Marine extinctions: the big ones go first*

There is strong evidence that populations of large marine animals have declined considerably in recent times. This decline is generally attributed to increasing fishing pressure, the effects of which in groups like sharks and rays are accentuated by the generally slow growth, late maturity and low reproductive output. Other major threats to oceanic life include habitat alteration, damage and loss from coastal developments, pollution, and the impacts of fisheries on the seabed and food species.For instance populations of Oceanic whitetip shark shrank by 93%, hammer heads by 89% and great whites by 80%. Some tuna species are sold at record values simply because they became so rare. The list continues and it is a new phenomenon that especially larger species are disappearing.

A new study by US researchers found that extinction threat in the modern oceans is strongly associated with large body size, whereas past extinction events were either nonselective or preferentially removed smaller-bodied taxa. Pelagic animals were victimized more than benthic animals during previous mass extinctions but are not preferentially threatened in the modern ocean. The differential importance of large-bodied animals to ecosystem function portends greater future ecological disruption than that caused by similar levels of taxonomic loss in past mass extinction events.

Many of these larger species are top predators that help to control the population sizes of smaller species. Removal of large-bodied predators can trigger trophic cascades affecting many other species. One of many examples is the explosive population growth of  the crown-of-thorns sea star (Acanthaster planci). Colleagues attribute this to the loss of giant triton (Charonia tritonis), one of the few species that can actually feed on the star fish. The snail shell is unfortunately massively harvested as decorative object.

The loss of large taxa may have caused more ecological disruption than the loss of comparable numbers of smaller taxa; indeed, loss of large animals may explain in part the multimillion-year delays in ecosystem recovery following these catastrophes. 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. And, unfortunately, the lack of correlation between the proportion of species assessed within higher taxa (phyla, classes, and orders) and the proportion considered threatened for marine animals suggests that the pessimistic projection of future genus losses may more closely approximate the true threat level than the optimistic projection. Without a dramatic shift in the business-as-usual course for marine management, our analysis suggests that the oceans will endure a mass extinction of sufficient intensity and ecological selectivity to rank among the major extinctions of the Phanerozoic (541 Ma to present).

*Title "stolen" from a Twitter feed by Jon Lefcheck