Should I Stay or Should I Go? Dispersal and Population Structure in Small, Isolated Desert Populations of West African Crocodiles

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.

Fine scale patterns of genetic partitioning in the rediscovered African crocodile,Crocodylus suchus(Saint-Hilaire 1807)

“…Recent analyses have clearly shown that this taxonomy is erroneous and that this taxon is comprised of two distinct species, Crocodylus niloticus and the newly rediscovered species Crocodylus suchus ( Hekkala et al, 2011 ; Oaks, 2011 ; Shirley et al, 2015 ). Fine-scale analyses of genetic partitioning within C. niloticus ( Hekkala et al, 2010 ), a previous localized study of C. suchus ( Velo-Antón et al, 2014 ), and our current analysis indicate considerably greater sub-structuring than previously recognized in African Crocodylus . Using Bayesian clustering of microsatellite markers, we identified a maximum of eight genetic clusters within the sampled C. suchus distribution.…”

“…Our study presents the first broad scale analysis of molecular variation within the newly resurrected species Crocodylus suchus ( Saint-Hilaire, 1807 ) and supports evidence for extensive landscape level genetic partitioning across western Africa. Despite known instances of long distance dispersal, genetic analyses for several species now indicate that crocodiles exhibit strong evidence for philopatry and restricted gene flow ( Hekkala et al, 2010 ; Hekkala et al, 2015 ; Shirley et al, 2014 ; Velo-Antón et al, 2014 ). This previously unrecognized level of genetic structuring raises doubts about crocodilian dispersal and recolonization potential in the face of ongoing exploitation, habitat loss and climate change.…”

Second Sahelian amphibian endemism suggested by phylogeography of Groove crowned Bullfrog ( Hoplobatrachus occipitalis ) in western Sahel and hints of polyploid species formation

“…The Djouk valley between Assaba and Tagant seems to be of particular importance for species persistence during arid phases, since it is where “highland” diversity is highest, but also considering similar evidence found in lizards (Gonçalves et al, ) and crocodiles (Velo‐Antón, Godinho, Campos, & Brito, ). As for the “lowland” lineage, the fact that only one mitochondrial allele is found in most of Mauritania (including Afollé and Adrar‐Atar) is consistent with a recent expansion, but the fact that all nuclear POMC alleles in Adrar‐Atar are private also suggests a refugium there.…”

“…The Djouk valley between Assaba and Tagant seems to be of particular importance for species persistence during arid phases, since it is where "highland" diversity is highest, but also considering similar evidence found in lizards (Gonçalves et al, 2018) and crocodiles (Velo-Antón, Godinho, Campos, & Brito, 2014). As for the "lowland" lineage, the fact that only one mitochondrial allele is found in most (Spieler & Linsenmair, 1998), and rapid development allowed by tadpole carnivory (Grosjean, Vences, & Dubois, 2004).…”

Differentiation of North African foxes and population genetic dynamics in the desert—insights into the evolutionary history of two sister taxa, Vulpes rueppellii and Vulpes vulpes

“…Presently, the Sahara desert may be a strong dispersal barrier, but since the onset of arid conditions, around 7-2.5 Mya (Schuster et al 2006;Swezey 2009), the Sahara went through a series of humid periods, harbouring mesic environments and wetlands that most likely allowed range expansions by more water-dependent species Drake et al 2011;Kröpelin et al 2008;Le Houérou 1997). During periods of increased aridity, those species would suffer population contractions and become isolated in refugial areas, such as mountains and oases Velo-Antón et al 2014).…”