Top predators are essential to the oceans

Source: Photographer: David Burdick

Gray Reef Shark (Carcharinus amblyrhynchos) Source:
Photographer: David Burdick

The global media was recently full of reports about the interaction that Australian pro-surfer Mick Fanning had with a shark during a competition in South Africa (see the video here). There is no doubt that the experience would have been terrifying and I’m very happy that Mick was not injured. The reactions across media and social media have been broad and varied. Many reports have reasonably pointed out that the rate of shark attacks is less than other more frequent dangers (like being killed by a cow) while some have, unreasonably in my opinion, postulated that we should “clear the ocean of sharks”.

There is no doubt that shark attacks are an emotionally charged event and that they sometimes have tragic outcomes. This has, in some instances, been used as an excuse to have shark culls. Rather than add to the chorus of voices stating how ridiculous this approach is (which it certainly is), I thought I would state something that seems to get forgotten: sharks are essential to the health of marine ecosystems and therefore essential to human life.

Why should we care about marine ecosystems? This seems like an inane question, but many people either don’t care or don’t understand how important healthy oceans are to our lives. Approximately 50% of the oxygen we breathe is produced in the ocean – can you skip every second breath? Over one billion people worldwide rely on seafood as their primary source of protein. Much of our food and pharmaceuticals relies on marine-based products. Basically, we can’t live without healthy marine ecosystems.

Why should I mention that? Because ecosystems rely on balance. When they are out of balance, they are unhealthy and become less productive. One of the services that top predators such as sharks provide to marine ecosystems is this balance. They control the species which would otherwise rapidly expand and dominate systems, lowering species diversity and productivity. A very good example of this in a terrestrial ecosystem is wolves in Yellowstone National Park in the USA. Wolves were hunted to local extinction in the area because they were thought to prey on livestock in the surrounding farmlands. In the absence of the wolves, however, elk populations expanded to the point where massive ecological damage was being done to the forests by grazing elk. Since reintroduction of the wolves, the forests have once again become healthy.

Another of the services that sharks perform is to “clean-up” marine ecosystem. Again, a terrestrial example that most people would be familiar with is lions in Africa catching the sick, diseased or old animals from herds. By removing these weaker individuals the lions are strengthening the herd as well as increasing the per capita resources available to the herd (by reducing its size).

Indeed, there is now a plethora of information on the benefits of top predators to the health and function of different ecosystems. We don’t seem to doubt this information for terrestrial systems, but our fear of sharks makes us irrational when it comes to marine systems. If one person is attacked by a shark the media goes crazy and we hear phrases like “shark cull”. If someone is killed by a cow…….. well, you’d never hear about it.

The reality is that in marine ecosystems the top predators are often sharks and these ecosystems cannot function properly without them. Humans need marine ecosystems to survive. Ergo, without sharks we can’t live as we currently do. While interactions with sharks can be terrifying and even tragic, we need to accept that the oceans are their habitat and we humans need them.

Ocean Acidification science: insightful and essential

Turfs overgrowing coralThe concentration of carbon dioxide in the atmosphere is rapidly increasing as we burn fossil fuels. Nobody doubts this. One of the emerging global consequences of this activity is Ocean Acidification (OA); approximately 30% of the CO2 that we emit into the atmosphere is dissolved into the oceans, forming carbonic acid and reducing the pH of the seawater. This is basic chemistry and can already be measured in many marine waters of the world.

The biological and ecological consequences of OA are, however, more complex to understand. Therefore, over the past two decades there has been a dramatic increase in the number of scientific studies investigating the effects of OA. Last week, a review of over 465 of these studies, written by Christopher Cornwall and Catriona Hurd, was published in the ICES Journal of Marine Science. They assessed a number of different scientific methods for rigour and concluded that overall OA science is well designed and executed, and provides useful insights into a complex problem.

There has been some good coverage of Cornwall and Hurd’s paper (e.g. in the journal Nature). Unfortunately, some media outlets misrepresent the findings of the paper. This is of great concern, as the inaccurate and sloppy journalism threatens an essential branch of marine science. I asked Cornwall to write something for this post:

Recently the Daily Mail reported that climate scientists are doom-mongering because their work is flawed. This report is misleading and only serves to introduce misinformation into the public arena.

The Daily Mail quotes an article in Nature by Cressey (2015) that highlights research by Cornwall and Hurd (2015).  Science is always evolving, and its aim is to improve both methods and theory in any given field, to be better equipped to answer the most complex of questions.   Cornwall and Hurd was merely a call for improvement in only one aspect of research amongst a multitude of methods.  The report in the Daily Mail misrepresents our findings.  There is overwhelming evidence that the effects of ocean acidification will impact our oceans through reductions in the growth and calcification rates of calcified organisms (e.g. shellfish, corals, etc., that make calcium carbonate ‘skeletons’), and an alteration of the behaviour of other marine invertebrates and fish.  This fact is unequivocal.  Rather than being “flawed”, the majority of ocean acidification studies have been carried out carefully, using a multitude of methods, and most provide extremely useful and insightful data on this complex problem.

Certainly, the consequences of OA are complex and modified by interactions with other stressors (e.g. nutrient pollution, global warming) and biotic interactions such as herbivory, competition and habitat complexity. This does not mean that the OA science to date is flawed, it simply means that we have more research to do to understand the future impacts. We need to understand all of the effects of OA, from the physiology of single organisms, through population dynamics and up to ecosystem-level interactions. This pattern of discovery is across all of science. For example, Einstein’s theory of relativity is complex. Physicists are making great discoveries but still have research to do. Ocean Acidification is no different.