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Sharkbites Saturday

Sharks and other ocean top predators: unlikely allies in combatting climate change?

Spiers, E.K.A., Stafford, R., Ramirez, M., Izurieta, D.F.V., Cornejo, M., Chavarria, J. 2016. Potential role of predators on carbon dynamics of marine ecosystems as assessed by a Bayesian belief network. Ecological Informatics, 36, 77-83. http://doi.org/10.1016/j.ecoinf.2016.10.003

Sharks provide more than just a toothy grin…

Healthy marine food webs provide food and livelihoods to millions of people across the globe, yet healthy food webs with viable populations of top predators like sharks may do more than just put dinner on the table. Researchers from the UK and Ecuador recently used statistical tools to suggest that healthy top predator populations may even reduce the amount of carbon produced by a food web.

How is this even possible? Well, imagine you’re making a delicious smoothie using a kitchen blender; it’s an older model, so you have your hand on top of the lid to ensure it stays on. Your hand here acts as a shark would in a marine food web; sharks and other top predators apply predation pressure and make sure no trophic level below them explodes (Fig. 1).

Fig. 1. Sharks and other top predators maintain balance in food webs by regulating the populations of creatures below them in the food web. Image: Encyclopedia Britannica

Too many mackerel? Sharks step in to munch them. Surplus skipjack tuna? Sharks save the day and devour some. Keeping populations in check is a vital job, as food web imbalances can lead to unintended consequences that reduce the number of organisms a given food web can support or provide as food for humans. However, despite their vital role in regulating food webs, sharks and other top predators are in steep decline; some estimates suggest up to 100 million sharks per year are slaughtered.  The authors specifically note that shark finning, or the practice of killing mass numbers of sharks only for their fins, is a particularly damaging type of top predator removal that is throwing marine food webs drastically off-kilter

Using statistics to fill in the blanks

Elizabeth Spiers and colleagues recognized that food web modeling often looks at population boom or bust trends in response to top predator removal, yet little work has looked at how these food web boom or busts impact carbon dynamics within such marine food webs. By carbon dynamics, I am referring to the production or removal of carbon-based compounds like carbon dioxide within a system. Such cycling is important to climate dynamics because carbon dioxide (CO2), produced through cellular respiration of carbon-based organic molecules and fossil fuel burning, is a greenhouse gas that causes warming of the Earth’s atmosphere.

The researchers used a Bayesian belief network to answer their questions about carbon export within a typical marine food web; a Bayesian belief network is a statistical way of incorporating prior knowledge and probability to predict possible outcomes. This set-up is a bit more complicated than simply guessing the probability of heads or tails in a coin toss, as this network incorporates the complex relationships within a food web (Fig. 2).

Fig. 2. The probability network Spiers and her colleagues set up tries to represent the complex interactions that are involved in a general marine food web. Image: Spiers et al. 2016

At each point in the network, probabilities were assigned describing how each food chain link would react to changes in other food chain links; these probabilities were based on previous research. The authors made some simplifying assumptions when calculating these input probabilities for ease of modeling; for example, they didn’t incorporate the strength of each food web interaction into the probability network. The model was run using different fishing scenarios, including the removal of top predators such as sharks, and each point (aka food chain link) in the probability network responded to the probabilities of the points surrounding it, based on its own assigned probability. This domino game of probabilities produced an overall model predicting how the entire food web’s biomass and subsequent carbon export would respond to top predator removal.

Save sharks, help slow down climate change?

Their simplified model produced some head-turning results. They found that when top predators such as sharks are removed from a marine food web, overall biomass and carbon production by the food web increases (Fig. 3). The likelihood of this outcome was found to be about 60% based on their modeling exercise.

Fig. 3. When sharks and other large predators like sailfish or large tuna are removed from a typical marine food web, lower food chain links produce appreciably more carbon
Image: Spiers et al. 2016

The authors preface their results as preliminary and simplified, noting that they used a number of assumptions that wouldn’t necessarily apply in a real food web. However, they do point out that these results provide yet another reason why the preservation of top predators like sharks is so important; without top predators ensuring food web balance, the overall food web produces more carbon dioxide, adding a greater carbon dioxide burden to a planet already inundated with carbon dioxide from fossil fuel burning. Although slaughtering sharks is a bad idea for a number of reasons, this research provides promising statistical support behind the hypothesis that sharks and other top predators may play an underappreciated role in climate change dynamics due to the way they regulate biomass and subsequent CO2 production within marine food webs.

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