Modeling rendille household herd composition

Previous analysis of Rendille household herd composition revealed a transition from camel to cattle ownership for sedentary impoverished Rendille pastoralists of northern Kenya. In an attempt to delineate determinants of livestock holdings, logistic regression analysis of 112 household herds from the Rendille settlement of Korr, Marsabit District, Kenya was undertaken. Results indicated that household wealth, measured in present livestock holdings, past drought losses, and livestock sales, formed better predictors of cattle ownership than did household characteristics pertaining to labor supply, wage earners, age-set membership, and birth order of household head. These results are discussed in light of pastoral strategies designed to minimize risk.     

Drought and economic differentiation among Ariaal pastoralists of Kenya

This paper examines the effects of the 1984 drought upon household wealth differences in a community of Ariaal pastoralists of northern Kenya. The database consists of 1985 post-drought livestock counts and informants' statements of species-specific drought loss, compared to 1976 livestock counts on the same 38 households. The analysis confirms the hypothesis that the drought resulted in increased household wealth inequalities. It is suggested that the combination of differential herd growth, differential participation in the cash market, and differential loss to the drought has contributed to a polarization within Ariaal of rich and poor, resulting in rural proletarianization and urban migration.     

Comparative study of two meromictic basins of Lake Banyoles (Spain) with sulphur phototrophic bacteria

The annual limnological dynamics of two meromictic basins of Lake Banyoles (C-III and C-IV) have been studied and compared on the basis of their physical, chemical and biological characters. Stability values calculated for both basins gave 865 g cm cm⁻² and 495 g cm cm⁻² for C-III and C-IV respectively. These values are in agreement with the fact that C-IV was almost completely mixed during winter. In this basin, during stratification, the monimolimnion increased in thickness as the stability increased. Isolation of the respective monimolimnia resulted in the development of anoxic conditions and the accumulation of sulphide in both C-III and C-IV, which favoured the development of dense populations of sulfur phototrophic bacteria. The purple sulphur bacterium Chromatium minus and the green sulphur bacterium Chlorobium phaeobacteroides were identified as the main components of these photosynthetic populations. The different depths at which the O₂/H₂S boundary was situated in both basins (and consequently the different light intensity reaching this zone) determined the growth of these bacteria. Light intensities at the chemocline of C-IV reached values up to 5% of surface incident light. In contrast, in C-III this variable was sensibly lower, with values depending on season and seldom reaching 1%. Phototrophic bacteria were consequently found earlier in C-IV than in C-III, where no significant concentrations were found until August. Finally stability is discussed as an important factor controlling chemical and biological dynamics in meromictic lakes.     

A major difference between the divergence patterns within the lines-1 families in mice and voles

L1 retroposons are represented in mice by subfamilies of interspersed sequences of varied abundance. Previous analyses have indicated that subfamilies are generated by duplicative transposition of a small number of members of the L1 family, the progeny of which then become a major component of the murine L1 population, and are not due to any active processes generating homology within preexisting groups of elements in a particular species. In mice, more than a third of the L1 elements belong to a clade that became active approximately 5 Mya and whose elements are > or = 95% identical. We have collected sequence information from 13 L1 elements isolated from two species of voles (Rodentia: Microtinae: Microtus and Arvicola) and have found that divergence within the vole L1 population is quite different from that in mice, in that there is no abundant subfamily of homologous elements. Individual L1 elements from voles are very divergent from one another and belong to a clade that began a period of elevated duplicative transposition approximately 13 Mya. Sequence analyses of portions of these divergent L1 elements (approximately 250 bp each) gave no evidence for concerted evolution having acted on the vole L1 elements since the split of the two vole lineages approximately 3.5 Mya; that is, the observed interspecific divergence (6.7%-24.7%) is not larger than the intraspecific divergence (7.9%-27.2%), and phylogenetic analyses showed no clustering into Arvicola and Microtus clades.      

Molecular phylogeny and divergence times of drosophilid species

The phylogenetic relationships and divergence times of 39 drosophilid species were studied by using the coding region of the Adh gene. Four genera--Scaptodrosophila, Zaprionus, Drosophila, and Scaptomyza (from Hawaii)--and three Drosophila subgenera--Drosophila, Engiscaptomyza, and Sophophora--were included. After conducting statistical analyses of the nucleotide sequences of the Adh, Adhr (Adh-related gene), and nuclear rRNA genes and a 905-bp segment of mitochondrial DNA, we used Scaptodrosophila as the outgroup. The phylogenetic tree obtained showed that the first major division of drosophilid species occurs between subgenus Sophophora (genus Drosophila) and the group including subgenera Drosophila and Engiscaptomyza plus the genera Zaprionus and Scaptomyza. Subgenus Sophophora is then divided into D. willistoni and the clade of D. obscura and D. melanogaster species groups. In the other major drosophilid group, Zaprionus first separates from the other species, and then D. immigrans leaves the remaining group of species. This remaining group then splits into the D. repleta group and the Hawaiian drosophilid cluster (Hawaiian Drosophila, Engiscaptomyza, and Scaptomyza). Engiscaptomyza and Scaptomyza are tightly clustered. Each of the D. repleta, D. obscura, and D. melanogaster groups is monophyletic. The splitting of subgenera Drosophila and Sophophora apparently occurred about 40 Mya, whereas the D. repleta group and the Hawaiian drosophilid cluster separated about 32 Mya. By contrast, the splitting of Engiscaptomyza and Scaptomyza occurred only about 11 Mya, suggesting that Scaptomyza experienced a rapid morphological evolution. The D. obscura and D. melanogaster groups apparently diverged about 25 Mya. Many of the D. repleta group species studied here have two functional Adh genes (Adh-1 and Adh-2), and these duplicated genes can be explained by two duplication events.      

Sulfate reduction and S-oxidation in a moorland pool sediment

In an oligotrophic moorland pool in The Netherlands, S cycling near the sediment/water boundary was investigated by measuring (1) SO₄ ^2– reduction rates in the sediment, (2) depletion of SO₄ ^2– in the overlying water column and (3) release of³⁵S from the sediment into the water column. Two locations differing in sediment type (highly organic and sandy) were compared, with respect to reduction rates and depletion of SO₄ ^2– in the overlying water.Sulfate reduction rates in sediments of an oligotrophic moorland pool were estimated by diagenetic modelling and whole core³⁵SO₄ ^2– injection. Rates of SO₄ ^2– consumption in the overlying water were estimated by changes in SO₄ ^2– concentration over time in in situ enclosures. Reduction rates ranged from 0.27–11.2 mmol m^–2 d^–1. Rates of SO₄ ^2– uptake from the enclosed water column varied from –0.5, –0.3 mmol m^–2 d^–1 (November) to 0.43–1.81 mmol m^–2 d^–1 (July, August and April). Maximum rates of oxidation to SO₄ ^2– in July 1990 estimated by combination of SO₄ ^2– reduction rates and rates of in situ SO₄ ^2– uptake in the enclosed water column were 10.3 and 10.5 mmol m^–2 d^–1 at an organic rich and at a sandy site respectively.Experiments with³⁵S^2– and³⁵SO₄ ^2– tracer suggested (1) a rapid formation of organically bound S from dissimilatory reduced SO₄ ^2– and (2) the presence of mainly non SO₄ ^2–-S derived from reduced S transported from the sediment into the overlying water. A³⁵S^2– tracer experiment showed that about 7% of³⁵S^2– injected at 1 cm depth in a sediment core was recovered in the overlying water column.Sulfate reduction rates in sediments with higher volumetric mass fraction of organic matter did not significantly differ from those in sediments with a lower mass fraction of organic matter.Corresponding author     

Ant versus bird exclusion effects on the arthropod assemblage of an organic citrus grove

1. Predation‐exclusion experiments have highlighted that top‐down control is pervasive in terrestrial communities, but most of these experiments are simplistic in that they only excluded a single group of predators and the effect of removal was evaluated on a few species from the community. The main goal of our study was to experimentally establish the relative effects of ants and birds on the same arthropod assemblage of canopy trees. 2. We conducted 1‐year long manipulative experiments in an organic citrus grove intended to quantify the independent effects of bird and ant predators on the abundance of arthropods. Birds were excluded with plastic nets whereas ants were excluded with sticky barriers on the trunks. The sticky barrier also excluded other ground dwelling insects, like the European earwig Forficula auricularia L. 3. Both the exclusion of ants and birds affected the arthropod community of the citrus canopies, but the exclusion of ants was far more important than the exclusion of birds. Indeed, almost all groups of arthropods had higher abundance in ant‐excluded than in control trees, whereas only dermapterans were more abundant in bird‐excluded than in control trees. A more detailed analysis conducted on spiders also showed that the effect of ant exclusion was limited to a few families rather than being widespread over the entire diverse spectrum of spiders. 4. Our results suggest that the relative importance of vertebrate and invertebrate predators in regulating arthropod populations largely depends on the nature of the predator–prey system.     

Unexpected side effects in biocrust after treating non‐native plants using carbon addition

Carbon addition has been proposed as an alternative to herbicide and manual removal methods to treat non‐native plants and reduce non‐target effects of treatments (e.g. impacts on native plants; surface disturbance). On Mojave Desert pavement and biocrust substrates after experimental soil disturbance and carbon addition (1,263 g C/m² as sucrose), we observed declines in lichens and moss cover in sucrose‐treated plots. To further explore this unforeseen potential side effect of using carbon addition as a non‐native plant treatment, we conducted biocrust surveys 5 and 7 years after treatments, sampled surface soils to observe if treatments additionally affected soil filamentous cyanobacteria, and conducted laboratory trials testing the effects of different levels of sucrose on cyanobacteria and desert mosses. Sucrose addition to biocrust plots reduced lichen and moss cover by 33–78% and species richness by 40–80%. Sucrose reduced biocrust cover in biocrust plots to levels similarly detected in pavement plots (<1%). While cyanobacteria in the field did not appear to be affected by sucrose, laboratory tests showed negative effects of sucrose on both cyanobacteria and mosses. Cyanobacteria declined by 41% 1 month after exposure to 5.4 g C/m² equivalent solutions. We detected injury to photosynthesis in mosses after 96 hour exposure to 79–316 g C/m² equivalent solutions. Caution is warranted when using carbon addition, at least in the form and concentration of sucrose, as a treatment for reducing non‐native plants on sites where conserving biocrust is a goal.     

Characterizing soil seed banks and relationships to plant communities

Estimates of soil seed banks are important to many ecological investigations and plant conservation, yet seed banks are among the most difficult plant community attributes to accurately quantify. To compare extraction and emergence seed bank characterization methods, we collected 0- to 5-cm soil seed bank samples and measured plant community composition in six microsite types (below different perennial plant species and interspaces) at 10 field sites in the Mojave Desert, USA. Extraction detected five times more species sample^?1 and orders of magnitude greater seed density than emergence, though evaluating viability of extracted seed was not straightforward. Only 13 % of 847 tested seeds from extraction emerged in follow-up assays. Considering all sites, species detection was more similar between methods: 21 taxa for emergence and 28 for extraction. Results suggest that: (i) capturing microsite variation is critical for efficiently estimating site-level desert seed banks; (ii) method comparisons hinged on the scale of analysis for species richness, as differences in species detection between methods diminished when increasing resolution from the sample to the regional scale; (iii) combining data from all seed bank methods provided the strongest correlation with vegetation; and (iv) improving knowledge of seed germinability is important for advancing both seed bank methods, including for extraction to evaluate the proportion of extracted seeds that are viable. Multifactor approaches that balance several effectiveness measures (e.g., both seed density and species detection at multiple scales) and procedural challenges are most likely to accurately represent complexity in tradeoffs for choosing methods to quantify soil seed banks.