Storage or Run-of-river Reservoirs: Exploring the Ecological Effects of Dam Operation on Stability and Species Interactions of Fish Assemblages.

Water level variation has an important role in the biology of fish species, driving behavior, feeding, and reproduction both in natural and modified environments. In reservoirs, different dam operation schemes result in alternative patterns of water level fluctuations. Storage (STR) reservoirs accumulate water and can vary the water level unpredictably, whereas this variation is more discrete in run-of-river (ROR) reservoirs. For this reason, ROR reservoirs are commonly presumed to be less environmentally harmful than STR reservoirs. We used multivariate autoregressive models (MAR) to compare the stability and species interactions of fish assemblages from two reservoirs under alternative operation schemes, using long-term data (15 years). We hypothesized that the lower variability of water level in the ROR reservoir would coincide with a more stable fish assemblage than in the STR reservoir. Contrary to our expectation, the MAR properties related to resilience and resistance indicated that the fish assemblage from the ROR was less stable than that from the STR reservoir. This suggests that the absence of water level variation limits the potential direct (movement and reproduction of fish) and indirect (primary production and nutrient input) benefits for fish that arise from the temporal environmental heterogeneity. Most importantly, this study highlights the need to reexamine the implications of ROR reservoirs on the health of aquatic communities. At least for fish, management actions should include varying the water level in a regime as similar as possible to the natural flow regime of the river, in order to improve the state of assemblages.

Connectance and nestedness as stabilizing factors in response to pulse disturbances in adaptive antagonistic networks.

Understanding how network architectures are related to community robustness is essential to investigating the effects of disturbances on biological systems. Regarding the perturbations that are observed in disturbance regimes, frequency and intensity are two main descriptors, specifically for those events with short duration. Here, I used the architecture of 45 real-world weighted bipartite networks to assess whether network size, connectance, and nestedness are related to the effects of pulse disturbances in antagonistic communities. Networks were simulated under five scenarios with different combinations of frequency and intensity of perturbations. The dynamics of resource-consumer interactions followed the adaptive interaction switching behavior, which is the key topological process underlying most of the architectures of antagonistic webs. As opposed to most studies considering the effects of disturbances as species extinctions explicitly, the effects of disturbances here were modeled as changes in the abundance of consumers following immediate reductions in the abundance of resources. Simulations revealed that community robustness to pulse disturbances increased with both connectance and nestedness overall, with no effect of network size. Community networks with highly connected and nested topologies were more robust to disturbances, particularly under high frequency and intensity perturbations. By considering disturbances that are not directly related to species' extinctions, this study provides valuable insights that connectance and nestedness have an important stabilizing role in ecological networks.

Is coexistence between non-native and native Erythrinidae species mediated by niche differentiation or environmental filtering? A case study in the upper Paraná River floodplain

The limiting similarity theory predicts that divergence in the functional traits of native and introduced species is an essential component in species establishment, as introduced species must occupy a niche that is unoccupied by resident species. On the other hand, the environmental filtering hypothesis predicts convergence between introduced and native species, as both possess traits that make them adapted to the local abiotic environment. Morphology, spatial co-occurrence, diet, feeding selectivity, and niche breadth and overlap of Erythrinidae were evaluated to detect possible mechanisms acting in the coexistence between non-native and native species. Native (Hoplias sp. B and Hoplias cf. malabaricus) and non-native (Hoplerythrinus unitaeniatus and Hoplias mbigua) species presented differences in morphological traits, spatial co-occurrence, diet, selectivity, and niche breadth and overlap. The mechanisms mediating species coexistence seem to vary according to species. The absence of spatial and feeding overlap suggests that non-native species H. unitaeniatus occupy a different niche than native species, supporting its successful establishment without eliminating the native species. However, low feeding overlap and similar morphologies between non-native and native species of Hoplias point to environmental filters; in this case, the non-native H. mbigua is able to establish due to similarities in functional traits.(AU)

Teorias sobre nicho ecológico afirmam que divergência em traços funcionais entre espécies nativas e introduzidas são essenciais ao estabelecimento da espécie introduzida, pois estas devem ocupar um nicho não utilizado pelas espécies residentes. Por outro lado, a teoria de filtros ambientais afirma a convergência entre espécies introduzidas e nativas, pois ambas possuem traços funcionais que fazem com que essas espécies estejam mais adaptadas as variáveis ambientais. Foram avaliadas a morfologia, ocorrência espacial, dieta, seletividade alimentar, sobreposição e largura de nicho em Erythrinidae para detectar possíveis mecanismos atuando na coexistência de espécies nativas e não nativas. Espécies nativas (Hoplias sp. B e Hoplias cf. malabaricus) e não nativas (Hoplerythrinus unitaeniatus e Hoplias mbigua) apresentaram diferenças em todos os atributos testados. Os mecanismos mediando a coexistência de espécies nativas e não nativas parecem variar de acordo com a espécie considerada. A ausência de sobreposição espacial e na dieta sugere que a espécie não nativa H. unitaeniatus ocupa um nicho distinto que as espécies nativas, favorecendo seu estabelecimento sem eliminar as espécies nativas, apontando para segregação de nicho. Por outro lado, morfologias similares, porém com diferentes dietas foram observadas para as espécies de Hoplias, sugerindo que as espécies coexistem devido à filtros ambientais, assim, neste caso, a espécie não nativa H. mbigua é capaz de se estabelecer no novo ambiente devido à similaridades em traços funcionais.(AU)