Abstract
The maintenance of both spatial and genetic connectivity is paramount to the long-term persistence of small, isolated populations living in environments with extreme climates. We aim to identify the distribution of genetic diversity and assess population sub-structuring and dispersal across dwarfed desert populations of Crocodylus suchus, which occur in isolated groups, usually less than five individuals, along the mountains of Mauritania (West Africa). We used both invasive and non-invasive sampling methods and a combination of mitochondrial DNA (12 S and ND4) and microsatellite markers (32 loci and a subset of 12 loci). Our results showed high genetic differentiation and geographic structure in Mauritanian populations of C. suchus. We identified a metapopulation system acting within four river sub-basins (high gene flow and absence of genetic structure) and considerable genetic differentiation between sub-basins (F ST range: 0.12–0.24) with rare dispersal events. Effective population sizes tend to be low within sub-basins while genetic diversity is maintained. Our study suggests that hydrographic networks (temporal connections along seasonal rivers during rainy periods) allow C. suchus to disperse and maintain metapopulation dynamics within sub-basins, which attenuate the loss of genetic diversity and the risk of extinction. We highlight the need of hydrographic conservation to protect vulnerable crocodiles isolated in small water bodies. We propose C. suchus as an umbrella species in Mauritania based on ecological affinities shared with other water-dependent species in desert environments.
Highlights
Habitat connectivity is important for populations living in climate-extreme environments, which are naturally patchy and have temporally variable resources [1]
The highest values of genetic diversity were observed in Gorgol el AkhdarGarfa for most of the nuclear genetic diversity measures (A–R, Ho and He; Table 1) and in the Karakoro-Kolimbinesub-basin for the mean number of alleles per locus and number of private alleles (Table 1)
Relict populations or individuals of C. niloticus present in West Africa cannot be excluded since the sample sizes and geographic areas surveyed are still limited [8,21]
Summary
Habitat connectivity is important for populations living in climate-extreme environments, which are naturally patchy and have temporally variable resources [1]. The migration movements of such species are usually constrained by extensive barriers, they disperse mostly through a limited number of natural corridors such as hydrographic basins These basins represent important passageways for the dispersal of numerous relictual aquatic species characterized by small and fragmented distributions [3]. The current status of relict Crocodylus suchus [6] populations across the SaharaSahel exemplifies the threats associated with loss of landscape connectivity for aquatic vertebrates with low population densities and restricted dispersal abilities [5]. This species was historically widespread over North Africa but was extirpated throughout much of its original distribution. Anthropogenic pressures in the first half of the last century left only a few relict populations in Chad and Mauritania [8,9]
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