Scientists are increasingly convinced that the world is barrelling towards a “sixth mass extinction” event. They fear species are going extinct at a rate that far exceeds what one would expect to see naturally, as a result of a major perturbation to the system.
A new study published last week in Science magazine confirms that in the world’s oceans threats of extinction are not apportioned equally among all species — rather, the larger ones, in terms of body size and mass, are uniquely imperilled right now, according to Chris Mooney, who analysed the study for The Washington Post.
From sharks to whales, giant clams, sea turtles, and tuna, the disproportionate threat to larger marine organisms reflects the “unique human propensity to cull the largest members of a population,” the authors of the study have said.
Geoscientist Jonathan Payne of Stanford University, the study’s lead author, has termed it surprising there was no similar kind of pattern in any of the previous mass extinction events examined.
The conclusion is that this pattern has not happened before in the half billion years of the animal fossil record. The researchers conducted the work through a statistical analysis of 2,497 different marine animal groups at one taxonomic level higher than the level of species — called “genera.” The finding was that increases in an organism’s body size were strongly linked to an increased risk of extinction in the present period — but this was not the case in the Earth’s distant past.
“The extreme bias against large-bodied animals distinguishes the modern diversity crisis from all potential deep-time analogues,” the researchers wrote.
“These losses in the ocean are paralleling what humans did to land animals some 50,000 to 10,000 years ago, when we wiped out around half of the big-bodied mammal species on Earth, like mammoths, mastodons, saber-tooth cats and the like,” said Anthony Barnosky, executive director of Stanford Jasper Ridge Biological Preserve, who reviewed the study for the Post.
Barnosky was the co-author of a study published last year that found an “exceptionally rapid loss of biodiversity over the last few centuries, indicating that a sixth mass extinction is already under way.”
A particular problem, according to Payne, is that if all the top predators are taken out, then the species they used to prey upon can run amok and explode in population, having large reverberating effects on the entire ecosystem.
“The preferential removal of the largest animals from the modern oceans, unprecedented in the history of animal life, may disrupt ecosystems for millions of years even at levels of taxonomic loss far below those of previous mass extinctions,” the authors wrote.
The current study underscores that ecosystem risks are not being principally driven by a changing climate — yet. Rather, they are being driven more directly by which species humans hunt and fish, and where they destroy ecosystems to build homes, farms, cities, and much more. But as climate change worsens, it will compound what is already happening.
The research is heartening for those who care about ocean conservation – precisely because human-driven large animal extinctions in the sea are not as advanced as they are on land, there is still a huge amount of biological life that we can save.