Browsing by Author "Brown, Valerie K."
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- Arthropod community structure in pastures of an island archipelago (Azores): looking for local–regional species richness patterns at fine-scalesPublication . Borges, Paulo A. V.; Brown, Valerie K.The arthropod species richness of pastures in three Azorean islands was used to examine the relationship between local and regional species richness over two years. Two groups of arthropods, spiders and sucking insects, representing two functionally different but common groups of pasture invertebrates were investigated. The local-regional species richness relationship was assessed over relatively fine scales: quadrats (= local scale) and within pastures (= regional scale). Mean plot species richness was used as a measure of local species richness (= α diversity) and regional species richness was estimated at the pasture level (= γ diversity) with the ‘first-order-Jackknife’ estimator. Three related issues were addressed: (i) the role of estimated regional species richness and variables operating at the local scale (vegetation structure and diversity) in determining local species richness; (ii) quantification of the relative contributions of α and β diversity to regional diversity using additive partitioning; and (iii) the occurrence of consistent patterns in different years by analysing independently between-year data. Species assemblages of spiders were saturated at the local scale (similar local species richness and increasing β-diversity in richer regions) and were more dependent on vegetational structure than regional species richness. Sucking insect herbivores, by contrast, exhibited a linear relationship between local and regional species richness, consistent with the proportional sampling model. The patterns were consistent between years. These results imply that for spiders local processes are important, with assemblages in a particular patch being constrained by habitat structure. In contrast, for sucking insects, local processes may be insignificant in structuring communities.
- Effect of island geological age on the arthropod species richness of Azorean pasturesPublication . Borges, Paulo A. V.; Brown, Valerie K.Species richness of six pasture arthropod assemblages (total arthropod species, total herbivore species, sucking and chewing herbivores, total predatory species and spiders) were regressed against several geographical variables (area, distance from the nearest mainland, maximum elevation and geological age of the islands) of three Azorean islands (S. Maria, Terceira and Pico). The species were sampled by the fixed-quadrate size sampling method and the results obtained are consistent with the geological age hypothesis, i.e. the species richness of the six indigenous arthropod assemblage’s increases with the geological age of the islands, both at local and regional scales. Higher values of indigenous and endemic species richness were consistently found on the older island (S. Maria), and the lowest values on the most recent island (Pico). Moreover, when considering the age of Faial (an older island probably once connected with Pico) as a estimate of the age of Pico, correlations between species richness and island age were improved, thereby strengthening the relationship. The older island (S. Maria) has more specialized herbivores and a greater proportion of herbivores in relation to predatory arthropods. Ecological and biogeographical studies in the Azores should take into account the effects of the time each island has been available for colonization and evolution.
- Estimating species richness of arthropods in Azorean pastures: the adequacy of suction sampling and pitfall trappingPublication . Borges, Paulo A. V.; Brown, Valerie K.Se han investigado las ventajas del uso combinado de técnicas de succión (Vortis) y trampas de caída, a la hora de estimar la riqueza de especies de artrópodos en pastizales antrópicos y seminaturales de tres islas oceánicas de las Azores de origen geológico reciente. Se ha comprobado, además, la capacidad de las trampas de caída y de succión para describir la composición faunística y la riqueza de especies según distintos grupos taxonómicos y ecológicos. Teniendo en cuenta el porcentaje de especies colectado sobre el total estimado, ambas técnicas de muestreo resultaron fiables en lo que se refiere a su eficacia, ofreciendo buenas estimas de la riqueza de especies de artrópodos. Usando los dos métodos a la vez, se obtuvieron valores de eficacia cercanos al 90% para algunos grupos taxonómicos. Las técnicas de succión o de caída por separado fueron fiables para estimar la riqueza de especies de artrópodos predadores (todas las especies) y arañas. Sin embargo, ambas técnicas son necesarias para conseguir estimaciones fiables en el caso de los grupos herbívoros. En términos de presencia-ausencia, los grupos que se alimentan de savia, los chupadores y los herbívoros masticadores mostraron diferentes patrones, con una proporción de colecta más alta en los primeros mediante las técnicas de succión, mientras que la mayoría de las especies masticadoras se pudieron colectar mediante las trampas de caída. Un patrón similar fue obtenido en el caso de las arañas, en donde las especies tejedoras fueron colectadas solo gracias a las técnicas de succión, mientras que la mayoría del resto de las especies se capturaron gracias al uso de trampas de caída. Se concluye que estos dos métodos de colecta deben considerarse complementarios y no alternativos. Por tanto, en estudios realizados en pastizales y sistemas agrícolas, se recomienda el uso simultáneo de ambas técnicas a fin de obtener estimas fiables de la riqueza de especies y la composición taxonómica de artrópodos.
- Phytophagous insects and web-building spiders in relation to pasture vegetation complexityPublication . Borges, Paulo A. V.; Brown, Valerie K.We investigate the potential use of structural characteristics of vegetation as predictors of the densities of pasture arthropods in three Azorean islands. Two types of upland pastures, recent sown pastures (3-4 yr old) and wet semi-natural old pastures (>35 yr old), were studied in three Azorean islands (S. Maria, Terceira and Pico). Three arthropod assemblages, of particular importance in pasture habitats, were assessed: insect forb-feeders, insect grass-feeders and web-building spiders. These are numerically abundant and represent a range of feeding strategies. Point quadrates for plants and suction (Vortis) for arthropods were used as sampling methods. Several vegetation indices were investigated, but only three (cover abundance of perennial forbs, cover abundance of perennial grasses and the total vegetation alpha-diversity) were good predictors of invertebrate abundance. Vegetation structure, defined by a Height Index, was of minor predictive value. In sown and semi-natural pastureland, diverse and structurally complex pasture sites support more individual insect forb-feeders, grass-feeders and web-building spiders per unit area than less complex ones.