RESUMO
Deforestation is a primary driver of biodiversity change through habitat loss and fragmentation. Stream biodiversity may not respond to deforestation in a simple linear relationship. Rather, threshold responses to extent and timing of deforestation may occur. Identification of critical deforestation thresholds is needed for effective conservation and management. We tested for threshold responses of fish species and functional groups to degree of watershed and riparian zone deforestation and time since impact in 75 streams in the western Brazilian Amazon. We used remote sensing to assess deforestation from 1984 to 2011. Fish assemblages were sampled with seines and dip nets in a standardized manner. Fish species (n = 84) were classified into 20 functional groups based on ecomorphological traits associated with habitat use, feeding, and locomotion. Threshold responses were quantified using threshold indicator taxa analysis. Negative threshold responses to deforestation were common and consistently occurred at very low levels of deforestation (<20%) and soon after impact (<10 years). Sensitive species were functionally unique and associated with complex habitats and structures of allochthonous origin found in forested watersheds. Positive threshold responses of species were less common and generally occurred at >70% deforestation and >10 years after impact. Findings were similar at the community level for both taxonomic and functional analyses. Because most negative threshold responses occurred at low levels of deforestation and soon after impact, even minimal change is expected to negatively affect biodiversity. Delayed positive threshold responses to extreme deforestation by a few species do not offset the loss of sensitive taxa and likely contribute to biotic homogenization.
Assuntos
Conservação dos Recursos Naturais , Rios , Animais , Biodiversidade , Brasil , Ecossistema , PeixesRESUMO
High rates of deforestation, either in the past or the present, affect many of the ecological processes in streams. Integrating deforestation history and the current landscape structure enhances the evaluation of ecological effects of land-use change. This is especially true when contemporary landscape conditions are similar but the temporal path to those conditions differs. One approach that has shown promise for evaluating biodiversity responses over time and space is the β-diversity partitioning, which combines taxonomic and functional trait-based approaches. We tested hypotheses related to stream fish assemblages' turnover in watersheds with different environmental conditions and deforestation histories. We sampled fish from 75 watersheds in the Machado River basin, Brazil, and environmental factors were quantified at multiple scales. Taxonomic turnover was higher than expected by chance, whereas functional turnover was lower than expected by the observed taxonomic turnover, indicating that deterministic processes are structuring these assemblages. The turnover, and the environmental factors differed among watersheds with different deforestation histories. Besides being scale-dependent, turnover patterns are also likely dependent on land use dynamics and involve time-lags.(AU)
Altas taxas de desmatamento, seja no passado ou no presente, afetam muitos processos ecológicos em riachos. Integrar o histórico do desmatamento à estrutura atual da paisagem melhora a avaliação dos efeitos ecológicos da mudança do uso do solo. Isto é especialmente verdadeiro quando as condições da paisagem contemporânea são semelhantes, mas seguiram trajetórias distintas. Uma abordagem promissora para avaliar as respostas da biodiversidade ao longo do tempo e espaço é a partição da diversidade-β, que combina abordagens taxonômica e funcional. Testamos hipóteses relacionadas à substituição das assembleias de peixes de riachos em microbacias com diferentes condições ambientais e histórias de desmatamento. Amostramos 75 microbacias na bacia do rio Machado, fatores ambientais foram quantificados em múltiplas escalas. A substituição taxonômica foi maior do que esperada pelo acaso, enquanto a substituição funcional foi menor do que o esperado pela substituição taxonômica, indicando que as assembleias são estruturadas por processos determinísticos. As substituições e fatores ambientais diferiram entre microbacias com históricos de desmatamento distintos. Além de serem escala-dependentes, os padrões de substituição provavelmente são dependentes da dinâmica do uso do solo, envolvendo defasagem temporal.(AU)
Assuntos
Animais , Conservação dos Recursos Naturais/história , Determinismo Genético , Peixes/classificação , RiosRESUMO
Roads affect biodiversity by increasing mortality rates, habitat loss, and natural landscape fragmentation. Poorly installed culverts can impound streams, changing the environmental conditions, and affecting aquatic communities. We evaluated the effects of road crossings on the taxonomic composition and functional structure of fish assemblages in lowland eastern Amazonian streams, Brazil. We tested the hypothesis that the presence of road-derived impoundments affects assemblage taxonomic and functional composition and structure. Two predictions were addressed: (1) Species and functional group composition will differ in impounded reaches in relation to lotic reaches; (2) Assemblages in downstream lotic reaches will be richer in species and functional groups in relation to upstream lotic and impoundment reaches. We sampled five streams crossed by roads presenting impoundment formation, conducted by visual census (day, dusk, and night) in 200 m line transects. Assemblage composition from lentic reaches was different from lotic reaches, and Shannon diversity from downstream reaches was different from upstream and impoundment, however, beta diversity partitioning between lotic reaches showed higher nestedness contribution, reinforcing the role of impoundment in limiting fish dispersal between lotic reaches. These results suggest that impoundments impose environmental and dispersal constraints to fish, affecting their longitudinal distribution in streams fragmented by roads.(AU)
Estradas afetam a biodiversidade aumentando as taxas de mortalidade, perda de hábitat e fragmentação. Cruzamentos mal instalados represam riachos, alterando as condições ambientais e afetando as comunidades aquáticas. Avaliamos os efeitos deste impacto sobre a estrutura taxonômica e funcional das assembleias de peixes em riachos da Amazônia Oriental, Brasil. Testamos a hipótese de que a presença dos alagados derivados de estradas afeta a composição e estrutura taxonômica e funcional das assembleias. Nossas predições foram: (1) A composição de espécies e grupos funcionais é diferente entre trechos alagados e lóticos; (2) Assembleias nos trechos à jusante serão mais ricas em espécies e grupos funcionais que os trechos à montante e alagados. Amostramos cinco riachos cruzados por estradas com formação de alagamentos por censo visual (dia, crepúsculo e noite) em transectos lineares de 200 m. A composição das assembleias dos trechos lênticos diferiu dos lóticos, e a diversidade de Shannon à jusante diferiu dos trechos montante e alagados, entretanto, nos lóticos, houve maior contribuição do aninhamento na partição da diversidade beta, reforçando o papel do represamento na limitação da dispersão de peixes entre estes. Sugerimos que os represamentos impõem restrições ambientais e de dispersão aos peixes, afetando sua distribuição longitudinal em riachos fragmentados por estradas.(AU)