We’ve known for a while now that humans are bad for marine life, but it can be hard to gauge in fundamental terms just how devastating our impact has been. A new study, published this week in the journal Science Advances, offers one striking way of understanding it: humanity has taken so much from marine biodiversity that we’ve actually broken one of the widest-ranging laws of nature.
From the blue whale – larger than any animal in the history of the planet – to the Myxozoa jellyfish – too small to be seen by the human eye – the Earth’s oceans are home to some of the weirdest, wildest life forms around. And for centuries, there’s been a hidden harmony governing them: a biomass distribution law that seemed to guarantee that, overall, size classes in the ocean were pretty evenly matched.
“[T]o a first approximation,” wrote Sheldon et al, the team who first hypothesized what’s become known as the size spectrum or Sheldon spectrum, in 1972, “roughly equal concentrations of material occur at all particle sizes … from bacteria to whales.” Essentially, they said, if we sort marine life into bins labeled by roughly equal logarithmic size classes – for instance, whales would be put in a bin marked “10-100 meters” (33-328 feet); dolphins and tiger sharks would go in the “1-10 meter” (3-33 feet) bin, and so on – then those bins should, with only a few exceptions, end up being the same size. To give a standard example: a bacterium may be 23 orders of magnitude smaller than a blue whale, but there are also about 23 orders of magnitude more bacteria in the ocean than blue whales.
Quite a few studies have tried to verify the Sheldon spectrum over the years, and most have found it to hold up. But the ocean is a huge place, and none have yet been able to take into account the full range of organisms that inhabit it. That was something the team behind this week’s findings wanted to change.
“One of the biggest challenges to comparing organisms spanning bacteria to whales is the enormous differences in scale,” explained lead author Dr Ian Hatton. “The ratio of their masses is equivalent to that between a human being and the entire Earth. We estimated organisms at the small end of the scale from more than 200,000 water samples collected globally, but larger marine life required completely different methods.”
Using a range of ocean observation techniques such as sonar and satellite observations, as well as the most recent meta-analyses, the team tested Sheldon’s hypothesis: first, they evaluated it in its “pristine” state before 1850, “before industrial-scale human capture of fish and marine mammals,” the study explains, using a combination of established historical reconstructions and marine ecosystem models.
“We were amazed,” said study co-author Dr Eric Galbraith. “[E]ach order of magnitude size class [contained] approximately 1 gigaton of biomass globally.”
Under pristine conditions, they had found, the Sheldon spectrum held up. But then they tested the hypothesis under current circumstances.
“[The] whole-ocean pattern is not immune to human impacts,” explains the study. “Despite marine mammal and wild fish catches amounting to less than 3 percent of annual human food consumption … fish more than 10 grams [0.4 ounces] in size and marine mammals are likely to have been reduced in biomass by about [60 percent] and the largest size classes appear to have experienced a near 90 percent reduction in biomass since 1800.”
Even under extreme climate change scenarios, the authors say, these losses are orders of magnitude worse than we should expect – and they throw into sharp relief the outsized impact humans are having on marine ecosystems.
“Prior work has pointed out that humans are now the top predator in the marine ecosystem, having extracted most of the predatory fish and mammals that previously occupied the upper ranges of the size spectrum,” notes the study. “This raises the question: Do humans now play the same role previously played by the predators we have removed? Have we simply inserted ourselves into the marine size spectrum and now act as a functionally equivalent top predator? The answer is clearly no.”
“Clearly, humans have not merely replaced the ocean’s top predators but have instead, through the cumulative impact of the past two centuries, fundamentally altered the flow of energy through the ecosystem.”