Marine turtles: Facing danger at every turn
Carlos Drews
This lecture was adapted from a presentation detailing WWF's Latin America and Caribbean Marine Turtle Conservation and Action Plan.
Most experts recognize seven species of marine turtles: the green, hawksbill, loggerhead, Kemp's ridley, olive ridley, leatherback, and flatback. All but one - the flatback - can be found in the Latin American and Caribbean region, and all are threatened by extinction.
Both human induced and natural factors present challenges to the survival of sea turtles. These highly migratory, unique reptiles spend their life at the coast and off shore, combining terrestrial and aquatic habitat requirements - depending on their needs at particular life stages - that often span the political limits of several nations. Marine turtles require several decades to reach sexual maturity and are potentially long lived, dispersing and migrating over vast areas.
The huge size of individual home ranges and the vast area requirements of marine turtles during their life cycle are unusual for existing reptiles. Consequently, marine turtle conservation actions must be sustained over decades, carried out over vast areas, be relevant to diverse marine and terrestrial environments, and involve international cooperation.
The causes of decline and the present and future threats to marine turtles are diverse. Three realms, however, have been recognized as main threats to marine turtles worldwide: habitat destruction and alteration, overexploitation for meat, hides, eggs and shells, and incidental capture in fisheries. Currently, human induced mortality is having a greater impact on marine turtle populations than natural mortality.
Under natural conditions, turtles suffer high hatchling, post-hatchling and juvenile mortality, but those that survive the early days grow into long-lived animals with very low adult mortality. Unfortunately, conditions nowadays are far from "natural" and turtles suffer mortality at all stages of their life cycle, leading to increasingly regular population crashes.
The number of female leatherbacks nesting on the Pacific beaches of Mexico has declined more than tenfold in less than a decade; the number of nesting loggerheads in eastern Australia has declined by 50 to 80% since the mid-1970s; Kemp's ridley nearly went extinct. The list goes on, and makes depressing reading.
The causes of the population declines are many and varied, but have their roots in two basic characteristics of turtle biology which render populations particularly vulnerable to the pressures described above: (1) reproduction is highly localized in beaches allowing easy access to eggs and nesting females, and rendering this critical habitat vulnerable to alteration through coastal development, and (2) their slow maturation hides the effect of overexploitation for decades.
Turtle populations can be destroyed from the "bottom up" by over-exploitation of the eggs, and destruction of nesting sites. For example, as far as we know, green turtles take 30 to 50 years to reach sexual maturity and remain reproductive for about 20 years. Adults are the visible component of a turtle population; their numbers are maintained by the gradual maturation of juvenile and sub-adult turtles. This will continue to happen, even if no eggs are laid or if all the eggs are collected.
It will be many decades before the number of adults begins to decline, but over time the reservoir of juveniles and sub-adults will become progressively depleted until there are no more recruits. These "last adults" will, in theory, survive for another 20 years during which time the situation may not seem too serious. In reality, however, the population is on the verge of extinction because once these adults die there will be no hatchlings, juveniles or sub-adults to replace them. If juvenile and adults are being killed, e.g. as bycatch, then this will simply happen more quickly.
The design of effective management and conservation strategies - particularly on a regional scale - is challenged by a number of factors, including gaps in the knowledge about marine turtle life history patterns and the actual conservation status of some of these turtle populations; a lack of understanding of current levels of exploitation and trade and the effects these are having on particular species or populations; and the unregulated and unmanaged nature of turtle catches and trade.
Over the next 10 years, the goal of WWF's global efforts is the reduction of threats to marine turtles from the loss and degradation of their critical habitats, from the impacts of unsustainable use, and from incidental capture (by-catch).
Based in the Central America program office, Carlos Drews is WWF's Latin America and Caribbean marine turtle coordinator. A native of Colombia, he has a Ph.D. in Zoology from Cambridge University.
