I'd argue that trying to solve a conservation crisis by causing another conservation crisis is perhaps not ideal.
"It turns out that the data showing this trophic cascade has major flaws in its underlying assumptions, and has been thoroughly rebutted. If you look closely at the data, it would suggest that cownose ray populations supposedly started to increase well after scallop populations began to collapse, almost as if something else caused the scallop populations to decline. (Dean Grubbs, who led the rebuttal, pointed out that this explanation only makes sense if you think that cownose rays can go back in time like the Terminator.) Also, cownose ray populations aren't increasing as much as these data seemed to show. What's instead happening is that existing cownose rays are migrating into new waters. Furthermore, shark populations haven't declined as much as these data seemed to show. The Grubbs rebuttal also notes that including more datasets complicates the supposedly clear pattern shown by the Myers paper. Finally, cownose rays don't really eat very many scallops. So although this is a well-known example of a trophic cascade that is often cited by environmentalists as a reason to protect sharks, it's a fundamentally flawed one.
"Another possible shark-driven trophic cascade might operate on coral reefs. Coral animals have a symbiotic relationship with tiny photosynthetic organisms called zooxanthellae. They live inside the corals and secrete sugars, which the corals eat. Without exposure to sunlight, zooxanthellae cannot photosynthesize, and the corals will starve. Happily, herbivorous fish like parrotfish help to graze fast-growing algae off of the corals, ensuring that sunlight can reach the zooxanthellae. Parrotfish are eaten by larger fish like grouper, which are eaten by (you guessed it) sharks. The decline of shark populations may cause predation release in grouper, which then eat more and more parrotfish. Fewer parrotfish means more algae growing on coral reefs, which means dying corals.
"This model seems to be essentially correct, but it's more complicated than that. It turns out that humans aren't just overfishing the sharks, but also the groupers, and in some cases even the parrotfish, Algae also grows on the coral for because of warmer waters or nutrient blooms, not simply because parrotfish populations are declining. Additionally, corals face other threats besides algae overgrowth. And while it's true that reef sharks often occupy a pretty similar seep on the food chain as groupers, some larger sharks eat small and medium sized groupers. So do sharks keep coral reefs healthy? They certainly play important roles in maintaining coral reef health under many circumstances, but as you can see, there are many variables to consider.
"A 2013 paper claimed to find true evidence of a trophic cascade on coral reefs. Specifically, the authors argued that Pacific coral reefs that had been heavily fished were home to fewer sharks and more mediumsized predators (called mesopredators) than protected reefs, which had more sharks and fewer mesopredators like groupers. On fished reefs with more mesopredators, the authors found fewer herbivorous fishes. Is this a case of a trophic cascade, with declines in sharks indirectly leading to declines in herbivores? Not so fast -- a 2016 paper claims that the pattern isn't quite so clear. This rebuttal argues that the difference in shark populations between fished and protected reefs isn't as significant as claimed in the 2013 paper. Furthermore, it argues that some of the fish species the 2013 paper authors counted as mesopredators shouldn't have been created as such because sharks don't eat those species. That rebuttal got a rebuttal, which got another rebuttal -- such is often the way of science. As of this writing, there hasn't been any conclusive evidence of trophic cascades driven by the loss of sharks on coral reefs -- in fact, an early 2021 paper found pretty strong evidence of the lack of trophic cascades on the Great Barrier Reef -- but the search is ongoing.
"Other possible shark trophic cascades include a reef shark -----> octopus -----> rock lobster food chain. Overfishing reef sharks in Australia seems to have led to an explosion in numbers of their octopus prey, which ace all the rock lobsters and damaged one fishery. Yet another possibly shark-driven trophic cascade involves seals. Fewer sharks means more seals, which eat a lot more fish. Trophic cascades are powerful forces in nature, but they're also really hard to detect because food webs are so large and complicated. I'd guess that even though some of the most popular examples of shark-driven trophic cascades may be flawed, it's very likely that some real cascades caused by sharks are out there.
"Some conservation activists have taken things too far, incorrectly asserting that, because of trophic cascades, the crash of shark populations could be directly responsible for the extinction of all life on Earth. According to this argument, which got its highest-profile mention in the documentary Sharkwater, phytoplankton, the base of the ocean food web, produce about half of all oxygen on Earth. If we lose sharks, the reasoning goes, this will destabilize the whole ocean, kill all the phytoplankton, and result in the loss of half of all oxygen on the planet, killing everything -- including us. Let me note again here that this is not correct, but it's an example of using trophic cascade theory for conservation advocacy.
"Trophic cascades are, generally speaking, more likely to occur in simpler ecosystems with more straightforward food chains. If you have five species that serve similar ecological roles as top predators, losing one probably won't disrupt the whole system because the other four can still keep mid-level predator populations in check. If you have only one top predator, losing its ecological role is more likely to disrupt the whole system. The examples described above range from hotly debated to thoroughly debunked, and I share them just to illustrate the general principle despite their particular imperfections. Despite their flaws, these high-profile examples are still useful to think about, if only because something like this is probably happening somewhere."