Grazing effects on patchy dryland vegetation in northern Patagonia   总被引：1，自引：0，他引：1  

Alejandro J Bisigato  Mónica B Bertiller 《Journal of Arid Environments》1997,36(4):639-653

In this study the spatial patterns and dynamics of vegetation patches along a grazing gradient in the steppe ofLarrea divaricataandStipaspp. in NE Patagonia (Argentina) are described. A general effect of grazing is the reduction of total plant cover resulting from the decrease in cover of perennial grasses (Stipa speciosa, Poa ligularis, Stipa tenuis) and some tall shrubs (Chuquiraga hystrix, Bougainvillea spinosa, Lycium chilense). Dwarf shrubs (Nassauvia fueguianaandJunellia seriphioides) increase their cover under medium and/or high grazing pressures. Plant species are spatially grouped into patches which alternate with areas of bare soil. Eleven types of vegetation patch differing in the dominant plant functional type or species, floristic richness and size were identified with different relative frequency along the grazing gradient. Based on these results, it is postulated that grazing forces the replacement of large patches dominated by tall shrubs with high species richness, byLarrea divaricatapatches or small dwarf shrub patches with low species richness and the extinction of grass patches. This results from: (1) disruption of local balances of species deletions and additions; (2) fragmentation of large patches; and (3) formation of new vegetation patches. These changes lead to differing plant spatial organization and heterogeneity along the grazing gradient which may be described by characteristic arrays of vegetation patches.  相似文献   

The use of higher taxa to assess the benthic conditions in the southern Gulf of Mexico     

Nayeli Domínguez-Castanedo  Ricardo Rojas-López  Vivianne Solís-Weiss  Pablo Hernández-Alcántara  & Alejandro Granados-Barba 《Marine Ecology》2007,28(S1):161-168

  相似文献   

Demersal trawling waste as a food source for Western Mediterranean seabirds during the summer     

Martinez-Abrain  Alejandro; Maestre  Raquel; Oro  Daniel 《ICES Journal of Marine Science》2002,59(3):529-537

  相似文献   

Community Structure of the Subtidal Macroinfauna in an Estuarine Mussel Bed in Southern Chile     

Eduardo  Jaramillo  Carlos  Bertrán Alejandro  Bravo 《Marine Ecology》1992,13(4):317-331

  相似文献   

Editorial to the second PECS 2004 special issue of Ocean Dynamics     

Paulo Salles  Alejandro Souza 《Ocean Dynamics》2006,56(3-4):151-152

  相似文献   

Reply to the Discussion to “Proterozoic–Early Paleozoic evolution in western South America—a discussion” by Astini and Rapalini     

Florencio G. Aceolaza  Hubert Miller  Alejandro J. Toselli 《Tectonophysics》2003,366(1-2):149-150

  相似文献   

A synthesis of the early life history of the anglerfish, Lophius piscatorius (Linnaeus, 1758) in northern British waters   总被引：1，自引：0，他引：1  

Hislop  John R.G.; Gallego  Alejandro; Heath  Michael R.; Kennedy  Fiona M.; Reeves  Stuart A.; Wright  Peter J. 《ICES Journal of Marine Science》2001,58(1):70-86

  相似文献   

Development of a foraminifera-based transfer function in the Basque marshes, N. Spain: Implications for sea-level studies in the Bay of Biscay     

Eduardo Leorri  Benjamin P. Horton  Alejandro Cearreta 《Marine Geology》2008,251(1-2):60-74

In order to reconstruct former sea level we have developed a foraminifera-based transfer function using three models based on a modern dataset of 59 samples and 23 species obtained from four Basque marshes in Northern Spain. The relationship between observed and foraminifera-predicted elevation illustrated the strong performance of the transfer function (r²_jack ranges from 0.74 to 0.81). These results indicated that precise reconstructions of former sea levels are possible (error ranges from 0.11 to 0.19 m). The transfer function was used to calibrate the foraminiferal assemblages collected from a 50 cm salt marsh core. We placed the foraminifera-based reconstructions into a temporal framework using ¹³⁷Cs, Pb concentrations, and ²¹⁰Pb-derived sediment accumulation rates. The resulting relative sea-level curve is in good agreement with regional tide-gauge data. Both instrumental data and microfossil records suggest a rate of relative sea-level rise of approximately 2 mm yr^− 1 for the 20th century.  相似文献   

Effects of fish manipulation on the plankton community in small hypertrophic lakes from the Pampa Plain (Argentina)     

Alejandro Sosnovsky  Rolando Quirs 《Limnologica》2009,39(3):219-229

Trophic cascade hypotheses predict that fish will affect the structure and biomass of pelagic plankton communities. In order to investigate trophic cascade effects from fish down to phytoplankton, whole-lake studies were performed in five hypertrophic (mean total phosphorus (TP) concentrations higher than 1000 mg m⁻³) shallow lakes located in the Pampa Plain. The main climatic characteristic of this region is the alternation between periods of drought and flood, with corresponding changes of lake depth and conductivity of lake water. All lakes were studied from April to December 2000. Samples were taken of their physical and chemical characteristics and biotic communities, focusing on the zooplankton community. Fish were manipulated in four lakes (Capurro, Longinotti, Vedia 1, Vedia 2), while the fifth (Lake Vedia 3) was left undisturbed as a reference system. High abundance of planktivorous minnows (Jenynsia multidentata and Cheirodon interruptus) dominated the fish community in the reference lake. In the manipulated lakes, fish stocks were largely reduced in late autumn (May). During winter, Capurro, Longinotti and Vedia 1 were stocked with a visual planktivore, the pampean silverside (Odontesthes bonariensis, Atherinidae). Fish stocking was 24, 33 and 19 kg ha⁻¹, respectively. In contrast, no fish were stocked in Lake Vedia 2. Following fish removal, large Daphnia appeared in these lakes and grazed intensively on the phytoplankton. In contrast, no Daphnia were found in the reference lake (Vedia 3). The stocking of O. bonariensis in lakes Capurro, Longinotti and Vedia 1 led to a decrease in the percentage of large cladocerans and a rise in the phytoplankton biomass:TP ratio. Moreover, the lakes mentioned were stocked with different quantities of silversides over different periods of time. These differences were reflected temporarily in the plankton dynamics, affecting mainly larger sized zooplankton. Nevertheless, the presence of Daphnia was short lived in the lake where fish had been removed and no O. bonariensis were stocked. Competition for resources and recruitment of remaining fish probably caused a collapse in the zooplankton biomass. Our results support the idea that fish predation on zooplankton and its effect on phytoplankton is very intense in small pampean lakes. Fish removal was short lived, however. This could be because in small pampean lakes fish recolonization is favored, and minnows are highly prolific. Moreover, if manipulation of the trophic structure of lakes is undertaken in the pampean region, high nutrient loading from the watershed, climate and hydrology should also be taken into account.  相似文献   

Assemblages of megabenthic gastropods from Uruguayan and northern Argentinean shelf: Spatial structure and environmental controls     

Alvar Carranza  Fabrizio Scarabino  Alejandro Brazeiro  Leonardo Ortega  Sergio Martínez 《Continental Shelf Research》2008

We analyzed the structure of the megabenthic gastropod assemblages on the Uruguayan and northern Argentinean shelf and slope. Our analysis determined that there are two major biologically distinct assemblages which occurred in a 210,000 km² area showing conspicuous environmental gradients and large frontal areas: (a) an assemblage associated with the zone under the influence of the freshwater discharge of Río de la Plata and the shallow waters of the inner shelf and (b) an assemblage associated with marine zone in the outer shelf, which includes Magellanic (Subantarctic) and subtropical faunas. A multivariate analysis demonstrated a significant correlation between the environmental and biological matrix. This evidence suggests a noticeable effect of the physical environment on the spatial structure of the assemblage. We suggest that the current distribution patterns are caused by two different processes operating together: while processes operating at ecological time scales (e.g. differential tolerances to salinity and depth) determine most of the structure observed at the inner shelf, the presence of two contrasting water masses over the outer shelf determine a biogeographic boundary for the benthic fauna, linked to shifting climatic factors influencing species niche dynamics over evolutionary time scales. Thus, at the spatial scale here considered, ecological and historical processes must be considered when attempting to understand which factors determine the current structure of benthic assemblages at regional scales.  相似文献