Fish assemblages of the Casiquiare River, a corridor and zoogeographical filter for dispersal between the Orinoco and Amazon basins

Abstract

Aim The aim of this study was to determine whether the Casiquiare River functions as a free dispersal corridor or as a partial barrier (i.e. filter) for the interchange of fish species of the Orinoco and Negro/Amazon basins using species assemblage patterns according to geographical location and environmental features.Location The Casiquiare, Upper Orinoco and Upper Negro rivers in southern Venezuela, South America.Methods Our study was based on an analysis of species presence/absence data and environmental information (11 habitat characteristics) collected by the authors and colleagues between the years 1984 and 1999. The data set consisted of 269 sampled sites and 452 fish species (> 50,000 specimens). A wide range of habitat types was included in the samples, and the collection sites were located at various points along the entire length of the Casiquiare main channel, at multiple sites on its tributary streams, as well as at various nearby sites outside the Casiquiare drainage, within the Upper Orinoco and Upper Rio Negro river systems. Most specimens and field data used in this analysis are archived in the Museo de Ciencias Naturales in Guanare, Venezuela. We performed canonical correspondence analysis (CCA) based on species presence/absence using two versions of the data set: one that eliminated sites having < 5 species and species occurring at < 5 sites; and another that eliminated sites having < 10 species and species occurring at < 10 sites. Cluster analysis was performed on sites based on species assemblage similarity, and a separate analysis was performed on species based on CCA loadings. ResultsThe CCA results for the two versions of the data set were qualitatively the same. The dominant environmental axis contrasted assemblages and sites associated with blackwater vs. clearwater conditions. Longitudinal position on the Casiquiare River was correlated (r 2 = 0.33) with CCA axis-1 scores, reflecting clearwater conditions nearer to its origin (bifurcation of the Orinoco) and blackwater conditions nearer to its mouth (junction with the Rio Negro). The second CCA axis was most strongly associated with habitat size and structural complexity. Species associations derived from the unweighted pair-group average clustering method and pair-wise squared Euclidean distances calculated from species loadings on CCA axes 1 and 2 showed seven ecological groupings. Cluster analysis of species assemblages according to watershed revealed a stronger influence of local environmental conditions than of geographical proximity.Main conclusions Fish assemblage composition is more consistently associated with local environmental conditions than with geographical position within the river drainages. Nonetheless, the results support the hypothesis that the mainstem

Composición y estructura de peces de la microcuenca del río Anchique, Tolima, Colombia

“…Posteriormente, se hallaron los índices de composición del sustrato (S) y complejidad estructural del hábitat (S), siguiendo la metodología propuesta por Winemiller, López-Fernández, Taphorn, Nico and Duque (2008), los cuales estandarizaron el índice (S) por medio de la siguiente fórmula: Donde, S son los índices de composición del sustrato y complejidad estructural del hábi-tat, x i representa cada uno de los componentes del sustrato y estructura del hábitat por cada estación de muestreo, y N representa el núme-ro de componentes combinados del sustrato o estructura del hábitat observados en las estaciones de muestreo; los valores cercanos a 0 indican mínima complejidad y cercanos a 1 máxima complejidad (Winemiller et al, 2008).…”

Guianacara dacrya, a new species from the rio Branco and Essequibo River drainages of the Guiana Shield (Perciformes: Cichlidae)

“…ROM 86127and see Willis et al, 2010). To our knowledge, only one genetic study has shown a significant role for the Rupununi savannas as a corridor between the Amazon and Essequibo basins (Lovejoy & de Araújo, 2000), but it is likely that the rainy season connection between the two basins affects different taxa in different ways, and that ecological differences between basins play a major role in determining which species are able to cross the divide, as is the case in the connection between the Amazon and Orinoco basins through the Casiquiare River (Winemiller et al, 2008, Willis et al, 2010.…”

“…Understanding this dual pattern of distribution requires a clear taxonomic understanding of species in the area as a requisite to distinguish those that are able to move freely between the basins and those who are limited in their movements by ecological or other kinds of filters (e.g., Winemiller et al, 2008). We examined specimens of Guianacara from both the Essequibo and the Takutu-Ireng-Branco basins in Guyana and Brazil and determined that populations on either side of the Rupununi savannas are morphologically indistinguishable from each other, but are clearly diagnosable as a species different from all other described Guianacara.…”

Peixes da Estação Ecológica Serra Geral do Tocantins, bacias dos Rios Tocantins e São Francisco, com observações sobre as implicações biogeográficas das "águas emendadas" dos Rios Sapão e Galheiros

“…Sua ictiofauna é uma das mais bem conhecidas da América do Sul (Britski et al 1984, Sato & Godinho 1999, Alves & Pompeu 2001, tendo sido uma das primeiras a receber um estudo monográfico detalhado (Lütken 1875, republicado, em tradução para o português em 2001). Uma compilação realizada pelos autores indicou a existência de no mínimo 180 espécies de peixes nativas ocorrendo nessa bacia (não incluindo a cabeceira comum entre os rios Sapão e Galheiros não pode ser de forma alguma comparada à famosa conexão existente entre os rios Negro e Orinoco, o Rio Casiquiare, que é um grande efluente do Rio Orinoco que desemboca no Rio Negro, e que efetivamente conecta os trechos superiores desses dois grandes sistemas hidrográficos (Winemiller et al 2008), sendo apenas uma área de cabeceira comum.…”