Although it may be difficult to contemplate in this travel constrained COVID-19 era, try to imagine yourself on a holiday on a Caribbean island. One of your first priorities, after settling into your beach-side hotel, is likely to be to immerse yourself in the warm, clear tropical ocean and investigate the nearby coral reef that was advertised on your hotel’s website. You thus unpack your snorkeling gear, stroll to the beach, don mask, snorkel, and fins, and slip into the sea. During your explorations I am sure you would be delighted to encounter a Caribbean green turtle swimming through your field of view. You might take a few photos of this beautiful herbivore with your water resistant phone and, once back on the beach, send them off to a few of your friends and relatives back home to let them know what they are missing.
Now, try to be even more imaginative, and envision yourself transported back 500 years to the same beach. The hotel would not be there of course, but there might be a few buccaneers camping further back among the sea grape trees; and you would look a little out of place in your surf shorts, snorkelling gear, and phone in hand. The reef, however, would be there and beckoning, so into the water you get. What would you see? For one thing, I can guarantee you would see a lot more green turtles.
Historical perspective
Early European voyagers to the Caribbean Sea were astounded by the abundance of green turtles. For instance, a priest on Columbus’ second voyage, Andres Bernáldez, reported that the crew were stunned by the number of green turtles they encountered while sailing among the Jardines de la Reina in 1494, a group of islands off the coast of southeastern Cuba1. Bernáldez wrote:
Throughout that voyage they saw that there were many turtles and very large. But in those 20 leagues, they saw very many more, for the sea was all thick with them, and they were of the very largest, so numerous that it seemed that the ships would run aground on them and were as if bathing in them.
Andres Bernáldez, 1494
In his History of Jamaica2, published in 1774, Edward Long , a British colonist and historian, recounted that seamen in the late 1600s used the sounds made by masses of green turtles migrating along an 800 kilometre route from the Gulf of Honduras to the Cayman Islands as a navigational aid. He wrote:
…it is affirmed, that vessels, which have lost their latitude in hazy weather, have steered entirely by the noise which these creatures make in swimming, to attain the Caymana Isles.
Edward Long, a British colonist and historian, 1774
From our 21st century perspective these seem like extraordinary observations and you would be forgiven for thinking that they may be the exaggerated tales of early European explorers and colonists intent on glamourizing their experiences in foreign regions for a receptive audience back home. However, the marine ecologist, Jeremy Jackson, the founder of the scientific discipline of historical marine ecology, demonstrated that these were no myths, and that green turtles were indeed once extraordinarily abundant in the Caribbean.
Reconstructing historic population numbers
Jackson combined historical observations with modern marine biological knowledge to derive estimates of how many green turtles once swam in the pre-Columbian Caribbean. He first based his calculations on historical hunting data from the Cayman Islands3. He knew that when the English captured Jamaica in 1655 there was little established agriculture to support a growing colony, so they mounted expeditions to hunt green turtles that nested on Grand Cayman Island over 400 kilometres away. Historical records show that this hunt peaked between 1688 and 1730 when 120 to 150 men in about 40 sloops killed and brought back to Jamaica about 13,000 turtles per year.
We know from the biology of green turtles that their sex ratio is 1:1 and that a female will nest on a beach to lay her eggs on average every 2.5 years. From this, Jackson was able to calculate the proportion of the adult Cayman Island green turtle population that consisted of nesting females, which turns out to be about one-fifth of the total population. Jackson then assumed, conservatively, that the 13,000 turtles killed each year amounted to 1 per cent of all the nesting females on the beach. On this basis there were about 1.3 million female turtles nesting each year, which means the total Grand Cayman population was around 6.5 million adult turtles. Jackson then assumed there were five other green turtle nesting sites in the Caribbean of about the same size as Grand Cayman. This meant the total pre-Columbian adult green turtle population was around 39 million.
Jackson then used another approach, this time based on how much seagrass an adult green turtle needed to eat to survive, and the estimated total area of seagrass meadows in the Caribbean. He knew from contemporary research that about 1 km2 of seagrass could support about 10,000 adult green turtles. On the basis that there was, conservatively, 66,000 km2 of seagrass available, then the adult population of green turtles in the Caribbean would have been around 660 million. Although sharks and other predators of green turtles would also have been more abundant, they would have targeted mostly juveniles, since the adults are large and well-protected and thus largely immune to predation. Jackson thus argued that the adult population of green turtles would have been mostly regulated by the availability of their food.
One of Jackson’s PhD students, Loren McClenachan, approached the problem in another way and helped refine Jackson’s estimates4. Based on an exhausting search of historical records, she found evidence that there were 59 green turtle nesting beaches in the pre-Columbian Caribbean, 9 of which were major nesting sites equivalent in size to the Cayman Islands site. Assuming that the 9 major sites supported populations equivalent in size to that estimated by Jackson for Grand Cayman, and that the remaining sites supported populations that were 10 per cent of this size, McClenachan calculated that the historic population of adult green turtles in the Caribbean was around 91 million.
Admittedly, the work of Jackson and McClenachan is based on historical data and several assumptions and gives wide-ranging estimates of historical Caribbean green turtle numbers. To put this into perspective, however, there are currently around 300,000 adult green turtles throughout the Caribbean. Based on Jackson’s and McClenachan’s work, we can be confident that there were tens of millions of green turtles in the past. The exact number is largely irrelevant. If we take an historical figure of 91 million as reasonable, then the current population is about 0.33 per cent of historic numbers. Clearly, green turtles are now ecologically extinct in the Caribbean. What happened to them all?
The demise of Caribbean green turtles
An adult green turtle makes an excellent packet of food—a 100 kilograms or so of easily caught, nourishing, and tasty protein and fat. It also stores well—sailing crews could hold green turtles alive on deck or in a well of seawater for months to be used for food when other supplies ran out. It is thus not surprising that sailors and buccaneers made good use of this abundant food source and hunted green turtles throughout the Caribbean during the 1500s.
The establishment of large sugar plantations on Caribbean islands starting in the late 1500s made green turtle meat a staple food commodity. The plantations required a large work force and five million enslaved Africans were transported to the Caribbean to meet the demand. Without an effective agricultural base on the islands, green turtles became an important food for a rapidly growing population of colonists and slaves. Regular hunting expeditions were mounted to nesting sites where the vulnerable females were killed on the beach. By the 1750s green turtles were also hunted to meet the demand for a growing European market for green turtle soup which had become a sought-after delicacy.
Commercial hunting of Caribbean green turtles endured up into the early 1900s. Large green turtle canning operations existed in Texas and Florida in the late 1800s and early 1900s to meet demand for green turtle soup in New York, Boston, and European cities. Turtles were obtained initially from the Gulf of Mexico and the Florida regions, but hunting extended to many other parts of the Caribbean as these stocks dwindled.
Over 400 years of intense, largely unregulated, hunting decimated these once superabundant megavertebrates. Of the 59 historical nesting populations of green turtles throughout the Caribbean, hunting eliminated at least 17 of these and severely reduced the numbers of turtles at the remaining sites. An historical population of tens of millions of Caribbean green turtles was thus reduced to a remnant.
So, going back to your imaginary snorkel on a Caribbean coral reef 500 years ago, you would have been swimming with hundreds of green turtles, which would have made a magnificent sight. What were the ecological consequences of emptying the Caribbean Sea of green turtles?
Ecological consequences
Adult green turtles are herbivorous, grazing prodigiously on seagrass, particularly a species known appropriately as turtle grass. McClenachan and her colleagues estimated that a pre-Columbian population of 91 million green turtles would have removed the mature blades from 86 per cent of the total area of seagrass in the Caribbean each year and, in so doing, would have consumed up to 45 per cent of the annual production of these seagrass beds. In contrast, the few remaining green turtles in the Caribbean today are consuming a tiny fraction, around 0.1 per cent, of the total area and annual production of available seagrass.
Green turtles were thus enormously important megaherbivores of the Caribbean Sea with an ecological role much like the bison of the prairie ecosystem of North America or the wildebeest of the Serengeti ecosystem of east Africa. They would have kept the seagrass beds clipped like putting greens, which is supported by historical accounts of seagrass beds “cut near the roots” by grazing green turtles. They would also have thinned out the beds, thereby reducing direct competition among different species of seagrasses and allowing several species to coexist. Furthermore, they maintained the health of the seagrass beds by removing old blades that are colonized by microorganisms that promote the spread of seagrass disease. Green turtles were therefore key ecosystem engineers, playing a vital role in maintaining and structuring Caribbean seagrass habitat. Their ecological demise has transformed seagrass beds into mostly overgrown monocultures of unconsumed, diseased turtle grass containing large amounts of uneaten decomposing organic matter. We can summarize their changing ecological role schematically as follows:
Restoring the Caribbean green turtle population
Now let us return to the theme of our imaginary snorkelling explorations. You have gone back 500 years and glimpsed the past. Now imagine you are transported to the same beach 100 years into the future. What would you see? Well, we have no idea. Human activities have unravelled the natural ecological fabric of Caribbean marine ecosystems in many ways, not just by decimating green turtles. It is now up to us to decide what kind of future we want for the Caribbean Sea and implement a restoration strategy. Our glimpse of the past is crucial for this task, since it tells us what we might aim for. A part of any Caribbean Sea ecological restoration project will have to be to revive the population of green turtles. Given the human population size of the Caribbean today, and its scale of development and habitat loss, striving to go back to a time when there were 90 million green turtles would be romantic nonsense. But perhaps a goal of re-establishing over time a population of 30 million adults across half the Caribbean Sea is worth considering? If so, let us begin developing a plan to determine what is feasible and how we might go about doing it.
References
1Jane, C (editor) (1930). Select Documents Illustrating the Four Voyages of Columbus. Vols I and II. Hakluyt Society, London
2Long, E. (1774). The History of Jamaica, London. Reprinted by Ian Randle Publishers, Kingson, 2003
3Jackson, J. B.C. (1997). Reefs since Columbus. Coral Reefs 16, Suppl.: S23-S32. https://doi.org/10.1007/s003380050238
4McClenachan,L., Jackson, J.B.C.and Newman, M.J.H. (2006). Conservation implications of historic sea turtle nesting beach loss. Front. Ecol. Environ. 4(6): 290-296. https://repository.si.edu/bitstream/handle/10088/18767/stri_McClenachan_Jackson_and_Newman_2006.pdf?sequence=1&isAllowed=y
