Climatic Variables Do Not Directly Predict Spider Richness and Abundance in Semiarid Caatinga Vegetation, Brazil.

Spiders are abundant in tropical ecosystems and exert predatory pressure on a wide variety of invertebrate populations and also serve as prey for many others organisms, being part of complex interrelationships influenced directly and indirectly by a myriad of factors. We examined the influence of biotic (i.e., prey availability) and abiotic (i.e., temperature, precipitation, relative humidity, real evapotranspiration) factors on species richness and abundance during a two-year period in the semiarid Caatinga vegetation in northeastern Brazil. Data were analyzed through partial autocorrelation functions, cross correlations, and a path analysis. A total of 2522 spiders were collected with beating tray, pit-fall traps, and malaise traps, comprising 91 species and 34 families. Spider abundance peaked in the rainy season. Our results suggest that total invertebrate abundance has a direct influence on spider richness and abundance, whereas the effects of precipitation were mainly indirectly related to most spider assemblage parameters. The increase in vegetation cover with the rainy season in the Caatinga provides more breeding and foraging sites for spiders and stimulates their activities. Additionally, rainfall in arid and semiarid ecosystems stimulated the activity and reproduction of many herbivore and detritivore invertebrates dependent on plant biomass and necromass consumption, leading to an increase in spider prey availability.

A structured inventory of spiders (Arachnida, Araneae) in natural and artificial forest gaps at Porto Urucu, Western Brazilian Amazonia / Inventário estruturado de aranhas (Arachnida, Araneae) em clareiras naturais e artificiais em Porto Urucu, Amazônia Ocidental Brasileira

A preliminary survey of the spider fauna in natural and artificial forest gap formations at "Porto Urucu", a petroleum/natural gas production facility in the Urucu river basin, Coari, Amazonas, Brazil is presented. Sampling was conducted both occasionally and using a protocol composed of a suite of techniques: beating trays (32 samples), nocturnal manual samplings (48), sweeping nets (16), Winkler extractors (24), and pitfall traps (120). A total of 4201 spiders, belonging to 43 families and 393 morphospecies, were collected during the dry season, in July, 2003. Excluding the occasional samples, the observed richness was 357 species. In a performance test of seven species richness estimators, the Incidence Based Coverage Estimator (ICE) was the best fit estimator, with 639 estimated species. To evaluate differences in species richness associated with natural and artificial gaps, samples from between the center of the gaps up to 300 meters inside the adjacent forest matrix were compared through the inspection of the confidence intervals of individual-based rarefaction curves for each treatment. The observed species richness was significantly higher in natural gaps combined with adjacent forest than in the artificial gaps combined with adjacent forest. Moreover, a community similarity analysis between the fauna collected under both treatments demonstrated that there were considerable differences in species composition. The significantly higher abundance of Lycosidae in artificial gap forest is explained by the presence of herbaceous vegetation in the gaps themselves. Ctenidae was significantly more abundant in the natural gap forest, probable due to the increase of shelter availability provided by the fallen trees in the gaps themselves. Both families are identified as potential indicators of environmental change related to the establishment or recovery of artificial gaps in the study area.

Apresenta-se um inventário preliminar da fauna de aranhas em clareiras naturais e artificiais em "Porto Urucu", uma instalação produtora de petróleo e gás natural na bacia do Rio Urucu, Coari, Amazonas, Brasil. As amostras foram realizadas de modo ocasional e com um protocolo estruturado composto por um conjunto de técnicas de amostragem: guarda-chuvas entomológicos (32 amostras), amostras manuais noturnas (48), rede de varredura (16), extratores de Winkler (24) e armadilhas de queda (120). Um total de 4201 aranhas pertencentes a 43 famílias e 393 morfoespécies foi coletado durante a estação seca em julho de 2003. Excluindo as amostras ocasionais, a riqueza observada foi de 357 espécies. No teste de desempenho de sete estimadores, ICE (Incidence Based Coverage Estimator) gerou o melhor resultado, com 639 espécies estimadas. Para avaliar diferenças na riqueza de espécies associadas a clareiras naturais e artificiais, conjuntos de amostras do centro da clareira a até 300 m adentro da floresta adjacente foram comparados através da inspeção dos intervalos de confiança de curvas de rarefação baseadas no número de indivíduos de cada tratamento. A riqueza observada foi significantemente maior nas clareiras naturais combinadas com as florestas adjacentes do que nas clareiras artificiais combinadas com as matas do entorno. Além disso, uma análise de similaridade entre as faunas coletadas em ambos os tratamentos mostrou que existe diferenças consideráveis na composição de espécies. A abundância significativamente mais alta de Lycosidae nas florestas de clareiras artificiais é explicada pela presença de vegetação herbácea nas clareiras propriamente ditas. Ctenidae foi significantemente mais abundante nas florestas de clareiras naturais, provavelmente devido ao aumento da disponibilidade de abrigos causado pela queda de árvores nas clareiras propriamente ditas. Ambas estas famílias são identificadas como potenciais indicadores de mudanças ambientais relacionadas com o estabelecimento ou recuperação de clareiras artificiais na área de estudo.