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Understanding the diversity and community structure of arbuscular mycorrhizal fungi (AMF) is important for potentially optimizing their role in the functioning of terrestrial ecosystems. Land use can have profound effects on AMF abundance, productivity, and species composition, but such effects have been explored less in tropical landscapes where agriculture is often less intensive and wildlife conservation can be predominant. Here, we conducted a landscape-scale investigation to quantify the effects of land uses on the AMF diversity in the Greater Serengeti Ecosystem, Tanzania. Abundance, productivity, and diversity of the spores of AM fungal species were determined from field soils by morphological techniques. A total of 315 AM fungal spores belonging 9 species and four families were recorded. Land use had a significant ( P  < 0.05) effect on AM fungal spore density and biomass production of AM hyphae in in-growth mesh bags. The AM fungal spore density was statistically significant ( P  < 0.05) and positively correlated with soil N, organic carbon content and sand contents, but negatively correlated with increasing soil P and pH. After accounting for soil N, Organic carbon, P and pH, wildlife grazed areas had nearly double the spore densities compared to livestock grazed and crop agriculture areas. Likewise, wildlife grazed soils had a higher Shannon-Wiener diversity index (2.13 ± 0.05) than either livestock grazed soils (1.76 ± 0.10) or cultivated soils (1.59 ± 0.09). AM fungal families exhibited significant differences in their abundance among land use types. While Glomeraceae had high abundance under all land uses, Gigasporaceae and Scutellosporaceae exhibited the highest abundance in wildlife grazed sites and Acaulosporaceae had high abundance in livestock-grazed and crop agriculture sites. Species composition within families was mostly similar across land uses, with the exception of Scutellospora sinuosum and Acaulospora denticulata , which exhibited much greater abundance in wildlife grazed sites than in livestock-grazed or crop agriculture sites. These results suggest that protected area grassland sites subject to wildlife grazing held a more abundant, diverse, and compositionally distinct AM fungal community than both livestock-grazed and crop agriculture sites, even after accounting for the influence of land use on soil N, Organic carbon, P and pH that affect AM fungi. From the perspective of sustainable production, it is very important to ascertain tillage practices and moderate grazing that can promote mycorrhizal diversity and, therefore, contribute to the long-term sustainability of agro-pastoral ecosystems under tropical conditions. [ABSTRACT FROM AUTHOR]

Grass roots can harbor abundant endophytic N 2 -fixing microbes (diazotrophs), but their abundance and activity compared to those on legumes and in soil crusts is still unknown. Here, in a natural ecosystem, the Serengeti of East Africa, we explored whether herbivores and soil nutrients limited grass root endophyte diazotroph abundance and their root mass-specific and area-specific N 2 -fixation, as they often do for diazotrophs symbiotic with legumes and those free-living in soil. N 2 -fixation and copy number of the nitrogenase gene nifH was measured with stable isotope and molecular methods, respectively, for the dominant grass Themeda triandra , and legume, Indigofera volkensii , and in the top 5 cm of soil in a 16-year herbivore exclosure experiment across four sites that varied in mean annual rainfall and soil N, P, and moisture. T. triandra nifH gene copy number was highly variable across sites and individuals but often approached or exceeded that of I. volkensii roots and soils. T. triandra roots generally exhibited lower root mass-specific N 2 -fixation (activity), which was not reduced by herbivores and increased in drier soils. In contrast, I. volkensii activity was only reduced by herbivores and soil diazotrophs were mostly inactive. T. triandra exhibited greater area-specific N 2 -fixation than I. volkensii , due to its much greater root biomass, but this difference was reduced by herbivores. Grass-associated endophytic diazotrophs may fix far more N 2 in natural systems than previously realized, and may be limited by different factors those affecting symbiotic legume and free-living soil diazotrophs. [ABSTRACT FROM AUTHOR]

Mycorrhizas influence plant productivity, community composition, nutrient dynamics and soil structure, yet the factors determining the abundance and diversity of these symbioses in natural ecosystems are poorly understood. We studied arbuscular mycorrhizal (AM) fungi (Glomeromycota) in long-term grazed and un-grazed plots at eight sites forming natural gradients of soil properties and rainfall in the Serengeti National Park, Tanzania. Four metrics of AM fungal abundance and diversity were examined: (1) abundance and species composition of spores, (2) standing crop of AM hyphae in the soil, (3) production of new AM hyphae in buried mesh bags and (4) a bioassay of AM fungal inoculum potential. Our first goal was to test the predictions of the Functional Equilibrium Model that AM fungi should be most abundant in grazed plots (because of increased nutrient demands caused by overcompensation) and at sites with the lowest soil phosphorus (P) and rainfall. Our second goal was to use multi-group structural equation models (SEMs) to investigate the interrelated influences of grazing, soil properties, rainfall and root biomass on the abundance and species composition of AM fungal communities. Two multi-group SEMs were generated: one to examine abundance patterns of AM fungi and another to examine patterns in the spores or auxiliary cells of different families of AM fungi. We observed an extremely high abundance and diversity of AM fungal spores. Only one prediction of the Functional Equilibrium Model was supported; production of new hyphae was highest in the lowest P soils. The abundance and species composition of AM fungi varied across sites but the only variable that responded to grazing was the abundance of Glomeraceae spores, with higher abundance in fenced (un-grazed) plots. Multi-group SEMs revealed that rainfall, total soil P, soil organic matter and soil texture were all important predictors, but the relative importance of each factor varied depending upon the AM metric or taxon observed. Our findings suggest niche differentiation among AM fungal taxa and demonstrate that climate and soil properties are important predictors of the abundance and community composition of these important, hidden components in the Serengeti ecosystem. [ABSTRACT FROM AUTHOR]

Abstract: We examined how elevated atmospheric [CO2] and higher rate of nitrogen (N) input may influence grasshopper growth by changing food plant quality and how such effects may be modified by species diversity of the plant community. We reared grasshopper nymphs (Melanoplus femurrubrum) on Poa pratensis from field-grown monocultures or polycultures (16 species) that were subjected to either ambient or elevated levels of CO2 and N. Grasshopper growth rate was higher on P. pratensis leaves grown in monocultures than in polycultures, higher on P. pratensis grown under elevated than under ambient [CO2], and higher on P. pratensis grown under elevated than under ambient [N]. The higher growth rate observed on P. pratensis exposed to elevated [CO2] was, however, less pronounced for polyculture- than monoculture-grown P. pratensis. Growth rate of the grasshoppers was positively correlated with leaf [N], [C], and concentration of soluble carbohydrates+lipids. Concentration of non-structural carbohydrates+lipids was higher in leaves grown under elevated than under ambient [CO2], and the difference between P. pratensis grown under ambient and elevated [CO2] was greater for monoculture- than polyculture-grown P. pratensis. In addition, leaf N concentration was higher in P. pratensis grown in monocultures than in polycultures, suggesting that plant species richness, indirectly, may influence insect performance by changed nutritional value of the plants. Because we found interactive effects between all factors included ([CO2], [N], and plant species diversity), our results suggest that these parameters may influence plant–insect interactions in a complex way that is not predictable from the sum of single factor manipulations. [Copyright &y& Elsevier]

Abstract: The high-altitude rangelands of the Trans-Himalaya represent a grazing ecosystem which has supported an indigenous pastoral community for millennia alongside a diverse assemblage of wild herbivores including burrowing mammals (pikas and voles). Pastoralists consider the small mammals to cause rangeland degradation and as competitors for their livestock, and actively eradicate them at many places. We present data on the ways in which small herbivores like pikas and voles mediate plant community dynamics. Vegetation cover and plant species richness were compared on and off both active and abandoned small mammal colonies. Plant species richness was higher inside colonies (about 4–5 species/plot) than outside (about 3 species/plot) whereas vegetation cover was only marginally lower (52% compared to 60%). Soil disturbance due to small mammals is seen to be associated with higher plant diversity without causing dramatic decline in overall vegetation cover. Such disturbance-mediated dynamics and vegetation mosaics produce a rich array of testable hypotheses that can highlight how small mammals influence assembly processes, succession, and dominance hierarchies in plant communities in this arid ecosystem. So, eradicating small mammals may lead to declining levels of diversity in this ecosystem, and compromise ecosystem-functioning. Changes in traditional pastoral practices and overstocking are more likely to be responsible for degradation. We emphasize that eradicating small mammals can lead to loss of diversity in this ecosystem and it is not a solution for the degradation problems. [Copyright &y& Elsevier]

Abstract: A fully factorial pond experiment was designed using two irradiance levels and two phosphorus concentrations to investigate irradiance and phosphorus effects on the growth of three submerged macrophytes: common waterweed (Elodea canadensis), Eurasian water milfoil (Myriophyllum spicatum), and water stargrass (Zosterella dubia). Results revealed that higher irradiance (230μmols−1 m−2 vs. 113μmols−1 m−2 at 2m depth) had significant positive effects on submerged macrophyte growth: increasing the number of individuals (seven-fold), the number of species surviving (two-fold), aboveground biomass (11-fold), belowground biomass (10-fold), and total biomass (11-fold), whereas elevated sediment phosphorus (2.1–3.3mgg−1 vs. 0.7mgg−1 dry sediment) did not have any significant impact. However, responses to irradiance differ among macrophyte species due to their morphology and physiology. Waterweed increased in numbers of individuals and total biomass under high irradiance while biomass per individual remained the same (∼0.02g). The other species increased both in numbers and biomass per individual. These results suggest that increased irradiance rather than decreased phosphorus loading is the main driver of changes in submerged macrophytes in North American temperate lake ecosystems. [Copyright &y& Elsevier]