Payne, J. L., Bush, A. M., Heim, N. A., Knope, M. L., & McCauley, D. J. (2016). Ecological selectivity of the emerging mass extinction in the oceans. Science. DOI: 10.1126/science.aaf2416.
The diversity of animal life on our planet is declining rapidly, leading many to believe we are heading towards a “sixth mass extinction”. We are losing animals at an alarming rate (dozens of species per day) and are predicted to lose around 30 to 50% of all animals by mid-century. Past mass extinctions were caused by natural events like asteroid impacts, volcanic eruptions, or natural climate shifts, but the current extinction is largely a result of human interference. Through human-induced climate change, habitat destruction, pollution, and over-hunting, we are speeding up extinction rates on land and in the ocean.
The first step in understanding and hopefully avoiding future impacts of extinction, is to determine the most vulnerable animals. This task seems daunting, as there are millions of animals on Earth, spanning a wide range in size and occupying diverse habitats. In this study, Payne et al. (2016) set their focus on the ocean to try and predict the impacts of extinction on two major groups of marine animals – molluscs and vertebrates. Using a giant database (over 2,400 groups of species), they examined how an animal’s level of extinction threat (via the IUCN) correlated with traits like body size, habitat, motility (i.e. how much the animal moves around), and feeding mode (predators vs. non-predators). They even included extinct animals from the fossil record in their study, to compare future extinction intensity to previous extinction events.
Payne et al. (2016) found the future threat of extinction for both molluscs and vertebrates to be higher than it has been since the last major extinction, over 60 million years ago (Fig. 1). This finding provides additional evidence that marine animals are facing serious threats of extinction, even under the most optimistic scenarios. But, what kinds of animals in the ocean are under the most pressure?
Out of all the traits examined, body size was most closely associated with extinction in living vertebrates and molluscs (Fig. 2A). The larger the animal, the higher its risk of extinction – this spells trouble for large vertebrates like sharks, dolphins and whales. Extinction threat was also positively correlated with motility, though to a lesser extent than body size. The other two examined traits, habitat and feeding type, were not correlated with any extinction threat (Fig. 2B and D).
The effects of this current extinction on marine vertebrates and molluscs is unique compared to the fossil record. The previous five mass extinctions were not at all related to animal body size or movement. Instead, past extinction events seemed to be more linked to the habitat animals occupied, hitting pelagic (open water animals) the hardest. This modern extinction might follow a unique trend because, unlike previous extinctions caused by catastrophic natural forces, this extinction appears to be more influenced by human impacts.
Discussion and significance:
Many scientists agree that we are currently within a “sixth mass extinction”, largely the result of human activity. Payne et al. (2016) identified body size to be the major indicator of extinction of vertebrates and molluscs in today’s oceans. So, why does modern extinction in the ocean seem to favor large-bodied marine animals? The answer may lie with the decline in marine populations due to overfishing (check out this review on overfishing). Fisheries have a tendency to target the tastiest species, which often happen to also be the biggest (such as tuna, grouper or shark). These large animals reproduce slowly and thus their populations cannot rebound as quickly to overfishing.
Large-bodied marine animals sit atop the food web, meaning their removal could have serious ramifications for smaller animals and the surrounding ecosystem. For example, if apex predators like sharks are removed from the ocean, their prey may grow unchecked and disrupt the natural order of the marine food web.
There is one silver lining coming out of this study: we have the ability to directly halt overfishing and lessen the impacts of extinction, especially on the loss of large-bodied animals. Make healthy seafood choices to lessen your impacts on marine species and help keep the oceans clean by taking small steps like recycling. The future of marine species lies in our hands, and we can help to make sure they do not go extinct in our lifetime.
Keep the conversation going, talk with each other and be informed on overfishing policies. Check out these websites for more information on overfishing and the modern extinction: