Manatees & Caribbean Conservation: The Threat of Low Genetic Diversity
Across the Caribbean, protected coastal waters have served as primary feeding and breeding grounds for the endangered Antillean manatee. Unfortunately, these same coastal waters are also a popular “habitat” for humans. In the past, the overlap between human and manatee habitat allowed for manatee hunting and threatened the survival of these gentle marine mammals. Today, however, threats are much more inadvertent and are often related to coastal development, degraded habitats and boat strikes.
In the state of Florida, decades of research on the species’ biological needs have helped conservationists address threats to its survival. For example, low wake zones and boater education have protected manatees from boat strikes, and many of their critical winter refuges are now protected. The Florida population has grown steadily, thus increasing from approximately 1,200 in 1991 to more than 5,000 in 2010. It is conceivable that in Florida manatees may one day be reclassified as “threatened” rather than “endangered.”
Yet, in other parts of the Caribbean, threats still loom. This includes small, isolated manatee populations found on islands that can be more susceptible to extinction and lack of genetic diversity. To ensure the species’ long-term viability, scientists have turned their sights to the overall population dynamics of manatees throughout the Caribbean. Molecular genetics has provided new insights into long-term threats the species faces. Fortunately, the emerging field of conservation genetics provides managers with tools and strategies for protecting the species’ long-term viability.
Slow, Fat & Dumb: Science Busts Stereotypes
Basic research on manatee biology, physiology and health primarily began in Florida in the late 1960s and started with one of the first critical questions: How and why were manatees dying? Since the majority of manatees in areas of Florida have boat strike scars on their bodies, a long-held, common misconception existed that they are physically (or intellectually) too slow to move out of the way of speeding boats. However, research shows manatees face unique biological challenges in avoiding boats. For starters, manatees must come to the surface to breathe every few minutes for their entire lifetime, which can last more than 60 years.
Manatees do typically perform a diving behavior when a boat approaches. However, boats are often moving too fast for the manatee to have time to dive out of the way. Manatees have brains evolved for long-term memory of large coastlines, fresh water sources and protected bays. Just like their closest cousin, the elephant, manatees never forget, and biologists have shown them capable of learning and responding to information. Since manatees have a rotund, or fusiform, shape, they are often assumed to be pleasantly plump. However, manatees actually have very thick skin and a relatively thin fat layer. Compared to other marine mammals with large blubber layers, such as whales, manatees are as skinny as super models. Their large body and shape is used to house up to 150 feet of intestine and other organs needed to digest large amounts of their low-calorie diet of seagrass.
Threats to Manatee Survival
Beyond just analyzing the basic biology of manatees, research has provided key information on threats to the manatee’s long-term survival, which have enabled managers to determine how to focus conservation efforts. Throughout the years, major threats to manatees in Florida have proven to be boat strikes (about 25 percent of all reported Florida manatee deaths), red tide toxicity and cold stress syndrome. Red tide is caused by an algae bloom, which produces a neurotoxin. Manatees inhale the toxin as they breathe at the water’s surface or ingest the toxin while feeding on aquatic vegetation. In manatees, it often results in seizures and drowning.
Cold stress syndrome is also a major cause of natural manatee deaths in Florida. In 2010 282 deaths were attributed to cold stress – a number 10 times higher than the five-year average (FWRI: http://myfwc.com/research/manatee/rescue-mortality-response/mortality-statistics/cold-related-2009-2010/). Manatees have evolved in tropical climates and, unlike other marine mammals, their physiology is adapted to shed heat, not retain it. Because Florida is the northern extent of their winter range, Florida manatees are pushing their physiology to the extreme to tolerate cooler temperatures.
Adaptations include larger body size and use of natural and, more recently, artificial warm water sources to thermo-regulate when water temperatures drop below 68°F for extended periods. Cold stress syndrome resembles hypothermia and can occur chronically; it results in emaciation and frostbite-like lesions. The syndrome can also occur rapidly when manatees venture from warm water refuges to feed in extremely cold water or are caught away from warm water during a sudden cold spell.
Thanks in large part to these early research efforts, the Florida manatee population now benefits from protections designed to minimize the number of deaths each year. Florida, however, is home to only one species of manatee, the West Indian. The other two species are the Amazonian and the West African. Furthermore, the West Indian manatee includes not only the Florida subspecies but also the Antillean, found throughout the Caribbean basin. Initially, these taxonomic differences were based on biological differences researchers observed, but recent genetic work has confirmed the validity of these differences. Antillean manatees face some of the same threats as the Florida subspecies. For example, an increasing number of boat strikes are occurring in places like Belize and Puerto Rico. However, manatees are also susceptible to the less visible problem lurking in their genetic make-up: the threat of low genetic diversity.
Antillean Manatee Research
Where once the Antillean manatee thrived along the Caribbean, Central and South American coastlines, many now live in small, isolated groups sporadically distributed throughout the region. As local populations are reduced, gaps in distribution occur, thus resulting in isolated populations. These populations, especially the smallest in number or those with low genetic diversity, can encounter inbreeding depression. Inbreeding depression is as bad as it sounds, and it refers to a cycle of related individuals interbreeding and eroding genetic diversity with time. This can be detrimental to the population since diversity is necessary for adaptation to environmental change or response to disease processes. With limited genetic diversity, populations often develop reproductive issues and physiological mutations, which results in fewer surviving offspring. This genetic and demographic decline is known as the extinction vortex.
With current cooperative research efforts, understanding of the Antillean manatee in the Caribbean basin and South America is quickly growing. This is occurring as more countries make imperiled wildlife a priority. However, some populations remain understudied and lack adequate protections.
Genetic diversity is not typically the first threat that comes to mind when people think of endangered species. It is a much more silent, invisible threat to a species’ long-term survival than poaching and injuries caused by boat strikes or even deaths due to red tide and cold stress. In fact, it tends to affect a species more by suppressing its ability to reproduce and adapt. And, with time, it can silently chip away at the health of a population. In fact, the Florida panther is one well known example of this. A lack of genetic diversity was slowly taking its toll on South Florida’s iconic big cat when conservation biologists realized they had to act before the population became extinct. Research helped mount a genetic rescue of the critically imperiled Florida panther through introduction of cats from a different subspecies. This led to increased vigor and resulted in higher breeding rates.
Biologists worry that the Antillean manatee, the subspecies found in Puerto Rico and other parts of the Caribbean, has low genetic diversity across its range. Compared to the Florida manatee, Antillean populations are much smaller and less diverse. Low diversity has been identified in both the mitochondrial and nuclear genomes,thus suggesting the populations have been small and predominantly isolated for generations. The three Antillean populations in which nuclear genome markers have been studied so far had similar levels of diversity as the North Atlantic right whale, considered one of the most endangered marine mammals in the world. The low genetic diversity and small populations sizes of Antillean manatees is attributed, in part, to centuries of hunting, which has only recently been curtailed in the early 20th Century.
Genetics as a Conservation Tool
Molecular genetics is a relatively new field with powerful tools for informing conservation and wildlife management. To aid conservation efforts, genetic studies focusing on manatees have increased at a rapid pace. These genetic tools can assist in many applications similar to crime scene investigations, such as detection of illegally harvested and mislabeled species in food items. Examples include endangered whale meat in Japan and endangered sturgeon caviar. Molecular markers can also track individuals using unique genetic fingerprints, identify pedigree and paternity relationships, and understand how different populations are related for long-term preservation.
One important focus is determining relationships among manatee populations across their range. Genetics can reveal the degree of relatedness between individuals and populations by examining DNA sequences. Manatees have been known to disperse long distances and can easily travel across geographic country borders.
One example of how molecular genetic approaches can improve management decisions is in the field of reintroductions. Some countries are now assessing the possibility of reintroducing manatees into areas where the species was hunted to extinction. Knowledge of biology and ecological needs, such as suitable habitat, freshwater sources and seagrass beds, is important for these efforts. Additionally, comprehensive genetic characterization of the extinct population and manatees selected for introduction is helpful. To avoid inbreeding, the reintroduced population should be founded with manatees having unique and high amounts of genetic diversity. Close monitoring of resultant offspring and intermittent reintroduction of new individuals with unique diversity could help to maintain a genetically healthy population.
Other methods in molecular genetics can be used to address questions on an evolutionary time-scale, dealing with past relationships and geographic distributions. These relationships can include identification of distinct relatives, related species, hybridization and isolated populations. Information can also be gleaned about geographic barriers, such as mountains or rivers, and patterns of dispersal into new areas.
Antillean Manatees in the Caribbean Basin
Little is known about many of the threatened Antillean populations. In much of the developed coastal areas, they face threats in the form of watercraft mortality and habitat destruction. However, information, such as population size and relationship to nearby populations, is often lacking. For instance, it is estimated that on the island of Jamaica fewer than 10 manatees exist, although very seldom have they been observed during the last couple of decades. Additionally, Belize is considered the stronghold of the Antillean manatee with its strong conservation protections and large population size (estimated to be more than 1,000). However, its neighbor to the south, Guatemala, has few protections and a very small or extinct manatee population – likely due to hunting pressures. This is a classic example of the source and sink relationship.
Habitat destruction most often occurs in protected, shallow bays or estuaries and includes removal of mangroves for salt production or aquaculture farming, especially shrimp farming. This results in reduced foraging area; and when mothers cannot access calm, protected waters, it can directly affect infant manatees. Cases of cow and calf separation are on the rise and results when births occur in areas with higher wave action. Construction of resorts, residences and industrial buildings along the coast can also deter manatees from utilizing critical habitat or utilizing travel-corridors. Warm or freshwater resources can also be reduced through human development, further limiting the habitat needed to survive cold winters or to obtain freshwater. These alterations of the environment can also have secondary effects like increases in diseases or pathogens.
Puerto Rico’s Antillean Manatees
The Puerto Rico island population of manatees is coastally marine and dependent on sources of freshwater. Manatees get their freshwater from river mouths, run-offs and water treatment plant outfalls. Humans and manatees have co-inhabited this area for a long time. The earliest accounts of manatees in Puerto Rico date as far back as the 1590s and include reports of Taino and Carib Indians, as well as early Spanish explorers, using manatee meat as an important food source. In fact, in the 16th Century, the Catholic Pope formally declared manatees to be a “type of fish” in order to allow Spaniards to comply with the precept of not consuming meat on Fridays. Later on, buccaneers roamed the Caribbean Sea as pirates. (The term buccaneer derives from the Caribbean Arawak word buccan, a wooden frame for smoking meat, preferably manatee.)
The island of Puerto Rico is one of the most densely populated islands in the world with approximately 1,000 people per square mile. Much of the population is concentrated in the coastal areas, especially in the capital city of San Juan. Increased human impacts and the high rate of development throughout Puerto Rico have strongly affected the environment. The protected, shallow bays where humans concentrate also make up the primary areas of manatee habitat. Watercraft traffic and human presence have increased in bays where manatees previously sought food, freshwater and sheltered areas for rest, reproduction and to care for their young. Unlike many of the well-protected manatees in Florida, manatees in Puerto Rico are typically wary of humans, perhaps due to the recent history of hunting. And, they may not utilize resources near areas of high human activity.
Other issues facing the Puerto Rico manatee population include their geographic isolation from other populations. The Florida population is the largest in the region and was anticipated to function in such a way as to supplement the small Puerto Rico population. If manatees migrated from Florida to Puerto Rico, the small population size and low genetic diversity could be improved. However, in a recent study using genetic tools, manatees from Puerto Rico and Florida were determined to be two separate genetic stocks.
The study indicated that no detectable movement or breeding has occurred between the two populations throughout the last few millennia. A key management concern is, in fact, the ability of Puerto Rico manatees to rebound from population declines. Recovering from population decline can be much slower when there is no supplementation from nearby populations. The geographic barriers in the Antillean region of the Caribbean limit the potential for increased population numbers of genetic diversity by the addition of manatees into the Puerto Rico population.
Among other findings of the study was the identification of two distinct populations within Puerto Rico itself. These two populations were shown to infrequently interbreed. The two genetically different groups provide diversity that may actually improve long-term prospects for manatee survival in Puerto Rico.
Similar to other West Indian population, the study found the overall genetic diversity in the Puerto Rico population was very low. As mentioned, low diversity decreases a species’ capacity to adapt and respond to environmental change, and may reduce the chance of rebounding after a significant loss of the population. Genetic diversity is important for imperiled species to weather threats to their survival, including random or rare events, such as disease, hurricanes or habitat destruction. When a population size drops to low numbers, diversity of its gene pool also shrinks. Even after a population’s numbers begin to climb again, population decline leaves a legacy of reduced genetic diversity known as a bottleneck. This renders the population more vulnerable to impacts of future events.The manatee population in Puerto Rico is small, estimated to contain approximately 338 individuals, and falls under protection of the U.S. Endangered Species Act, along with the Florida populations. Since threats, habitat and population size of the Puerto Rico population differ from Florida, it could be beneficial for Puerto Rico to be considered separately for management actions. This is a noteworthy point since, as mentioned, Florida manatees do not migrate to Puerto Rico to provide supplementation to the smaller population.
The Future for Manatee Conservation Genetics
It is an exciting time in the field of manatee genetics. An important stepping stone includes the sequencing of the manatee genome in 2012, which opened the door to expanded research in evolution, health, conservation and physiological responses. As genetic sequencing costs decrease and capabilities increase, expanded studies of evolutionary differences between the Florida and Puerto Rico manatee can be undertaken. For example, how have the populations adapted to cooler temperatures or lack of fresh water? Also, genetic adaptations to red tide could be identified in manatees more often exposed or better able to survive exposure. Examination of functional genes could better quantify degree of inbreeding and population fitness. Although no strong physiological or genetic implications of decreased survivability or fecundity have been identified in Florida, reductions in numbers or genetic diversity could lead to increased inbreeding and reduced population fitness. Continued monitoring efforts are needed to ensure sustainability of the population. With implementation of new technologies, the future for research and conservation seems bright for Antillean manatees. Increased attention by the public and many governments has led to strong protections and continued cooperative research for the imperiled populations. Genetic studies are continuing for Antillean manatees, such as in the critically endangered Brazil population and the other two species of manatee, the Amazonian and West African. Continued efforts by dedicated biologists and future-looking managers will help to conserve these gentle giants and allow them to live a tranquil existence alongside humans.