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      DROUGHT AND RANGELAND GRASSHOPPER SPECIES DIVERSITY

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      The Canadian Entomologist

      Cambridge University Press (CUP)

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          Abstract

          Rangeland grasshopper species richness was monitored at 10 sites in Montana, U.S.A., during 1986 through 1992, which included an extreme drought year (1988). We observed significant post-1988 drought reductions in rangeland grasshopper species richness in the eastern and south-central region of Montana where drought intensity has been increasing during the past 20 years. In the north-central region, which also experienced the 1988 drought but showed no long-term drought trend, we did not observe a post-drought reduction in overall rangeland grasshopper species richness. Thus, in terms of rangeland grasshopper species richness, the potential impact of a severe drought in a given year may depend on the timing of such an event within longer-term climatic cycles. Our findings suggest that as regional drought intensity increases temporally, there may be an increased likelihood that a single extreme drought year will not only have a profound impact on abundance but may also result in significant long-term reduction of grasshopper species richness. Our results support the hypothesis that resource limitation is a very important factor in structuring rangeland grasshopper communities in space and time.

          Résumé

          La richesse en espèces de criquets dans les zones de pâturage a été évaluée à 10 endroits dans le Montana, É.-U, de 1986 à 1992, intervalle qui recouvre une année d’extrême sécheresse (1988). Nous avons observé des réductions importantes de la richesse en espèces de criquets après la sécheresse de 1988 dans les zones de pâturage de l’est et du centre sud du Montana où l’intensité de la sécheresse a subi une augmentation au cours des 20 dernières années. Dans le centre nord, qui a également subi la sécheresse de 1988, mais où il ne s’est pas produit d’augmentation à long terme de l’intensité de la sécheresse, il n’y a pas eu de réduction particulière de la richesse en espèces après la sécheresse de 1988. Il semble donc que l’impact d’une sécheresse ponctuelle intense sur la richesse en espèces de criquets dans les zones de pâturage soit tributaire du cycle du climat où elle se situe. Nos résultats indiquent qu’une augmentation graduelle de l’intensité de la sécheresse dans une zone donnée peut non seulement faire augmenter la probabilité qu’une année de sécheresse extrême ait un impact important sur l’abondance, mais peut également faire en sorte que cette année de sécheresse entraîne une réduction à long terme de la richesse en espèces de criquets. Nos résultats corroborent l’hypothèse selon laquelle la restriction des ressources constitue un facteur important d’organisation spatiale et temporelle des communautés de criquets dans les zones de pâturage.

          [Traduit par la Rédaction]

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          Most cited references 32

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          The commonly observed high diversity of trees in tropical rain forests and corals on tropical reefs is a nonequilibrium state which, if not disturbed further, will progress toward a low-diversity equilibrium community. This may not happen if gradual changes in climate favor different species. If equilibrium is reached, a lesser degree of diversity may be sustained by niche diversification or by a compensatory mortality that favors inferior competitors. However, tropical forests and reefs are subject to severe disturbances often enough that equilibrium may never be attained.
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            Patterns of tree abundance and dispersion in a tropical deciduous (dry) forest are summarized. The generalization that tropical trees have spaced adults did not hold. All species were either clumped or randomly dispersed, with rare species more clumped than common species. Breeding system was unrelated to species abundance or dispersion, but clumping was related to mode of seed dispersal. Juvenile densities decreased approximately exponentially away from adults. Rare species gave evidence of poor reproductive performance compared with their performance when common in nearby forests. Patterns of relative species abundance in the dry forest are compared with patterns in other forests, and are explained by a simple stochastic model based on random-walk immigration and extinction set in motion by periodic community disturbance.
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              Latitudinal Variations in Organic Diversity

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                Author and article information

                Journal
                applab
                The Canadian Entomologist
                Can Entomol
                Cambridge University Press (CUP)
                0008-347X
                1918-3240
                August 1994
                May 31 2012
                August 1994
                : 126
                : 04
                : 1075-1092
                10.4039/Ent1261075-4
                © 1994

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