The sensitivity of complex dynamic food webs to the loss of top omnivores.

Theoretical studies have attempted to predict the effects of losing species and their relative importance to community-level processes, but little attention has been paid to specific interaction types such as omnivory and the trophic level of extinct species. Here, we use complex food web models to assess the importance of omnivores to community-level stability. We simulated food webs with varying size and complexity so we could remove omnivores from different trophic levels and track the fates of communities. Results show that food webs were more sensitive to the loss of omnivores from higher trophic levels, aside from the effects of network size and complexity on food web stability. These results underline the importance of omnivory by introducing dynamic switching across multiple energy paths. We expect this paper to shed light on additional factors that are fundamental when modelling extinctions such as the trophic level of lost species.

Adjustments in population and reproductive dynamics of native and non-native congeneric species during 26 years after invasion

We aimed to improve the understanding of the establishment of Serrasalmus marginatus (non-native), which was followed by a decrease in the abundance of Serrasalmus maculatus (native) in the upper Paraná River floodplain. We estimated age, mortality rate, length and age at first maturity and variations in gonad development in three time-periods along a 26-year truncated time scale for both species. Population and reproduction parameters of both species showed substantial fluctuations among periods. Most age classes were sampled in all time-periods, but with considerable difference in abundance, with predominance of older individuals in the second time-period and younger individuals in the third time-period for both species. The mortality rates decreased for both species in the second time-period, but increased for the native in the third time-period. Length and age at first maturity decreased in the second time-period for both species, increasing the number of mature individuals on their populations. In the third time-period, the number of immature individuals increased for both species. We suggest that species experienced stressful conditions during cooccurrence and this have resulted in physiological responses in both species, reflecting in population and reproductive adjustments that may have relaxed competitive interactions between them, optimizing survival, reproductive effort and coexistence.(AU)

Nosso objetivo foi aumentar o conhecimento sobre o estabelecimento de Serrasalmus marginatus (não nativa), que foi seguido pela diminuição na abundância de Serrasalmus maculatus (nativa) na planície de inundação do alto rio Paraná. Estimamos a idade, taxa de mortalidade, comprimento e idade de primeira maturação e variações no desenvolvimento gonadal em três períodos ao longo de 26 anos para as espécies. Os parâmetros populacionais e de reprodução das espécies mostraram flutuações substanciais. A maioria das faixas etárias foi amostrada em todos os períodos, mas com diferenças consideráveis na abundância, predominando indivíduos mais velhos no segundo período e indivíduos mais jovens no terceiro período para ambas as espécies. A taxa de mortalidade diminuiu para as espécies no segundo período, mas aumentou para a espécie nativa no terceiro período. O comprimento e a idade de primeira maturação diminuíram no segundo período para as espécies, aumentando o número de indivíduos adultos. No terceiro período, o número de indivíduos imaturos aumentou para ambas as espécies. Sugerimos que as espécies passaram por condições estressantes durante a coocorrência, resultando em respostas fisiológicas que desencadearam ajustes populacionais e reprodutivos que podem ter minimizado a interação competitiva entre elas, otimizando a sobrevivência, o esforço reprodutivo e a coexistência.(AU)

Effect of land-use types on the ecomorphological structure of fish assemblage in distinct mesohabitats of neotropical streams / Efeito dos tipos de uso do solo na estrutura ecomorfológica da assembleia de peixes em distintos mesohábitats de riachos neotropicais

Abstract The use and occupation of land by human population substantially influence environmental variables and fish assemblage in streams. However, there is little knowledge on how these changes affect the ecomorphological structure of fish assemblage in mesohabitats. Therefore, we aim to assess whether the land-use types affect the ecomorphological structure of fish assemblage in distinct mesohabitats. Environmental and ichthyofaunistic data were collected in three mesohabitats (rifles, runs, and pools) of five rural and five urban streams. Twenty-one ecomorphological indices were obtained from the mean of linear morphological measurements and areas of the fishes. Subsequently, the Euclidean distance was calculated, based on the ecomorphological indices, between each pair of species, to measure the ecomorphological distances for the mesohabitats of the rural and urban streams. The results show that the urban environment is more harmful to streams than the rural one, due to changes in the environmental variables and decrease in species richness. The main environmental changes found in urban streams were the decrease in canopy cover by riparian vegetation and dissolved oxygen, and the increase in electrical conductivity and bed silting. Also, there was a significant decrease in the morphological similarity between fish species in the mesohabitats of urban streams compared to rural ones. Therefore, we can conclude that the urban environment leads to the loss of morphologically similar fish species in the mesohabitats, with only a few functionally distinct species remaining.

Resumo O uso e ocupação do solo pela população humana influencia substancialmente as variáveis ambientais e a assembleia de peixes em riachos. No entanto, há pouco conhecimento de como estas alterações afeta a estrutura ecomorfológica da assembleia de peixes em mesohábitats. Portanto, objetiva-se avaliar se o tipo do uso do solo afeta a estrutura ecomorfológica da assembleia de peixes nos distintos mesohábitats. Dados ambientais e ictiofaunísticos foram coletados em três mesohabitats (corredeiras, rápidos e remansos) de cinco riachos rurais e cinco urbanos. Vinte e um índices ecomorfológicos foram obtidos a partir das médias das medidas morfológicas lineares e áreas dos peixes. Posteriormente, a distância Euclidiana foi calculada, baseada nos índices ecomorfológicos, entre cada par de espécies, afim de mensurar as distâncias ecomorfológicas para os mesohábitats dos riachos rurais e urbanos. Os resultados mostram que o ambiente urbano é mais danoso aos riachos do que o rural, devido a diminuição da riqueza de espécies e as alterações nas variáveis ambientais. As principais mudanças ambientais encontradas em riachos urbanos foram a diminuição da cobertura do dossel pela vegetação ripária e do oxigênio dissolvido e o aumento da condutividade elétrica e do assoreamento do leito. Além disso, houve uma diminuição significativa da similaridade morfológica entre as espécies de peixes nos mesohábitats de riachos urbanos em comparação com os rurais. Portanto, podemos concluir que o ambiente urbano leva à perda de espécies de peixes morfologicamente similares nos mesohábitats, restando apenas algumas espécies funcionalmente distintas.

Population structure and reproductive biology of Astyanax gymnodontus (Characiformes: Characidae), a poorly known endemic fish of the Iguaçu River basin, Brazil.

Success in fish breeding depends on reproduction intensity, periodicity and the place where it occurs. Information about fish species reproduction is important to assist managers, and to determine conservation and management strategies. The fish assemblage of the Iguaçu River basin is already known for its high endemism, and despite this privilege, the large number of dams built along it, threat this particular biodiversity. Astyanax gymnodontus is an endemic fish species and studies on its population structure and reproductive biology are important, since they represent the first step for further community studies. Our objective was to evaluate some aspects of the population structure and reproductive biology of A. gymnodontus in the influence area of Salto Santiago dam, Iguaçu River, Paraná State, Brazil. Sampling was made monthly from July 2003 to June 2005, and bimonthly from July 2005 to March 2013, at five sites in the influence area of Salto Santiago dam. Fishes were collected using 10 m length gillnets with meshes ranging from 2.5 to 6.0 cm between non-adjacent knots and trammel nets with inner meshes of 6.0 cm between non-adjacent knots. Nets were arranged on surface, bottom and margins of each site, exposed for 24 h. Additional drags on littoral areas were performed from January to March and October to December from 2009 to 2011, with 50.0 m nets, 0.5 cm mesh size, for juveniles capture. We captured and analyzed 21 932 individuals, being 9 249 females and 12 683 males, representing 42.2 % and 57.8 %, respectively. The average body length was 8.8 cm for females and 8.3 cm for males. The average weight was 18.8 g for females and 16.0 g for males. Sex ratio calculated for the entire period was 1.8 males/female. Males were more abundant than females in 73.2 % of samples and significant differences were observed in 35.3 % of samples. The estimated length at first maturity (SL(50)) was 6.4 cm for females and 6.2 cm for males. We suggest that sexual differences in body length and weight, and sex ratio occurred as a result of sexual differences in energy allocation. Females spend more energy on body growth, which means more eggs production and higher fecundity. As an integrated response, males spend more energy on sperm production, in competition with other males, leading to a smaller body size. Differences in estimated SL(50) might be an indicator of this sexual difference in energy allocation. The reproduction period was from September to February, with greater intensity at the beginning of this period, and the sites of highest reproductive intensity were those immediate downstream from dams, and therefore with more riverine characteristics.