Gaze out from the deck of a boat and you will see an ocean that was, in Henry David Thoreau’s phrase, “equally wild and unfathomable always”. There’s a stark contrast in appearance here between the apparently rugged and pristine ocean and our landscapes, so obviously and extensively modified by humans. As well as wild, of course the oceans are vast – they cover some 70% of Earth’s surface, and with an average depth approaching 4km they make up 99% of our planet’s living space.
As for the organisms that inhabit our oceans – well, everyone knows they are phenomenally abundant, geographically widespread, and enormously fertile, right? Together, the vastness of the oceans and the exuberance of marine life makes the concept of marine extinction little more than a far-fetched idea.
A decade ago, one of us (Dulvy) decided to take a fresh look at some of these assumptions, reviewing what was then known about marine extinctions, and questioning the easy contrasts often made between marine (widespread, abundant, safe) and non-marine (specialist, rare, threatened) organisms.
Now, the other one of us (Webb) has published a new study in the journal Current Biology that attempts to put on an equal footing extinction risk across both marine and non-marine environments.
We did this by summarising data compiled in the International Union for Conservation of Nature’s Red List of Threatened Species across all major taxonomic groups, separately for marine and non-marine species. This could only have been possible through a decade-long concerted effort by the union to ramp up its assessment of marine organisms.
On the face of it, our results appear to support the view that marine species are at less risk. Simple views of such statistics have previously been used to confirm expectations that extinction risk is lower in the oceans.
Focusing on the unknown
When we dig deeper into these numbers, however, a rather different picture emerges. Our comparisons can only consider those species which have had their conservation status formally assessed on the red list. Despite 50 years of effort by tens of thousands of volunteers, during which more than 70,000 species have been assessed, we still know the status of just 3% of known marine and 4% of known non-marine species.
Across all these assessed species, extinction rates in non-marine groups are around twice those in marine groups – a much lower figure than previously thought.
Digging further still, the pattern changes markedly when we focus only on the best studied groups. This generally means the more charismatic, recognisable animals such as turtles or seabirds. While there are many more species of sea snails or crustaceans out there waiting to be discovered, taxonomists are unlikely to discover many new seabirds.
Naturally we expect the most robust estimate of extinction and threat rates for these best-studied groups. And of these animals on average 20-25% of species are threatened with extinction, regardless of whether they live on land or in sea.
The ‘extinction-proof’ myth
Marine extinction risk has ramped up rapidly in the past 50 years, to converge upon the level of risk seen on land. There are of course some caveats to consider. Most significantly, very few marine groups meet our criteria for being “well known”, and most of those that do are atypical.
Some 90% of the 225,000 or so known sea species are invertebrates, with the diversity of fish more than matched by many less eye-catching groups. But the well-known groups are mainly vertebrates (seabirds, marine mammals, fish, turtles) or otherwise tied to our coastal seas (mangroves, seagrasses, corals).
Well-known groups of land animals are similarly biased towards the charismatic and convenient but – and here is a genuine marine-terrestrial contrast – most terrestrial biodiversity is contained within fewer groups to start with. Thus, while flowering plants and insects together cover much of the diversity of life on land, life in the sea is spread more evenly across dozens of major groups.
Indeed, a striking finding of this new study is that 64 of the 88 major groups of marine life, together containing almost a third of known marine species, have had no members at all assessed. Clearly the final word on marine extinction rates cannot be written until some at least of these gaps are filled in.
This task falls entirely to the volunteer scientists working in their spare time with little credit, and less funding. Securing this work is vital to ensure that we understand the true research and conservation priorities rather than that provided by the increasingly barren narrative litany of ocean calamities.
Despite the lack of overall representation of the marine tree of life in the new study, one group that is included is the archetypal “extinction-proof” marine fish. Thomas Henry Huxley, Darwin’s Bulldog himself, pronounced with authoritative confidence at the 1883 Fisheries Exhibition in London “probably all the great sea fisheries, are inexhaustible; that is to say, that nothing we do seriously affects the number of the fish,” a preconception that still clings stubbornly to 21st century discourse.
The emerging evidence over the past decade shows that abundant/widespread/fertile paradigm of marine life histories is the exception rather than the rule. So are marine species safe from harm? This new work confirms that the precautionary working hypothesis is, “no more than those on land”.
Tom Webb receives funding from the Royal Society and NERC.
Nicholas Dulvy does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.