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Book Review

Turn off some lights for the turtles: using statistics to make turtle conservation tangible

DOI: http://dx.doi.org/10.1016/j.jeem.2016.02.003

The kinder side of the ocean, explained

By human standards, the ocean might be a little too kind, even a borderline pushover. The ocean doesn’t charge for swims and surf sessions, or any time spent enjoying its reaches.  Wetlands and marshes don’t invoice communities for protecting them during storms (“Pay to the Order of Sal T. Marsh”? Have yet to see this bill). Fish don’t ask fishermen to cough up a few bucks before swimming into their nets. The ocean doesn’t charge for any of the goods or ecosystem services we as humans derive from it; it is what is known as a common resource. As users of this common resource, its up to us to regulate our use of the ocean and the taking of its flora and fauna. Unfortunately, we humans have proven to be pretty bad at common resource management, leading to the famous idea of the “tragedy of the commons”. Smarter people than me have expounded on this at length, but suffice it to say, this idea says that users of a common resource have difficulty regulating their use because there is no resource oversight. Individual greed compounds as more people use the resource, often leaving it depleted or destroyed in the long run. However, when common resources (like the ocean and its creatures, such as sea turtles or fish) are assigned monetary or temporal value, the “tragedy of the commons” often gets a little less tragic, as people recognize the worth of the given common resource and are more eager to protect or regulate it.

Enter modern economics, holding hands with population biology

Bajan and French researchers found this “money talks” logic to be research-worthy, and decided to frame one element of sea turtle conservation in terms of economic value. The researchers identified that nighttime light pollution is a key factor in sea turtle population health; light pollution around coastal margins deters turtles from coming to land to nest, increases the likelihood of human interference during nesting, and/or interferes with turtles finding their way back to the sea (Fig. 1). Light-caused disorientation is particularly a problem for hatchlings, as they try to scramble to the sea without being eaten; this is dicey enough without added light making them more visible to predators.

Fig. 1. Light pollution obstructs clear nighttime skies, substituting high levels of artificial light during what should be dark hours. This image shows the night sky before and after a large blackout in the US, when 55 million people lost power and the skies approached natural darkness. Image credit: Todd Carlson, via darksky.org

Fig. 1. Light pollution obstructs clear nighttime skies, substituting high levels of artificial light during what should be dark hours. This image shows the night sky before and after a large blackout in the US, when 55 million people lost power and the skies approached natural darkness. Image credit: Todd Carlson, via darksky.org

The strategy

Scientists/researchers Michael Brei and his colleagues pulled together some data describing sea turtle nests from three different species on 156 beaches in Guadeloupe (Fig. 2). Their data included nest location, nesting activity, associated monitoring efforts, local economic activity, and satellite-derived nighttime light images. Brei and coauthors then worked some statistical magic to derive some key information from these datasets. The authors specifically used econometric modeling to relate and manipulate economic and population data surrounding their sea turtle study subjects. Here, modeling involved making assumptions about how the observations were collected, as well as making more educated guesses to fill in missing variables. They then created a set of equations that fits the data and also predicts or informs more in-depth insight about the data at hand.

Fig. 2. Guadeloupe is a small Caribbean Island, previously a French colony. Researchers used data from 156 beaches on this island, as well as satellite imagery providing light pollution information, in econometric models that provide some information about the current and future effect of light pollution on sea turtles. Image credit: WorldAtlas.org

Fig. 2. Guadeloupe is a small Caribbean Island, previously a French colony. Researchers used data from 156 beaches on this island, as well as satellite imagery providing light pollution information, in econometric models that provide some information about the current and future effect of light pollution on sea turtles. Image credit: WorldAtlas.org

Statistics says a lot…

They found that nighttime light does have a significant negative impact on turtle nesting, likely because the light deters mom turtles from nesting, while also confusing hatchlings trying to reach the sea; this was intuitive given local-scale results seen elsewhere. They also found that light pollution was a significant stressor on the turtle population overall. Using population modeling and making some basic assumptions to feed into their calculations, they found that light pollution may be preventing the birth and survival of approximately 1805 turtles in the Caribbean. By using information from turtle hospitals and rehab centers, researchers were able to calculate the cost of raising an adult turtle to adulthood. Using this information, they found that if the number of missing turtles were to be made up by raising turtles in captivity, it would cost between $6.2 – 287.9 million dollars to repopulate the Caribbean with those turtles lost due to light pollution in the area. They qualified this price tag by saying this is a low estimate of true cost of the lost turtles, as it does not take into account lost ecosystem services related to the lost turtles. Furthermore, they decided to make some simple assumptions to estimate if light pollution could be contributing to possible extinction of the three species they examined. The researchers made some simplifying assumptions about turtle reproduction and survival, and produced a basic estimate of turtle population dynamics with these simplifications. They then added nighttime light pollution into the equation; their simple model suggests that nighttime pollution speeds up possible extinction of each examined species, due to its interference with turtle reproduction on Caribbean beaches (Fig. 3).

Table derived from Brei et al. 2016 describing projected time to extinction for each species included in the study. Brei and his colleagues used a simple model to predict how light pollution could effect long term population health; they found light pollution speeds up possible extinction for all three examined species, with extinction approaching faster if the model is tweaked to account for a greater impact of light on hatchling survival.

Fig. 3. Table derived from Brei et al. 2016 describing projected time to extinction for each species included in the study. Brei and his colleagues used a simple model to predict how light pollution could effect long term population health; they found light pollution speeds up possible extinction for all three examined species, with extinction approaching faster if the model is tweaked to account for a greater impact of light on hatchling survival.

So what does all this math actually mean?

To reiterate, the above described math is a simplification; the authors fill in variables they don’t know with existing data or best guesses, but this doesn’t approximate exactly what is happening to turtle populations in real-time. It is just a best estimate using information that we already possess. This does however frame the effects of light pollution on turtle populations in two variables humans relate to: time and money. This research suggests that light population is negatively impacting turtle populations, and it would cost a fortune for humans to try to recover this lost population using our turtle-raising capabilities. The study also says that light pollution likely has a significant long-term impact on turtle population survival, since it messes up their reproductive activities; light pollution speeds up possible extinction of these already threatened species. Imagine your great grandkids living in a world without sea turtles! Overall, this work is important in its valuation of turtle populations in variables humans can readily grasp. It also is important because it shows that a preventable stressor could alter the course of a species’ survival; simple daily activities can help the health and survival of sea turtles and other creatures that are impacted by artificial light. Turn off indoor lights, put motion sensors on outdoor lights to prevent them from running when no one is around, and perhaps most importantly, educate yourself and your neighbors about light pollution!

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