In this study, we investigated the influence of environmental variables (predictor variables) on the species richness, species diversity, functional diversity, and functional redundancy (response variables) of stream fish assemblages in an agroecosystem that harbor a gradient of degradation. We hypothesized that, despite presenting high richness or diversity in some occasions, fish communities will be more functionally redundant with stream degradation. Species richness, species diversity, and functional redundancy were predicted by the percentage of grass on the banks, which is a characteristic that indicates degraded conditions, whereas the percentage of coarse substrate in the stream bottom was an important predictor of all response variables and indicates more preserved conditions. Despite being more numerous and diverse, the groups of species living in streams with an abundance of grass on the banks perform similar functions in the ecosystem. We found that riparian and watershed land use had low predictive power in comparison to the instream habitat. If there is any interest in promoting ecosystem functions and fish diversity, conservation strategies should seek to restore forests in watersheds and riparian buffers, protect instream habitats from siltation, provide wood debris, and mitigate the proliferation of grass on stream banks. Such actions will work better if they are planned together with good farming practices because these basins will continue to be used for agriculture and livestock in the future.

ABSTRACT. Feeding opportunism of Knodus moenkhausii (Teleostei, Characidae): an abundant species in streams of the northwestern in the state of São Paulo, Brazil. In the present investigation we studied the feeding biology of Knodus moenkhausii (Eigenmann & Kennedy, 1903) in streams of the Upper Paraná river system in the state of São Paulo. In eight streams (R1-R8), K. moenkhausii consumed 18 items, of which algae, ephemeropteran nymphs, and dipteran larvae were the most frequent and dominant autochthonous items, as were terrestrial insects, hymenopterans, and spiders regarding allochthonous items. In the R2 stream, K. moenkhausii showed distinct diet, mostly due to depth, substrate type, and presence of riparian vegetation. In the R9 stream (monthly sampled for one year), 15 items were consumed, of which terrestrial insects predominated all year round; dipteran larvae and algae were least relevant during December-January (hot and wet period) and June-July (cold and dry period), respectively. During snorkeling observations in this stream, we recorded individuals performing drift feeding, picking at relatively small preys close to the substrate and submerged vegetation, and surface picking. The large diversity of consumed items -conditioned to habitat and seasonal variations -and the diversity of feeding tactics allow us to consider K. moenkhausii a rather opportunistic fish species. Such opportunism apparently reflects on the species abundance, showing its ability to allocate a significant part of its energy to reproduction, even in physically impacted habitats by human actions.KEYWORDS. Nektonics, omnivory, feeding tactics, seasonality. (Eigenmann & Kennedy, 1903) em riachos do Alto rio Paraná no Estado de São Paulo. Em oito riachos (R1-R8), K. moenkhausii se alimentou de 18 itens, dos quais algas, ninfas de efemerópteros e larvas de dípteros foram os itens autóctones mais freqüentes e dominantes; fragmentos de insetos terrestres, himenópteros e aranhas foram os itens alóctones mais freqüentes e dominantes. No riacho R2, K. moenkhausii apresentou dieta distinta dos demais riachos, principalmente em função da profundidade, tipo de substrato e da presença de vegetação ripária. No riacho R9, amostrado mensalmente durante um ano, foram identificados 15 itens, dos quais insetos terrestres predominaram ao longo do ano; larvas de dípteros e algas foram pouco expressivas nos períodos de dezembro-janeiro (período mais quente e chuvoso) e junho-julho (período mais frio e seco). No riacho R9 foram realizadas observações subaquáticas durante mergulho livre, onde observamos a cata de itens na coluna d'água junto do substrato, da vegetação submersa e na superfície da água. A elevada variedade de itens consumidos -condicionada às variações do hábitat e sazonais -e a prática de diversas táticas nos permitem considerar K. moenkhausii uma espécie oportunista quanto ao uso dos recursos alimentares. Este oportunismo aparentemente se reflete na abundância da espécie, demonstrando boa capacidade em alocar parte significativa de su...

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The habitat use and feeding habits of a set of nektonic fish species often found in small low-gradient streams in Brazil were investigated. The core species in the present study was Knodus moenkhausii (Eigenmann & Kennedy, 1903), the most abundant of five species in the nektonic guild. Records of the species in 22 streams indicate that K. moenkhausii is associated, on a mesohabitat scale, with runs, and on a microhabitat scale, with sandy bottoms, intermediate depth, and open sites without coverage or submerged vegetation. During snorkeling observations, two additional nektonic registered species showed spatial segregation from K. moenkhausii with respect to foraging microhabitats. Feeding habits of the observed nektonic species were significantly correlated with riparian vegetation. The six species studied appear to use different resources, as suggested by a non-metric ordination of diet, and sites with characteristic riparian vegetation. The studied species may be considered generalists with a tendency to insectivory, with K. moenkhausii feeding on the widest variety of resources. Such opportunism explains its ability to successfully occupy instream habitats with low complexity

The loss of riparian forests can disrupt the structure and function of lotic ecosystems through increased habitat homogenization and decreased resource diversity. We conducted a field experiment and manipulated structural complexity and basal resource diversity to determine their effect on multiple aspects of community and food-web structure of degraded tropical streams. In-stream manipulations included the addition of woody debris (WD) and the addition of wood and leaf packs (WLP). The addition of structural complexity to degraded streams promoted detritus retention and had a positive effect on stream taxonomic richness, abundance and biomass. At the conclusion of the experiment, abundance and richness in the WD-treated reaches increased by over 110% and 80%, respectively, while abundance and richness in the WLP-treated reaches increased by over 280% and 170% respectively. Wood debris and leaves were consumed only by few taxa. Detritivorous taxa were the most abundant trophic guild at the beginning and at the end of the experiment. Food webs in treated reaches were relatively more complex in terms of links and species at the conclusion of the experiment, with highest maximum food chain length in the WD treatments and highest number of trophic species, links, link density, predators and prey at the WLP treatment. Despite differences observed in diet-based food webs, there was little variation in isotopic niche space, likely due to the high degree of omnivory and trophic redundancy, which was attributed to the importance of fine detritus that supported a broad range of consumers. Even in these degraded streams, aquatic taxa responded to the addition of increased complexity suggesting that these efforts may be an effective first step to restoring the structure and function of these food webs.