Changes of taxonomic and trophic structure of fish assemblages along an environmental gradient in the Upper Beni watershed (Bolivia)

Abstract

The distribution and the diet of 28 fish species were evaluated, during the dry season, in 12 streams of the Upper Beni watershed (Amazon basin, Bolivia). The 12 streams were of similar size (stream width and water depth) but situated on a gradient of altitude in the Andean and sub-Andean areas. The environmental conditions in the stream changed in relation to the altitude. As altitude decreased, slope and water velocity also decreased, while temperature, conductivity, pH and the proportion of pools increased. Although the diets of the species were mainly based on two aquatic autochthonous food resources, invertebrates and sediment, species were classified into five trophic guilds: detritivores, algivores, piscivores, invertivores-omnivores and aquatic specialist invertivores. In all streams invertivores dominated or co-dominated with detritivores. The trophic structure of the assemblages, however, changed in relation to the environmental gradient. The fish species richness increased and the trophic composition became more diverse at lower altitudes, when slope decreased and temperature increased. At the same time, the relative number of invertivore species decreased, whereas the relative number of detritivore, algivore and piscivore species increased. Decreasing altitude appeared to play a role similar to increasing stream size along the longitudinal gradient. This could be explained by geomorphological and temperature variations that may generate environmental conditions favourable to an increase of productivity.

Dietary-morphological relationships in fish assemblages of small forested streams in the Bolivian Amazon

“…Invertivorous species composed the dominant guild of this assemblage. This result is in agreement with the ones already obtained for other tropical (Angermeier and Karr 1983;Bojsen and Barriga 2002;Silva 1993;Uieda et al 1997;Deus and Petrere-Junior 2003;Pouilly et al 2006;Ibanez et al 2007) and temperate forested streams (Schlosser 1982;Rahel and Hubert 1991;Oberdorff et al 1993Oberdorff et al , 2002 suggesting a possible convergence in trophic structure between temperate and tropical fish assemblages ). Trophic diversity of fish assemblages in such systems may be strongly related to food availability (Angermeier and Karr 1983), which in turn may be influenced by common environmental constraints.…”

“…3). Benthic fishes from the algivorous and mud feeder guilds were also characterized by relatively narrow heads and a ventral (Loricaridae) or oblique (Curimatidae) mouth orientation and this result is again well supported by other studies (Gatz 1979;Watson and Balon 1984;Winemiller et al 1995;Hugueny and Pouilly 1999;Pouilly et al 2006). Fishes from the herbivorous and piscivorous guilds generally displayed a large standard length while species from the aquatic invertivorous guild were mostly characterized by their small sizes.…”

“…This suggests that even if morphology may limit patterns of resource-use, these limits are broad enough to allow most fishes changing their choice of prey resources in response to local biotic and abiotic conditions. For example, Pouilly et al (2006) have analyzed the trophic structure of fish species in other neotropical streams of the Beni River basin of the Bolivian Amazon. We compared the trophic status and the diet breath (using Levin's standardized index) of four species common to both studies (i.e.…”

The contribution of environmental factors to fish assemblages in the Río Acahuapa, a small drainage in Central America

“…These changes of species composition along the altitudinal gradient of the river may respond to a gradient of environmental factors or changes in the dynamics of the habitat structure along the river. Although our environmental variables may be similar to other studies in small streams or tropical rivers (Pouilly et al, 2006), our data show a difference with respect to river velocity, whose values are high in the intermediate and low areas of the basin, while low areas of rivers usually tend have calm waters. This can be explained because the Río Acahuapa has many tributaries in the intermediate and low areas (Hernández et al, 2010), which allows these unusual characteristics.…”

“…For example, some biotic factors may explain the flow and spatial distribution of some species along the basin, such as migrations along the fluvial system for breeding and mating or search for shelter (Schlosser, 1991;McDowall, 1992;Matthews, 1998;Lucas, Baras, 2001;McDowall, 2007). Also, trophic aspects may also explain associations of fish and can be related to environmental aspects in rivers (Pouilly et al, 2006). Therefore, it is important to consider these and other additional factors to complete information on the ecological dynamics in the Río Acahuapa basin.…”