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In view of the imminent risks to primary nesting habitats, here we review studies of mitochondrial (mt) DNA diversity for the nesting populations of green and loggerhead turtles at X'cacel. Recent research has provided a relatively complete atlas of Atlantic mtDNA diversity in these species (Encalada et al. 1996, 1998), and so the potential consequences of rookery extinctions on intraspecific genetic diversity can be estimated.

Samples from green turtle (n = 20) and loggerhead turtles (n = 20) were obtained from the nesting beaches of X'cacel and nearby Isla Cozumel. Haplotype frequencies from these locales were not significantly different and so the two locations are considered here as a single population. Samples consisted of a few drops of blood taken from the dorsal cervical sinus of nesting females following a protocol by Owens & Ruiz (1980). A mtDNA region of 391 base pairs (bp) and 510 bp from the mtDNA control region were sequenced from loggerhead and green turtles, respectively (see Encalada et al. 1996, 1998 for extended description of methods).

These surveys found that X'cacel populations of Chelonia mydas and Caretta caretta are unique in several respects (Encalada et al. 1996, 1998). Although there was some sharing of haplotypes with other regional nesting colonies (especially Florida, USA), haplotype frequency comparisons show that both the green turtle and loggerhead nesting populations are independent population units. Hence the first conclusion available from genetic data is that the nesting colonies at X'cacel and adjacent beaches are isolated stocks. If these nesting areas are extirpated, they are not likely to be recolonized over the ecological time scales meaningful to conservation. They will rise and fall largely on their own, and any loss of genetic diversity is likely to be permanent.

For both the green and loggerhead turtles of the Atlantic, the highest levels of genetic diversity were observed at the X'cacel nesting beaches (haplotype diversity h = 0.82 for green turtles; h = 0.65 for loggerheads). In addition, both species exhibited a high degree of haplotype endemism. Four of the eighteen haplotypes observed in Atlantic green turtles were unique to X'cacel and adjacent beaches. Three of the ten haplotypes observed in Atlantic loggerheads were found only at this location. Hence nesting populations of X'cacel contribute substantially to the overall mtDNA diversity of Atlantic green and loggerhead turtles. This observation prompts our second conclusion, that loss of the X'cacel nesting colony would have a strong impact on overall genetic diversity. Extirpation of Quintana Roo green turtles would entail a 22% loss of mtDNA diversity in Atlantic populations, while extirpation of loggerheads from X'cacel would eradicate 30% of mtDNA diversity in Atlantic populations (Figure 1). By any criteria, the nesting populations at X'cacel are major reservoirs of mtDNA diversity. In terms of natural genetic resources, the nesting beaches of Quintana Roo may represent one of the crown jewels of Mexican biodiversity (Zurita et al. 1993).

Atlantic loggerhead turtle

Atlantic green turtle

Figure 1. Haplotype networks for Atlantic loggerhead and green turtles. Haplotypes endemic to the coastal area threatened by development (X'cacel and adjacent beaches), are indicated by skulls. For the loggerhead turtle, these include (clockwise from lower left), haplotypes J, I, and H (Encalada et al. 1998). In the green turtle these include (clockwise from lower left), haplotypes 15-18 (Enclalda et al. 1996).

How would the loss of genetic diversity at Quintana Roo affect the prospects for survival of Atlantic sea turtles? Genetic diversity is widely recognized as the insurance which allows species to cope with diseases, stress, and changing environmental conditions. While the specific consequences of this erosion of genetic diversity are difficult to predict, the extirpation of the most diverse nesting populations in the Atlantic Ocean can only be regarded with alarm.

It is important to note that loss of the mtDNA diversity at X'cacel may not directly influence natural selection, adaptation, and persistence of sea turtle populations. The mitochondrial control region is a noncoding region, and therefore does not contain the genes that are typically considered "the stuff of evolution" (see Lynch 1995). However, overall trends in genetic diversity in the mtDNA control region, especially processes such as erosion of diversity through population reductions, are likely to reflect parallel losses in genetic loci that affect long-term survival and adaptation.

Recent studies based on genetic markers and tag-recapture data have demonstrated that sea turtles, especially loggerhead turtles, travel much further than previously suspected. Adult females tagged at the Quintana Roo nesting beaches have been recovered in the Gulf of Mexico and Caribbean Sea (two females were caught off the coast of Honduras, and four subadults were caught in Cuba; Zurita et al. 1994). Subadult turtles from the Quintana Roo

nesting population have been detected on the east coast of the United States (Rankin-Baransky 1997). Juveniles have been detected in the farthest points influenced by the North Atlantic gyre, including the Azores, Madeiras, and the Mediterranean Sea (Bolten et al. 1998, Laurent et al. 1998). From this evidence, it is clear that the nesting populations of Quintana Roo contribute to marine ecosystems throughout North Atlantic and European waters. When viewed from this perspective, the loss of the Quintana Roo nesting areas could have repercussions across a vast geographic and political scale.

Of course the management of loggerhead and green turtles cannot be solely based on genetic considerations. The biologists that have been working with marine turtles in X'cacel for over a decade have highlighted the importance of this beach for the perpetuation of sea turtle nesting in Mexico, and have demonstrated the ecological importance of the area as a whole. Ecological roles, environmental education, and the needs of local people must remain paramount (Zurita et al. 1993; Prezas 1996; Prezas et al. 1998). To these primary considerations, we add a warning that the loss of nesting habitat at Quintana Roo, Mexico, would entail a substantial loss in Atlantic mtDNA diversity for green and loggerhead turtles. Genetic data also demonstrate that the loss of Quintana Roo nesting colonies would deplete marine ecosystems across thousands of kilometres, making the sale of X'cacel for tourist development a matter of serious international concern. Taken together, the ecological and genetic studies indicate that X'cacel represents a unique source of diversity and thus we urge the incorporation of this beach into long term management plans designed with the ultimate goal of permanent protection.

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