Today’s post comes from Salvador Herrando-Pérez (who, incidentally, recently submitted his excellent PhD thesis).
The hips of John Travolta, the sword of Luke Skywalker, and the teeth of Jaws marked an era. I still get goose pimples with the movie soundtrack (bass, tuba, orchestra… silence) solemnizing each of the big shark’s attacks. The media and cinema have created the myth of man’s worst friend. This partly explains why shark fishing does not trigger the same societal rejection as the hunting of other colossuses such as whales or elephants. Some authors contend that we currently live in the sixth massive extinction event of planet Earth (1) 75 % of which is strongly driven by one species, humans, and characterized by the systematic disappearance of mega-animals in general (e.g., mammoths, Steller’s seacow), and predators in particular, e.g., sharks (2, 3).
The selective extirpation of apex predators, recently coined as ‘trophic downgrading’, is transforming habitat structure and species composition of many ecosystems worldwide (4). In the marine realm, over the last half a century, the main target of the world’s fisheries has turned from (oft-large body-sized) piscivorous to planctivorous fish and invertebrates, indicating that fishery fleets are exploiting a trophic level down to collapse, then harvesting the next lower trophic level (5-7).
Myers et al. (8) illustrate the problem with the fisheries of apex-predator sharks in the northeastern coast of the USA. Those Atlantic waters are rife with many species of shark (> 2 m), whose main prey are smaller chondrichthyans (skates, rays, catsharks, sharks), which in turn prey on bottom fishes and bivalves. Myers et al. (8) found that, over the last three decades, the abundance of seven species of large sharks declined by ~ 90 %, coinciding with the crash of a centenary fishery of bay scallops (Agropecten irradians). Conversely, the abundance of 12 smaller chondrichthyes increased dramatically over the same period of time. In particular, the cownose ray (Rhinoptera bonasus), the principal predator of bay scallops, might today exceed > 40 million individuals in some bays, and consume up to ~ 840,000 tonnes of scallops annually. The obvious hypothesis is that the reduction of apex sharks triggers the boom of small chondrichthyans, hence leading to the break-down of scallop stocks.
Eating and being eaten
Trophic downgrading is an example of a trophic cascade. In simple terms, a trophic cascade represents a predator-prey (or parasite-host) relationship, the balance of which affects the abundance, biomass or productivity of a third species (9). Thus, fish species in a freshwater lake might enhance local pollination of terrestrial plants if, by feeding on aquatic larvae of dragonfly, diminish the abundance of adults of dragonfly that forage on pollinating bees (10).
Trophic cascades can originate from human action (e.g., trophic downgrading), natural factors, or both. Thus, at the beginning of last century, overhunting of Alaskan sea otters (Enhydra lutris) brought the species to the brink of extinction, but provoked the explosion of their preferred prey, sea urchins, which devoured kelp forests. Subsequently, kelp forests recovered after several decades of otter protection (11). Now some authors suggest that killer whale (Orcinus orca) predation on sea otters might jeopardize kelp forests again (12-14).
Most importantly, trophic cascades result not only from direct mortality of prey species, but due to change in prey behaviour avoiding their enemies (15). In Australia, tiger sharks (Galeocerdo cuvier) can shape local distribution and abundance of seagrass beds, because dugongs (Dugong dugon) avoid the lush shallow-water seagrass, in favour of deeper waters with less food, yet lower risk of shark encounters (16).
Globally, trophic cascades brought about by extirpation or introduction of predators or megaherbivores promote or exacerbate a range of severe, long-term environmental impacts such as wild fires, invasion of foreign species, disease bouts, CO2 emissions, water hypoxia or impoverishment of species richness (4). Those outcomes manifest the complexity of trophic chains, and the need to account for species’ ecological functions in our (largely failed) endeavour of regulating our encroachments on nature and ecosystem services.
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