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Amphibian diversity, distribution and conservation in the Ethiopian highlands : morphological, molecular and biogeographic investigation on Leptopelis and Ptychadena (Anura)

Mengistu, Abebe Ameha. Amphibian diversity, distribution and conservation in the Ethiopian highlands : morphological, molecular and biogeographic investigation on Leptopelis and Ptychadena (Anura). 2012, PhD Thesis, University of Basel, Faculty of Science.

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Official URL: http://edoc.unibas.ch/diss/DissB_10172

Abstract

Little is known about the diversity, distribution and population status of Ethiopian amphibians in general, and most of the existing knowledge is based on field data recorded about three decades ago or earlier. There are almost no genetic data available for molecular systematics studies. Species of the Tree Frogs (Leptopelis Günther 1859) and Grassland Frogs (Ptychadena Boulenger 1917) are solely distributed in Africa (mainly sub-Saharan), with 52 and 53 known species, respectively. Six species of Leptopelis and thirteen species of Ptychadena were recorded in Ethiopia. Together these two genera comprise 30% of all known species of amphibians in Ethiopia. Many of these species (five Leptopelis and five Ptychadena) are said to be endemic to the Ethiopian Highlands that are part of the Eastern Afromontane Biodiversity Hotspot. Geographically, the Ethiopian Highlands are bisected by the Rift Valley, and further fragmented by valleys and gorges, and comprise the highest concentration of elevated ground in Africa. Earlier taxonomic studies were mainly based on morphological and ecological studies, making it difficult to identify most species in museum collections or during field work. In addition, limited sampling from the geographic areas of each species made it difficult to make good estimates of the ranges of species, and to prioritize them for conservation.
The main objectives of this study are, using Leptopelis and Ptychadena as model taxa, 1) to explore the diversity and phylogeography of some amphibians in the Ethiopian Highlands and parts of the Rift Valley; 2) to preliminarily explain the geo-climatic events that are associated with the evolutionary history of these taxa; and 3) to re-evaluate the conservation status of species and their habitats by associating phylogenetic, biogeographic and ecological information. We tested hypotheses pertaining to correspondence of traditional morphological taxonomy versus molecular phylogeny, biogeographic distinctness of distribution patterns, evolutionary history of diversification, and evaluation of conservation status of species and populations in the highlands.
The field sampling conducted between 2006 and 2010 from several localities across and within the Ethiopian Rift Valley was substantial, but not enough, to fully assess the phylogeography of the above genera. This assessment, conducted for the first time for Ethiopian Leptopelis and Ptychadena, was done using partial mitochondrial DNA sequences of the 12S and 16S genes. Estimation of phylogenetic relationships and divergence times was made using Maximum Likelihood and Maximum Parsimony methods. Morphological observations were made on fresh collections, holotypes and non-type museum materials. Biogeographic patterns were assessed using geographic distribution data. Geographic and spatial data were combined with new phylogenetic groupings to assess geographic ranges and habitat status of populations and species. We followed a new approach of ‘elevation-based’ extent of occurrence (instead of the simple polygons applied in the IUCN Red List maps) to estimate geographic ranges. Results were compared and evaluated with previous reports and online database.
Despite the wide taxonomic variation between the studied genera (Leptopelis and Ptychaena), we found similarities in some of their major taxonomic problems, as well as evolutionary, biogeographic and conservation aspects. The similarities are in: overlaps in many morphological characters (conservatism) among different species; misleading variation in some physical features within a population/species (homoplasy); monophyly of highland endemic species; cryptic phylo-groups embedded within known clades; relatively low genetic distance between species and recent evolutionary divergence times; habitat status and threats for survival of populations.
In both genera, the reconstructed phylogenetic relationships showed that the highland endemic species form well-supported monophyletic groups: the ‘Ethiopian Highland Leptopelis species group’, and the ‘Ptychadena cooperi species group’. Cryptic diversity of highland endemics was revealed, highlighting the possibility of having six candidate species (at least two Leptopelis and four Ptychadena) awaiting description. Two presumed lowland endemic species of Ptychadena (P. filwoha and P. harenna) were genetically found to be conspecific with other widely distributed (non-endemic) lowland species (P. mascareniensis and P. anchietae, respectively).
Wide overlaps in several morphological features made identification of some closely related species populations difficult. New diagnostic features were identified to characterize some of the most difficult groups (e.g., presence of spicules in males of P. neumanni contrasted with absence in P. erlangeri or P. nana).
As compared with some other African species for which molecular data are available, relatively small evolutionary distances were found among the highland species within each genus, explained by a possibly very recent radiation as estimated in the corresponding divergence times. Diversification of the highland endemics was probably associated with formation of the Ethiopian Highlands by volcanic activities and uplifting, and accompanying climatic changes between 30 and 6 Mya.
We observed congruence of phylogenetic groups with clear patterns of geographic distribution, allowing us to identify distinct biogeographic categories that can potentially serve as units for conservation of Ethiopian amphibians. The Rift Valley and major river gorges appear important horizontal barriers delimiting geographic ranges of most species. There is no considerable vertical (altitudinal) segregation of the highland populations in the two genera.
The highland Leptopelis and Ptychadena comprise more diverse endemic species (16) than known before (10), contributing substantially to the biodiversity value of the Ethiopian Highlands. While range extension is revealed for some species in the current study, there is unfortunately a substantial reduction for others from what has been proposed previously. Land cover data and our field observation of many parts of the country for about two decades indicate that large parts of the suitable highland habitats for most species of Leptopelis and Ptychadena are severely degraded and fragmented by human activities. Lakes are drying, levels of rivers are decreasing, and the wooded and grassland vegetation of the highlands and the Rift Valley is being lost. Our revised assessment of populations showed that nine of the 16 studied species appear to be threatened at different levels. This implies an urgent need for revision of existing conservation status of these taxa to protect them in rapidly changing environments. Further work is needed in the areas of phylogeography, taxonomy, and natural history of populations and species in areas that are not covered in this study.
Advisors:Nagel, Peter
Committee Members:Loader, Simon and Gower, David
Faculties and Departments:05 Faculty of Science > Departement Umweltwissenschaften > Institut für Natur- Landschafts- und Umweltschutz > Biogeographie (Nagel)
Item Type:Thesis
Thesis no:10172
Bibsysno:Link to catalogue
Number of Pages:204 Bl.
Language:English
Identification Number:
Last Modified:30 Jun 2016 10:51
Deposited On:19 Nov 2012 14:23

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