The Experts below are selected from a list of 5328 Experts worldwide ranked by ideXlab platform
Julio R. Gutiérrez - One of the best experts on this subject based on the ideXlab platform.
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Bottom-Up Control of consumers leads to top-down indirect facilitation of invasive annual herbs in semiarid Chile.
Ecology, 2011Co-Authors: Jaime Madrigal, Peter L. Meserve, Douglas A. Kelt, Julio R. Gutiérrez, Francisco A. SqueoAbstract:The abundance of exotic plants is thought to be limited by competition with resident species (including plants and generalist herbivores). In contrast, observations in semiarid Chile suggest that a native generalist rodent, the degu (Octodon degus), may be facilitating the expansion of exotic annual plants. We tested this hypothesis with a 20-year data set from a World Biosphere Reserve in mediterranean Chile. In this semiarid environment, rainfall varies annually and dramatically influences cover by both native and exotic annual plants; degu population density affects the composition and cover of exotic and native annual plants. In low-rainfall years, cover of both native and exotic herbs is extremely low. Higher levels of precipitation result in proportional increases in cover of all annual plants (exotic and native species), leading in turn to increases in degu population densities, at which point they impact native herbs in proportion to their greater cover, indirectly favoring the expansion of exotic plants. We propose that Bottom-Up Control of consumers at our site results in top-down indirect facilitation of invasive annual herbs, and that this pattern may be general to other semiarid ecosystems.
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Thirteen Years of Shifting Top-Down and Bottom-Up Control
BioScience, 2003Co-Authors: Peter L. Meserve, Douglas A. Kelt, W. Bryan Milstead, Julio R. GutiérrezAbstract:Abstract For 13 years, we studied the role of biotic interactions, including predation, herbivory, and interspecific competition, in a semiarid thorn scrub community in north-central Chile. Using a large-scale field manipulation, we monitored changes in small mammals, plants, and vertebrate predators. We documented important “top-down” predation effects on some small-mammal species, and small-mammal effects on some plant species resulting from experimental exclusions. However, periodic El Nino Southern Oscillation (ENSO) events caused several high rainfall episodes during this interval, resulting in large “Bottom-Up” increases in both plants and animals. Therefore, we suggest that instead of exclusive top-down or Bottom-Up Control, this system undergoes shifting Control, with relatively greater importance of biotic interactions in wet years and of resource limitation in dry ones. Because intervals between ENSO events are long and responses are slow, long-term studies are essential for understanding such e...
Patrick L. Thompson - One of the best experts on this subject based on the ideXlab platform.
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Warming shifts top-down and Bottom-Up Control of pond food web structure and function
Philosophical Transactions of the Royal Society B: Biological Sciences, 2012Co-Authors: Jonathan B Shurin, Jessica L Clasen, Pavel Kratina, Hamish S Greig, Patrick L. ThompsonAbstract:The effects of global and local environmental changes are transmitted through networks of interacting organisms to shape the structure of communities and the dynamics of ecosystems. We tested the impact of elevated temperature on the top-down and Bottom-Up forces structuring experimental freshwater pond food webs in western Canada over 16 months. Experimental warming was crossed with treatments manipulating the presence of planktivorous fish and eutrophication through enhanced nutrient supply. We found that higher temperatures produced top-heavy food webs with lower biomass of benthic and pelagic producers, equivalent biomass of zooplankton, zoobenthos and pelagic bacteria, and more pelagic viruses. Eutrophication increased the biomass of all organisms studied, while fish had cascading positive effects on periphyton, phytoplankton and bacteria, and reduced biomass of invertebrates. Surprisingly, virus biomass was reduced in the presence of fish, suggesting the possibility for complex mechanisms of top-down Control of the lytic cycle. Warming reduced the effects of eutrophication on periphyton, and magnified the already strong effects of fish on phytoplankton and bacteria. Warming, fish and nutrients all increased whole-system rates of net production despite their distinct impacts on the distribution of biomass between producers and consumers, plankton and benthos, and microbes and macrobes. Our results indicate that warming exerts a host of indirect effects on aquatic food webs mediated through shifts in the magnitudes of top-down and Bottom-Up forcing.
Stijn Temmerman - One of the best experts on this subject based on the ideXlab platform.
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Top-down vs. Bottom-Up Control on vegetation composition in a tidal marsh depends on scale
PloS one, 2017Co-Authors: Kelly Elschot, Anke Vermeulen, Wouter Vandenbruwaene, Jan P. Bakker, Tjeerd J. Bouma, Julia Stahl, Henk Castelijns, Stijn TemmermanAbstract:The relative impact of top-down Control by herbivores and Bottom-Up Control by environmental conditions on vegetation is a subject of debate in ecology. In this study, we hypothesize that top-down Control by goose foraging and Bottom-Up Control by sediment accretion on vegetation composition within an ecosystem can co-occur but operate at different spatial and temporal scales. We used a highly dynamic marsh system with a large population of the Greylag goose (Anser anser) to investigate the potential importance of spatial and temporal scales on these processes. At the local scale, Greylag geese grub for below-ground storage organs of the vegetation, thereby creating bare patches of a few square metres within the marsh vegetation. In our study, such activities by Greylag geese allowed them to exert top-down Control by setting back vegetation succession. However, we found that the patches reverted back to the initial vegetation type within 12 years. At large spatial (i.e. several square kilometres) and temporal scales (i.e. decades), high rates of sediment accretion surpassing the rate of local sea-level rise were found to drive long-term vegetation succession and increased cover of several climax vegetation types. In summary, we conclude that the vegetation composition within this tidal marsh was primarily Controlled by the Bottom-Up factor of sediment accretion, which operates at large spatial as well as temporal scales. Top-down Control exerted by herbivores was found to be a secondary process and operated at much smaller spatial and temporal scales.
Oscar Iribarne - One of the best experts on this subject based on the ideXlab platform.
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Abiotic stress mediates top-down and Bottom-Up Control in a Southwestern Atlantic salt marsh.
Oecologia, 2009Co-Authors: Juan Alberti, Agustina Mendez Casariego, Pedro Daleo, Eugenia Fanjul, Brian R. Silliman, Mark D. Bertness, Oscar IribarneAbstract:Increasing evidence has shown that nutrients and consumers interact to Control primary productivity in natural systems, but how abiotic stress affects this interaction is unclear. Moreover, while herbivores can strongly impact zonation patterns in a variety of systems, there are few examples of this in salt marshes. We evaluated the effect of nutrients and herbivores on the productivity and distribution of the cordgrass Spartina densiflora along an intertidal stress gradient, in a Southwestern Atlantic salt marsh. We characterized abiotic stresses (salinity, ammonium concentration, and anoxia) and manipulated nutrients and the presence of the herbivorous crab Neohelice (Chasmagnathus) granulata, at different tidal heights with a factorial experiment. Abiotic stress increased at both ends of the tidal gradient. Salinity and anoxia were highest at the upper and lower edge of the intertidal, respectively. Nutrients and herbivory interacted to Control cordgrass biomass, but their relative importance varied with environmental context. Herbivory increased at lower tidal heights to the point that cordgrass transplants onto bare mud substrate were entirely consumed unless crabs were excluded, while nutrients were most important where abiotic stress was reduced. Our results show how the impact of herbivores and nutrients on plant productivity can be dependent on environmental conditions and that the lower intertidal limits of marsh plants can be Controlled by herbivory.
Laurent Itti - One of the best experts on this subject based on the ideXlab platform.
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computational modelling of visual attention
Nature Reviews Neuroscience, 2001Co-Authors: Laurent IttiAbstract:Five important trends have emerged from recent work on computational models of focal visual attention that emphasize the Bottom-Up, image-based Control of attentional deployment. First, the perceptual saliency of stimuli critically depends on the surrounding context. Second, a unique 'saliency map' that topographically encodes for stimulus conspicuity over the visual scene has proved to be an efficient and plausible Bottom-Up Control strategy. Third, inhibition of return, the process by which the currently attended location is prevented from being attended again, is a crucial element of attentional deployment. Fourth, attention and eye movements tightly interplay, posing computational challenges with respect to the coordinate system used to Control attention. And last, scene understanding and object recognition strongly constrain the selection of attended locations. Insights from these five key areas provide a framework for a computational and neurobiological understanding of visual attention.