Languishing Leatherbacks

Leatherback turtles, Dermochelys coriacea, are the largest of all sea turtles, tipping the scales at up to 900 kg. Unlike other sea turtles, the leatherback lacks a carapace covered with scutes; instead its carapace is covered by thick leathery skin that is embedded with small bones forming seven ridges running along its back. This turtle has a wonderful set of anatomical and physiological adaptations, such as huge flippers and an efficient circulatory system, that make it a powerful swimmer and deep ocean diver. Males spend their entire lives at sea, while females usually return to their birthplace along sandy beaches to dig nests and lay eggs.

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Leatherback female on the beach at Las Baulas National Park. Credit: Karla Hernández.

Unfortunately, from the perspective of conserving awesome animals in our world, some populations of leatherbacks are declining rapidly, and many are now listed as critically endangered by the IUCN Red List. Pilar Santidrian Tomillo wanted to know why leatherback populations in the Eastern Pacific Ocean have declined so much in recent years. Working at Las Baulas National Park in northwestern Costa Rica since 1993, Tomillo and her colleagues have tagged 1927 nesting females so they could measure survival and return rates to the nesting shoreline. They discovered an alarming trend of sharp decline as described by the graph below.

TomilloFig1Tomillo and her colleagues knew that many leatherbacks were killed every year as a consequence of bycatch – capture by fishing nets or lines cast by fishermen who are targeting other species. But leatherback bycatch is very difficult to monitor accurately, as few fishermen keep accurate records of dead turtles, and turtles may die after being entangled and subsequently freed. The researchers also suspected that climate variability could influence leatherback population size. El Niño Southern Oscillation (ENSO) is a large-scale atmospheric system that affects global climate. In leatherback foraging areas, El Niño years are associated with high atmospheric pressure and warm sea temperatures, while La Niña years are associated with low atmospheric pressure and cool sea temperatures. Importantly, cool sea temperatures stimulate upwelling of nutrient-rich water to the surface, increasing production of phytoplankton, thereby increasing the abundance of  jellyfish and other favored leatherback food items. So the researchers hypothesized that the leatherbacks might do better in La Niña years than in El Niño years.

But what do they mean by doing better? There are two important factors influencing population growth: survival and reproduction. Either one could be affected by climate. By recapturing marked individuals, Tomillo and her colleagues were able to measure both survival and one important aspect of reproduction, which is how often females return to lay eggs. Reproduction is a very energetically demanding process for leatherback females, as they must migrate long distances (often thousands of kilometers) from their feeding grounds, and their eggs are large and plentiful, so females require a huge investment in resources to reproduce. Consequently, at Tomillo’s field site, only 4.5% of females reproduced in consecutive years, while the average interval between reproductive events was 3.65 years.

Let’s consider leatherback survival. As you can see from the data below, annual survival probability is very variable from year to year, ranging from about 30% in 2012 to near 100% in several years. Disturbingly, the long-term trend is downward, and the overall mean adult survival rate of 0.78 is very low in comparison to viable populations of sea turtles. If survival rates do not increase, the future is very bleak for this population.

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Annual survival probability of adult females tagged at Las Baulas National Park. Vertical bars indicate 95% confidence intervals.

How does climate variation influence survival and reproduction? The Multivariate ENSO Index (MEI) measures ENSO strength, with positive numbers (X-axis on graphs below) indicating El Niño years (with poor food availability), and negative numbers indicating La Niña years (with good food availability). The researchers found no climate effect on survival (top graph below), but a high reproductive rate associated with La Niña events (bottom graph below).

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The question remains, why is survival so low? Climate does not appear to affect survival, so that brings us back to human impact. Tomillo and her colleagues recommend reducing bycatch levels and implementing beach conservation measures to eradicate egg poaching. They also warn us that increases in global temperatures reduce egg hatching success, and pose a severe stress to this and other critically endangered leatherback populations throughout the world.

note: the paper that describes this research is from the journal Ecology. The reference is Santidrian Tomillo, P., N. J. Robinson, A. SanzAguilar, J. R. Spotila, F. V. Paladino, and G. Tavecchia. 2017. High and variable mortality of leatherback turtles reveal possible anthropogenic impacts.  Ecology 98: 2170–2179. Thanks to the Ecological Society of America for allowing me to use figures from the paper. Copyright © 2017 by the Ecological Society of America. All rights reserved.

Scientists MARCH against MADNESS (in April)

Back in my formative college years, my friends and I would indulge in many spontaneous gatherings, in which the question of “What is reality” occupied the stage front and center. As consummate dabblers in multiplistic world views, we considered all conceivable answers, and chose none. But time shuffled on, and so did we, to new adventures in which the reality problem no longer seemed so central, nor so puzzling. We recognized that reality is observable, but that your senses might betray you sometimes. That seemed like enough.

Going back a bit before my formative college years, Copernicus upset this sensory world view by publishing (almost on his deathbed) “De Revolutionibus”, which proposed, and gave some evidence for the hypothesis that Earth revolved around a fixed sun (heliocentrism). Originally this provocative alternative was mostly ignored, but many years later it raised some serious issues (particularly for Galileo) as it became more seriously considered. One major objection was that the reality imparted by our senses told us that the world was standing still – otherwise would we not be blown away by a world that was racing around a sun (and rotating at a frenzied pace to boot)? A second major objection was that the consensus of scientists at the time believed that that the world was standing still and occupied the central position. A third major objection was that a heliocentric world view, if taken literally, seemed at odds with some parts of the Bible, in particular when Joshua asked the sun to stand still so the Israelites had more time to deal with the Gibeonites.

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Figure of the heavenly bodies – An illustration of the Ptolemeic geocentric system by Portuguese cosmographer Bartolomeu Velho, 1568 (Bibliotheque Nationale, Paris)

The reality of our senses is a powerful argument. Copernicus, Galileo and scientists to follow would need to come up with a great deal of physical evidence to sway humanity from the commonsense observation that Earth is still. They did so with astronomical observations (aided by the invention of the telescope) and by developing theories of inertia, momentum and gravity, which propelled our understanding of universal laws for many different applications. As the scientific evidence became more compelling, scientific consensus shifted, and now most people accept the Sun as the center of the solar system, with Earth as one of eight (or nine) orbiting planets, even though these people (including many scientists) don’t understand the physics or the underlying mathematics. Lastly, even fundamentalist Christians and Jews can now argue that Joshua was simply using the language of his time when he issued his request to the Sun.

Advancing in time to 1896, Svante Arrhenius published a paper that proposed that atmospheric CO2 levels could influence atmospheric temperatures in ways that are now familiar to us. Like Copernicus before him, Arrhenius’ ideas were initially rejected by most scientists, until new technology was applied to measuring atmospheric CO2 levels, and to developing models of how the atmosphere and climate interacted. Ultimately, these new approaches led to the development of a scientific consensus that climate is changing, that Earth’s surface is warming, and that human behavior is responsible. Over the past 60 years we have measured the changes, we have developed a more coherent scientific understanding of atmospheric processes, we have made mathematical models that generate projections and we have validated these models empirically. Unlike Copernicus and Galileo, we can observe climate change using the reality imparted by our senses (either online, in books, or by journeying to shorelines or to polar regions that are losing their cool). Unlike Copernicus and Galileo, the consensus of the scientific community is in our camp. Unlike Copernicus and Galileo, the Bible does not make any claims about climate or CO2. This reality should be a no-brainer!

But it isn’t, and I lack the omniscience to understand why this reality is being denied. One hypothesis is that the geocentric hypothesis has been replaced by the corporate-centric hypothesis, which states that corporations and their shareholders are at the center of the universe, and that financial earnings are the currency of reality. The power of this system is that these earnings, if they are maximized and judiciously applied, can be used to purchase some people’s perceptions of reality, so that their reality denies the scientific consensus. This new corporate-centric hypothesis denies scientific facts, and downgrades them with alternative facts that claim to be equally valid.

On April 22 we march across the globe to celebrate and affirm the reality of our senses, the truth of our observations, and the beauty of our complicated world. We celebrate a universe with no center, and a world with millions of different species that interact with each other and their environment in meaningful and mysterious ways. We celebrate the pursuit of rational inquiry into the processes underlying these interactions, and the deepening of our understanding of who we are as humans, and how we can, as scientists, apply real knowledge to allow our Earth to flourish.