Date Published: February 25, 2011
Publisher: Public Library of Science
Author(s): José C. Brito, Fernando Martínez-Freiría, Pablo Sierra, Neftalí Sillero, Pedro Tarroso, Brock Fenton. http://doi.org/10.1371/journal.pone.0014734
Abstract: Relict populations of Crocodylus niloticus persist in Chad, Egypt and Mauritania. Although crocodiles were widespread throughout the Sahara until the early 20th century, increased aridity combined with human persecution led to local extinction. Knowledge on distribution, occupied habitats, population size and prey availability is scarce in most populations. This study evaluates the status of Saharan crocodiles and provides new data for Mauritania to assist conservation planning.
A series of surveys in Mauritania detected crocodile presence in 78 localities dispersed across 10 river basins and most tended to be isolated within river basins. Permanent gueltas and seasonal tâmoûrts were the most common occupied habitats. Crocodile encounters ranged from one to more than 20 individuals, but in most localities less than five crocodiles were observed. Larger numbers were observed after the rainy season and during night sampling. Crocodiles were found dead in between water points along dry river-beds suggesting the occurrence of dispersal.
Research priorities in Chad and Egypt should focus on quantifying population size and pressures exerted on habitats. The present study increased in by 35% the number of known crocodile localities in Mauritania. Gueltas are crucial for the persistence of mountain populations. Oscillations in water availability throughout the year and the small dimensions of gueltas affect biological traits, including activity and body size. Studies are needed to understand adaptation traits of desert populations. Molecular analyses are needed to quantify genetic variability, population sub-structuring and effective population size, and detect the occurrence of gene flow. Monitoring is needed to detect demographical and genetical trends in completely isolated populations. Crocodiles are apparently vulnerable during dispersal events. Awareness campaigns focusing on the vulnerability and relict value of crocodiles should be implemented. Classification of Mauritanian mountains as protected areas should be prioritised.
Partial Text: The Sahara is the largest desert in the world and it is characterised by the occurrence of vast dune fields and featureless plains subjected to low precipitation levels and high temperature ranges . However, this apparently bare ecosystem has not always been like this. Since the onset of the Sahara, at about 7 M.Y.A , its range has largely fluctuated following closely periodical climatic oscillations. Several alternated phases of dry and humid climates have occurred allowing the expansion and contraction of the desert areas, respectively, through range shifts of the hyper-arid sand seas and featureless plains . At the Last Glacial Maximum (LGM, 18,000 yr), the Sahara was much larger and warmer than today, but during the mid-Holocene (7,000 yr) it was almost absent due to the higher levels of temperature and rainfall in comparison with the present day , . During this last humid phase, the arid plains and sand seas were replaced by lakes, grasslands and open savannas in many low altitude sites, and temperate xerophytic woods and warm mixed forests covered mountains –.
Ethics statement: Fieldwork in Mauritania developed with permission from the Ministère Délégué auprès du Premier Ministre, Chargé de l’Environment. Parc National du Banc d’Arguin, Nouakchott (Permit: 460/MDE/PNBA). There are no animal husbandry, experimentation and care/welfare concerns.
Populations of the Nile crocodile in the Sahara are currently known from three countries, Chad, Egypt and Mauritania (Figure 1). An appreciation on the status of populations and conservation issues affecting habitats are given below. Summary data on Saharan localities (excluding extant populations in Mauritania) are given in Table S1.
Nile crocodiles occur in the Sahara desert in fragmented populations throughout several mountains. Although the mechanisms explaining the presence of crocodiles in the Sahara are well understood (e.g.), in reality there is paucity of knowledge about distribution, demography, ecology, and conservation status of populations. Research priorities in Chad and Egypt should focus on studies quantifying population size and pressures exerted on habitats in the population present in guelta Archei and Lake Nasser (e.g.). Field surveys are also needed in the Tibesti where the presence of crocodiles is uncertain . The remoteness and isolation character of these mountains might have assured the persistence of crocodiles. Fine-scaled remote sensing techniques might be applied prior to fieldwork in order to identify suitable water localities for the occurrence of crocodiles –.