101,942 research outputs found
Spatial and temporal variation in crop diversity in agroforestry homegardens of southern Ethiopia
No full textA key assumption in many homegarden studies is that homegardens are ecologically and socio-economically sustainable due to their species diversity. The precise relation between diversity and sustainability is still heavily debated, however. A basic question is how diversity in homegardens can best be characterized in view of the various dimensions of species diversity and their variation in time and space. This paper assesses different types of species diversity in the homegardens of Sidama region of southern Ethiopia. In a survey of crop species in 144 homegardens a total of 78 cultivated crop species (excluding trees) belonging to 10 functional groups were recorded; there were on average 16 crop species and 8 functional groups per farm. Within homegardens, plots differ in species composition and crop diversity. Four types of homegarden systems are distinguished differing in both type and area-share of dominant species, relative orientation at subsistence or cash production and overall crop diversity. The gradual replacement of enset by maize and of coffee by more financially attractive cash crops khat and pineapple causes a decrease in overall crop diversity. Our data demonstrate that it is incorrect to consider homegardens as generic systems with a uniform distribution of species diversity: important within and between homegarden variation exists. Ecological and socio-economic sustainability is not just related to species diversity per se, but rather to more specific features such as presence of keystone species and diversity in functional species groups. Socio-economic sustainability in terms of adjustment to socio-economic change implies dynamics in species diversit
Amphibian diversity, distribution and conservation in the Ethiopian highlands : morphological, molecular and biogeographic investigation on Leptopelis and Ptychadena (Anura)
No full textLittle 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
Holocene opening directions along the axes of Red Sea (Afar) and Main Ethiopian rifts: an overview
No full textThe role of pre-existing structures in the origin, propagation and architecture of faults in the Main Ethiopian rift
No full textOpinion Dynamics Optimization by Varying Susceptibility to Persuasion via Non-Convex Local Search
No full textA long line of work in social psychology has studied variations in people's susceptibility to persuasion - the extent to which they are willing to modify their opinions on a topic. This body of literature suggests an interesting perspective on theoretical models of opinion formation by interacting parties in a network: in addition to considering interventions that directly modify people's intrinsic opinions, it is also natural to consider interventions that modify people's susceptibility to persuasion.In this work, motivated by this fact, we propose an influence optimization problem. Specifically, we adopt a popular model for social opinion dynamics, where each agent has some fixed innate opinion, and a resistance that measures the importance it places on its innate opinion; agents influence one another's opinions through an iterative process. Under certain conditions, this iterative process converges to some equilibrium opinion vector. For the unbudgeted variant of the problem, the goal is to modify the resistance of any number of agents (within some given range) such that the sum of the equilibrium opinions is minimized; for the budgeted variant, in addition the algorithm is given upfront a restriction on the number of agents whose resistance may be modified.We prove that the objective function is in general non-convex. Hence, formulating the problem as a convex program as in an early version of this work (Abebe et al., KDD'18) might have potential correctness issues. We instead analyze the structure of the objective function, and show that any local optimum is also a global optimum, which is somehow surprising as the objective function might not be convex. Furthermore, we combine the iterative process and the local search paradigm to design very efficient algorithms that can solve the unbudgeted variant of the problem optimally on large-scale graphs containing millions of nodes. Finally, we propose and evaluate experimentally a family of heuristics for the budgeted variant of the problem. © 2021 held by the owner/author(s). Publication rights licensed to ACM
Feeding ecology of free-living benthic nematodes
No full textMoens T, Bergtold M, Traunspurger W. Feeding ecology of free-living benthic nematodes. In: Abebe E, Andrassy I, Traunspurger W, eds. Freshwater Nematodes: Ecology and taxonomy. Wallingford: CAB International; 2006: 752
Letter, [Author unclear] to Paulina T. Merritt
No full textHandwritten letter to Paulina Merritt from an unknown author, October 1, 1876.
Quaternary faulting and volcanism in the main Ethiopian Rift
No full textThe Main Ethiopian Rift (MER) is associated with bimodal Quaternary magmatism. Field, remote sensing, and geochronology data are used to examine the relationships between axial acidic volcanoes and basaltic eruptions. Two main Quaternary magmatic episodes are recognizeable in MER: a) basaltic flows followed by ignimbrites and silicic centers in the rift floor (2-1 Ma) and b) axial silicic volcanoes and basalts since ~650 Ka. The first episode consists mainly of basaltic flows related to the Afar Stratoid and outcrops in the central and northern MER. Scattered silicic centers developed subsequently along the rift floor. In the second episode, spatial and temporal correlation between rift localization and silicic centers becomes more evident. The silicic centers are located at the intersection of the WFB with earlier structures, especially E-W faults. With ageing, these centers become faulted and allow basalts to erupt right through the volcanic edifice, suggesting a decrease in the amount of differentiation in the magma chambers, possibly due to their cooling. This style of evolution appears to be characteristic of continental rifts prior to the onset of drifting
Structure of Tendaho Graben and Manda Hararo Rift: implications for the evolution of the Red Sea propagator in Central Afar
No full textThe Red Sea and Aden rifts (or propagators) meet in Afar. Here we use remote sensing and field analyses to define the geology and structure of the southern part of the Red Sea propagator, in Central Afar. This consists of the NW-SE trending Tendaho Graben (TG) and the younger and active NW-SE trending Manda Hararo Rift (MHR), partly within TG. Tectonic and volcanic activity within TG developed mostly between ~1.8 to ~0.6 Ma, with a stretching factor ~1.1, an extension rate ~3.6 mm/yr and the fissural eruption of part (~7000 km3/Ma) of the Afar Stratoid sequence (mainly basaltic lava flows and ignimbrites). MHR, before terminating southward, has a ~1.04 and extension rate ~1.2 mm/yr, and is associated with the emission of ~600 km3/Ma of basalts in the last ~0.2 Ma. These data suggest that, after the exceptional amount of magma erupted between ~1.8 to ~0.6 Ma, magmatic and tectonic activity significantly decreased along the southern part of the Red Sea propagator in the last ~0.2 Ma. This decrease coincides with the on-land development and migration of the more active (inferred extension rate in the order of ~10 mm/yr, as proposed in previous studies) Aden propagator, suggesting that spreading in Central Afar mainly occurred along one active propagator at any one time
The Chaplygin gas as a model for modified teleparallel gravity?
No full textThis paper explores the possibility of treating the exotic Chaplygin-gas (CG) fluid model as some manifestation of an f(T) gravitation. To this end, we use the different cosmological CG equations of state, compare them with the equation of state for the modified teleparallel gravity and reconstruct the corresponding Lagrangian densities. We then explicitly derive the equation of state parameter of the torsion fluid and study its evolution for vacuum-torsion, radiation-torsion, dust-torsion, stiff fluid-torsion and radiation-dust-torsion multi-fluid systems. The obtained Lagrangians have, in general, matter dependence due to the matter-torsion coupling appearing in the energy density and pressure terms of the modified teleparallel gravity theory. For the simplest CG models, however, it is possibly to reconstruct f(T) Lagrangians that depend explicitly on the torsion scalar T only. The preliminary results show that, in addition to providing Chaplygin-gas-like solutions to the modified teleparallel gravitation, which naturally behave like dark matter and dark energy at early and late times respectively, the technique can be used to overcome some of the challenges attributed to the CG cosmological alternativ
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