Seasonal variation in fish trophic networks in two clear-water streams in the Central Llanos region, Venezuela

Food webs are not static entities; consumer resource interactions vary in both time and space, which complicates depiction and comparisons of food web structures. We estimated fish assemblage structure and diets in two clear-water streams in the Venezuelan Llanos region (Charcote and Charcotico) and constructed trophic networks (sub-webs defined by fishes as the principal consumers) during four periods of the annual flood pulse. As stream conditions changed from high-water to low-water, we hypothesized that: 1) the piscivore-non-piscivore ratio would increase; 2) dietary diversity would decrease; 3) interspecific dietary overlap would decline; 4) fewer allochthonous food items would be consumed; and 5) food-web connectance would increase. The piscivore-non-piscivore abundance ratio was higher in both streams during the low-water period. Dietary diversity declined as water levels dropped and availability of aquatic habitats and resources declined, but interspecific dietary overlap was not lower. Contrary to our hypothesis, average interspecific dietary overlap increased at Charcote as the dry season progressed, even though dietary overlap among species was significantly lower than expected by chance. We did not find strong support for our hypotheses regarding seasonal patterns of consumption of allochthonous resources and food web connectance, both of which revealed little seasonal variation.(AU)

Redes alimentares não são entidades estáticas; interações entre consumidores e recursos variam no tempo e no espaço, o que complica as representações das estruturas de redes alimentares. Nós estimamos a estrutura da assembléia de peixes e dieta em dois riachos de água clara na região venezuelana dos Llanos (Charcote e Charcotico) e construímos redes tróficas (sub-redes definidas por peixes como os principais consumidores) para quatro períodos do pulso anual de inundação. À medida que as condições dos riachos mudam de águas altas para águas baixas, nós hipotetizamos que: 1) a razão piscívoros e não piscívoros irá aumentar; 2) a diversidade da dieta irá diminuir; 3) a sobreposição alimentar interespecífica irá diminuir; 4) menos itens alimentares alóctones serão consumidos; e 5) a conectância da teia alimentar irá aumentar. A razão da abundância de piscívoros e não piscívoros foi maior em ambos riachos durante a estação de águas baixas. A diversidade da dieta declinou à medida que as águas baixavam e a disponibilidade de habitats aquáticos e recursos declinavam, mas a sobreposição alimentar interespecífica não foi menor. Contrária à nossa hipótese, a média de sobreposição alimentar interespecífica aumentou no Charcote à medida que a estação cheia progrediu, mesmo com a sobreposição alimentar significativamente menor do que o esperado ao acaso. Não encontramos forte suporte para nossas hipóteses relacionadas aos padrões sazonais de consumo de recursos alóctones e conectância da cadeia trófica, os quais revelaram pouca variação sazonal.(AU)

[The model of development of science by the example of limnology].

Limnology--the science about lakes is the young and relatively closed area of studies; its existence is owing to several hundreds of scientists. The International Society of Limnologists holds its meetings since 1922. We used materials of these meetings to find out the main stages of development of this science; among these stages there were both fast and relatively calm periods. Based on analysis of these data, we constructed a model of development of the science, the same data being used for tuning and verification of the model. We have suggested that the main regularities and of development of limnology can be extrapolated to other sciences. The main "acting person" in the model is population of scientists. Each scientist, with some probability, can propose new ideas as well as use in his elaborations some particular complex of the already accumulated knowledge and ideas. The model also takes into consideration how the scientific information is spreading, specifically some individual peculiarities of model scientists, such as age, experience, communicability. After the model parameters had been chosen in such a way that is described adequately the development of limnology, we performed a series of experiments by changing some of the characteristics and obtained rather unexpected results published preliminary in the short work (Levchenko V. F and Menshutkin V. V. Int. J. Comp. Anticip. Syst., 2008, vol. 22, p. 63-75) and discussed here in the greater detail. It is revealed, that the development of science is passing irregularly and sharply decelerated at low level of scientists communication and absence of scientific schools, and that the age of "scientific youth" of scientist begins usually only after 40 years.