The River Continuum Concept (RCC) assumes that autochthonous primary production in forest streams is limited by light and is insufficient to sustain secondary production by consumers; they must therefore depend on allochthonous carbon from the surrounding forest. Recent studies have, however, questioned the importance of allochthonous carbon in stream food webs. There is a growing body of evidence using stable‐isotope techniques that demonstrate the importance of algae (autochthonous production) in the food webs of tropical streams. The actual contributions of autochthonous and allochthonous resources are rarely evaluated accurately because few studies consider the diet and the trophic efficiencies of the components of the food web or measure primary and secondary production to estimate the energy flow. We estimated the annual net primary productivity of periphytic microalgae (NPP) and the secondary production of macroinvertebrates (SP) from empirical models and we used stable isotopes of carbon and nitrogen to quantify the flow of material in food webs of five forest streams in the Guapi‐Macacu catchment, Rio de Janeiro, Brazil. NPP ranged from 46 g to 173 g dry mass (DM) m−2, whereas SP ranged from 0.90 g DM m−2 to 2.58 g DM m−2. The contribution of allochthonous carbon to the SP was more important than autochthonous sources and varied from 56% to 74% of all basal energy flow assimilated by primary consumers. The annual ingestion rate of basal sources varied from 8.08 g DM m−2 to 26.57 g DM m−2, with the allochthonous material contributing 76% and 87% of this. The annual ingestion rate of autochthonous material varied from 1.2% to 5.5% of the NPP. The present work suggests that the principal energy source for macroinvertebrates in streams of the Guapi‐Macacu catchment came from the riparian forest, as predicted by the RCC. However, this dependence appeared not to be driven by an absolute lack of autochthonous NPP, which seemed more than sufficient to sustain the entirety of macroinvertebrate SP.
We investigated the niche breadth and overlap of the fish species occurring in four environments affected by the Coaracy Nunes reservoir, in the Amapá Brazilian State. Seasonal samples of fishes were taken using a standard configuration of gillnets, as well as dragnets, lines, and cast-nets. Five hundred and forty stomach contents, representing 47 fish species were analyzed and quantified. Niche breadth and overlap were estimated using indexes of Levins and Pianka, respectively, while interspecific competition was evaluated using a null model (RA3). ANOVA and the kruskal-Wallis test were used, respectively, to evaluate differences in niche breadth and overlap between areas. The data indicate that the majority of the fish species belong to the piscivore, omnivore, and detritivore guilds. These species have likely colonized the environments due to the availability of suitable feeding resources, and the favorable physical conditions created by the river damming. Overall, few species have ample niches, but most of them are highly specialized. Resources seasonal variation had little effect on the feeding behavior of most species in the study areas. The null models indicated that competition was not a factor determining on community structure.
We developed an Ecopath model in the Estuary of Sirinhaém River (SIR), a small-sized system surrounded by mangroves, subject to high impact, mainly by sugar cane industry and other farming industries in order to describe the food web structure and trophic interactions. In addition, we compared our findings with those of 20 available Ecopath estuarine models for tropical, subtropical and temperate regions, aiming to synthesize the knowledge on trophic dynamics and provide a comprehensive analysis of the structures and functioning of estuaries. Our model consisted of 25 compartments and its indicators were within the expected range for estuarine areas around the world. The average trophic transfer efficiency for the entire system was 11.8%, similar to the theoretical value of 10%. The Keystone Index and MTI (Mixed Trophic Impact) analysis indicated that the snook (Centropomus undecimalis and Centropomus parallelus) and jack (Caranx latus and Caranx hippos) are considered as key resources in the system, revealing its high impact in the food web. Both the species have a high ecological and commercial relevance, despite the unregulated fisheries. As result of the comparison of ecosystem model indicators in estuaries, differences in the ecosystem structure from the low latitude zones (tropical estuaries) to the high latitude zones (temperate system) were noticed. The structure of temperate and sub-tropical estuaries were based on high flows of detritus and export, while tropical systems have high biomass, respiration and consumption rates. Higher values of System Omnivory Index (SOI) and Overhead (SO) were observed in the tropical and subtropical estuaries, denoting a more complex food chain. Globally, none of the estuarine models were classified as fully mature ecosystems, although the tropical ecosystems were considered more mature than the subtropical and temperate ecosystems. This study is an important contribution to the trophic modeling of estuaries, Highlights ► Sirinhaém estuary, Northeast Brazil, is an immature and resilient ecosystem. ► The jack and, mainly, snook were key species in the Estuary of Sirinhaém River. ► Tropical estuaries were based in high biomass, respiration and consumption rates. ► The System Overhead Index was higher in tropical estuaries than in the other systems. ► TPP/TR was lower in tropical estuaries than in the other systems.
Fish and invertebrates are introduced in freshwaters around the world for commercial purposes, despite widely known impacts on food webs and biological invasions. As a proxy for artificial environments, we modeled a typical reservoir in a Brazilian semiarid region using an ecosystem approach. We compared the role of native and nonnative invasive species (NIS) in the food web, between dry and wet periods, and under the influence of an extreme drought period (from 2011 to 2015), simulating the variation in fish biomasses due to decreasing consumption. Key ecosystem groups were fishes (mainly NIS), birds, and insects. Nutrient cycling was dependent on invaders, while the trophic structure was detritus based during the drought. Biomass of detritivores was almost two times higher than herbivores, and native fish species decreased abruptly in response to invaders and volume variation. The dominance of low-trophic levels (TLII) and tilapia-Oreochromis niloticus (Linnaeus, 1758) and other tilapiines-resulted from interactions among invaders, feeding behavior on benthos, and environmental seasonality, tending toward biotic homogenization (''benthification'') at the ecosystem level. An increasing relevance of detritivores with cascading effects in ecosystems subject to drought, multiple
BackgroundEcosystem modeling applied to fisheries remains hampered by a lack of local information. Fishers’ knowledge could fill this gap, improving participation in and the management of fisheries.MethodologyThe same fishing area was modeled using two approaches: based on fishers’ knowledge and based on scientific information. For the former, the data was collected by interviews through the Delphi methodology, and for the latter, the data was gathered from the literature. Agreement between the attributes generated by the fishers’ knowledge model and scientific model is discussed and explored, aiming to improve data availability, the ecosystem model, and fisheries management.Principal FindingsThe ecosystem attributes produced from the fishers’ knowledge model were consistent with the ecosystem attributes produced by the scientific model, and elaborated using only the scientific data from literature.Conclusions/SignificanceThis study provides evidence that fishers’ knowledge may suitably complement scientific data, and may improve the modeling tools for the research and management of fisheries.
Recently, there is an increasing perception that the ecosystem approach gives important insights to support fisheries stock assessment and management. This paper aims to quantify energy flows in the Itaipu Reservoir (Brazil) and to simulate increase of the fishing effort of some species, using Ecopath with Ecosim software, which could allow inferences on stability. Therefore, two steady-state Itaipu models were built (1983-87 and 1988-92). Results showed that: a) there are no differences between models, and results on aging trends do not vary over time indicating that fishery does not alter the ecosystem as a whole; b) results of fisheries simulations are approximate to mono-specific stock assessment for the same species and periods; c) many authors believe that tropical ecosystems are environments where biotic and abiotic oscillations are annual and sometimes unexpected, but the results found for the Itaipu Reservoir indicate that stability was met after 16 years.Nos dias atuais, aumenta a percepção que a análise ecossistêmica fornece importantes esclarecimentos na avaliação e manejo de estoques pesqueiros. Este trabalho objetiva quantificar o fluxo de energia na teia trófica do reservatório de Itaipu e simular o aumento de pesca de algumas espécies usando o programa "Ecopath com Ecosim", que permite inferências sobre a estabilidade do sistema. Desta forma, dois modelos de "estado-estacionário" foram construídos (o primeiro para o período 1983-87 e o outro para 1988-92). Os resultados mostraram que: a) não há diferenças entre os modelos e os resultados sobre tendências de amadurecimento não variam com o tempo indicando que a pesca não altera o sistema como um todo; b) os resultados de simulação de pesca se aproximam de avaliações mono-específicas, realizadas por outros autores, com dados de mesmas espécies e períodos; c) muitos autores acreditam que reservatórios tropicais são ambientes submetidos a oscilações antrópicas que alteram grandemente sua dinâmica dificultando a estabilidade, mas os resultados para o reservatório de Itaipu indicam que a estabilidade foi encontrada 16 anos após a sua formação.
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