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  • 1
    Publication Date: 2001-12-01
    Description: The manner in which terrestrial ecosystems are regulated is controversial. The "top-down" school holds that predators limit herbivores and thereby prevent them from overexploiting vegetation. "Bottom-up" proponents stress the role of plant chemical defenses in limiting plant depredation by herbivores. A set of predator-free islands created by a hydroelectric impoundment in Venezuela allows a test of these competing world views. Limited area restricts the fauna of small (0.25 to 0.9 hectare) islands to predators of invertebrates (birds, lizards, anurans, and spiders), seed predators (rodents), and herbivores (howler monkeys, iguanas, and leaf-cutter ants). Predators of vertebrates are absent, and densities of rodents, howler monkeys, iguanas, and leaf-cutter ants are 10 to 100 times greater than on the nearby mainland, suggesting that predators normally limit their populations. The densities of seedlings and saplings of canopy trees are severely reduced on herbivore-affected islands, providing evidence of a trophic cascade unleashed in the absence of top-down regulation.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Terborgh, J -- Lopez, L -- Nunez, P -- Rao, M -- Shahabuddin, G -- Orihuela, G -- Riveros, M -- Ascanio, R -- Adler, G H -- Lambert, T D -- Balbas, L -- New York, N.Y. -- Science. 2001 Nov 30;294(5548):1923-6.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Center for Tropical Conservation, Duke University, Box 90381, Durham, NC 27708, USA. manu@duke.edu〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/11729317" target="_blank"〉PubMed〈/a〉
    Keywords: Animals ; Ants/physiology ; Anura/physiology ; Birds/physiology ; *Ecosystem ; Female ; *Food Chain ; Fresh Water ; *Geography ; Haplorhini/physiology ; Iguanas/physiology ; Lizards/physiology ; *Models, Biological ; Population Density ; Power Plants ; Reproduction ; Rodentia/physiology ; Spiders/physiology ; Swine/physiology ; Trees/*physiology ; Venezuela
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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  • 2
    Publication Date: 2002-01-26
    Description: The high alpha-diversity of tropical forests has been amply documented, but beta-diversity-how species composition changes with distance-has seldom been studied. We present quantitative estimates of beta-diversity for tropical trees by comparing species composition of plots in lowland terra firme forest in Panama, Ecuador, and Peru. We compare observations with predictions derived from a neutral model in which habitat is uniform and only dispersal and speciation influence species turnover. We find that beta-diversity is higher in Panama than in western Amazonia and that patterns in both areas are inconsistent with the neutral model. In Panama, habitat variation appears to increase species turnover relative to Amazonia, where unexpectedly low turnover over great distances suggests that population densities of some species are bounded by as yet unidentified processes. At intermediate scales in both regions, observations can be matched by theory, suggesting that dispersal limitation, with speciation, influences species turnover.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Condit, Richard -- Pitman, Nigel -- Leigh, Egbert G Jr -- Chave, Jerome -- Terborgh, John -- Foster, Robin B -- Nunez, Percy -- Aguilar, Salomon -- Valencia, Renato -- Villa, Gorky -- Muller-Landau, Helene C -- Losos, Elizabeth -- Hubbell, Stephen P -- New York, N.Y. -- Science. 2002 Jan 25;295(5555):666-9.〈br /〉〈span class="detail_caption"〉Author address: 〈/span〉Center for Tropical Forest Science, Smithsonian Tropical Research Institute, Unit 0948, APO AA 34002-0948, USA.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/11809969" target="_blank"〉PubMed〈/a〉
    Keywords: *Ecosystem ; Ecuador ; Environment ; Fourier Analysis ; Models, Biological ; Panama ; Peru ; Probability ; *Trees/classification/growth & development ; *Tropical Climate
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Computer Science , Medicine , Natural Sciences in General , Physics
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