Local effects of deforestation on stream fish assemblages in the amazon-savannah transitional area

The expansion of agriculture in the southeast Amazon-Savanah transitional area has greatly decreased forest cover and influenced stream structure and functioning. We assessed the effects of forest cover loss on the integrity of streams by assessing stream physical conditions and the taxonomic and functional diversity of fish assemblages in this transitional area. We hypothesize that low forest cover, especially adjacent to streams, results in poor physical conditions (e.g., warmer temperatures, less physical structure, etc.), which in turn will decrease the taxonomic and functional diversity of fish assemblages. We detected that loss of forest cover negatively affects natural stream conditions and reduces the functional diversity of fish assemblages, but we did not find a strong effect on taxonomic diversity. Ambush and stalking predators, diurnal surface pickers (groups that exhibit opportunistic life history strategies), grazers, pickers, and browsers (groups that exhibit equilibrium life history strategies) were the functional groups with the strongest relation to altered environments. These groups can explore different niches, both with natural characteristics or altered by human activities. Our results suggest that the preservation of riparian zones can minimize the loss of specialized fish species in assemblages of Amazon-Savannah stream systems.(AU)

A expansão da agricultura na área de transição Amazônia-Savana diminuiu muito a cobertura florestal, influenciando a estrutura e o funcionamento dos sistemas de riachos. Avaliamos o efeito da perda de cobertura florestal na integridade dos riachos avaliando as condições físicas do ambiente e a diversidade taxonômica e funcional das assembleias de peixes nesta área de transição. Nossa hipótese é que a baixa cobertura florestal, especialmente adjacente ao riacho, resulta em más condições físicas (por exemplo, temperaturas mais quentes, e menos estrutura física) o que por sua vez diminuirá os valores da diversidade taxonômica e funcional das assembleias de peixes. Detectamos que a perda de cobertura florestal afeta negativamente as condições naturais e reduz a diversidade funcional das assembleias de peixes, mas não encontramos um efeito para a diversidade taxonômica. Predadores de emboscada e espreita, catadores de superfície diurnos, pastores, catadores e navegadores foram os grupos funcionais que tiveram a relação mais forte com ambientes alterados. Esses grupos podem explorar diferentes nichos, tanto com características naturais quanto alterados pela ação antrópica. Nossos resultados sugerem a preservação da zona ripária a fim de evitar a perda de espécies especializadas das assembleias de peixes de riachos.(AU)

Acute toxicity of inorganic nitrogen (ammonium, nitrate and nitrite) to tadpoles of five tropical amphibian species.

Despite the higher diversity of amphibians and the increasing use of agrochemicals in tropical countries, knowledge on the ecotoxicity of such compounds to tropical amphibians remains very limited. The aim of this study was, therefore, to assess the acute lethal toxicity of three nitrogen salts (ammonium sulphate, sodium nitrate and sodium nitrite) to tadpoles of five tropical frog species: Rhinella ornata, Boana faber, B. pardalis, Physalaemus cuvieri, and P. olfersii. The order of sensitivity to the nitrogen salts for all five species was sodium nitrite > ammonium sulphate > sodium nitrate. There was not a single most sensitive species to all three nitrogen salts. However, differences in generated 4-d LC50 values between the most and least sensitive test species were small (a factor 2 to 6). A comparison with published toxicity values does not suggest an intrinsic higher, or lower, sensitivity of the tropical species tested as compared to their temperate counterparts. Reported nitrogen concentrations in sugarcane fields do not indicate a lethal risk to the amphibian species tested. Chronic-exposure and field studies are recommended to evaluate amphibian sensitivity under environmental-realistic multiple-stressor conditions.

Effects of deforestation on headwater stream fish assemblages in the Upper Xingu River Basin, Southeastern Amazonia

The expansion of the Amazonian agricultural frontier represents the most extensive land cover change in the world, detrimentally affecting stream ecosystems which collectively harbor the greatest diversity of freshwater fish on the planet. Our goal was to test the hypotheses that deforestation affects the abundance, richness, and taxonomic structure of headwater stream fish assemblages in the Upper Xingu River Basin, in Southeastern Amazonia. Standardized sampling surveys in replicated first order streams demonstrated that deforestation strongly influences fish assemblage structure. Deforested stream reaches had twice the fish abundance than reference stream reaches in primary forests. These differences in assemblage structure were largely driven by increases in the abundance of a handful of species, as no influence of deforestation on species richness was observed. Stream canopy cover was the strongest predictor of assemblage structure, possibly by a combination of direct and indirect effects on the provision of forest detritus, food resources, channel morphology, and micro-climate regulation. Given the dynamic nature of change in land cover and use in the region, this article is an important contribution to the understanding of the effects of deforestation on Amazonian stream fish, and their conservation.(AU)

A expansão da fronteira agrícola amazônica representa a mais ampla mudança na cobertura e uso da terra no mundo contemporâneo, e ameaça a integridade de ecossistemas de riachos que abrigam a maior diversidade de peixes de água doce do planeta. Nosso objetivo foi testar as hipóteses de que o desmatamento afeta a abundância, a riqueza e a estrutura taxonômica de assembleias de peixes em riachos de cabeceira na bacia do alto rio Xingu, no sudeste da Amazônia. Por meio de amostragens padronizadas realizadas em riachos de primeira ordem, demonstramos que o desmatamento influencia fortemente a estrutura das assembleias de peixes. Riachos em áreas desmatadas tiveram o dobro da abundância de peixes que riachos em áreas florestadas. Essas diferenças na estrutura das assembleias foram em grande parte causadas pelo aumento na abundância de algumas poucas espécies, já que nenhuma influência do desmatamento na riqueza de espécies foi observada. A cobertura de dossel sobre os riachos foi o mais forte preditor da estrutura das assembleias, possivelmente por uma combinação de efeitos diretos e indiretos relacionados à provisão de detritos florestais, recursos alimentares, morfologia dos canais, e regulação micro-climática. Dada a natureza dinâmica da mudança na cobertura e uso da terra na região, este artigo é uma contribuição importante para a compreensão dos efeitos a longo prazo do desmatamento nos peixes de riachos amazônicos e, portanto, para sua conservação.(AU)

Deforestation and stream warming affect body size of Amazonian fishes.

Declining body size has been suggested to be a universal response of organisms to rising temperatures, manifesting at all levels of organization and in a broad range of taxa. However, no study to date evaluated whether deforestation-driven warming could trigger a similar response. We studied changes in fish body size, from individuals to assemblages, in streams in Southeastern Amazonia. We first conducted sampling surveys to validate the assumption that deforestation promoted stream warming, and to test the hypothesis that warmer deforested streams had reduced fish body sizes relative to cooler forest streams. As predicted, deforested streams were up to 6 °C warmer and had fish 36% smaller than forest streams on average. This body size reduction could be largely explained by the responses of the four most common species, which were 43-55% smaller in deforested streams. We then conducted a laboratory experiment to test the hypothesis that stream warming as measured in the field was sufficient to cause a growth reduction in the dominant fish species in the region. Fish reared at forest stream temperatures gained mass, whereas those reared at deforested stream temperatures lost mass. Our results suggest that deforestation-driven stream warming is likely to be a relevant factor promoting observed body size reductions, although other changes in stream conditions, like reductions in organic matter inputs, can also be important. A broad scale reduction in fish body size due to warming may be occurring in streams throughout the Amazonian Arc of Deforestation, with potential implications for the conservation of Amazonian fish biodiversity and food supply for people around the Basin.