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1. Alien species often flourish and become invasive in urban ecosystems. How and why invaders succeed in urban systems is an important, yet poorly understood, question. We investigate whether the success of urban invaders is related to changes in species traits that enhance invasive potential. We also explore whether This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Length-frequency analyses showed that male HypostomuS ufinis are bigger than femalcs, but H. luetkeni showed no such differences. The sex ratio was estimated at 1 : 1 for both species.The temporal variation of sex ratio indicated a decrease in the male population when GSI was at its maximum value, suggesting that males are less prone to capture during the spawning season. According to the decrease of males during the reproductive period, paternal behaviour is proposed for both species. H. ufinis reached sexual maturity at a smaller size than H. luefkeni. Considering the length weight relationship and the mortality rate of the reproductive females in both species, the physiological consequences of delay or precocity of the first maturation are discussed. 9' 1995 Thc Fisherics Socicty of the British Isle.;Kcy words: size structure; sex ratio; onset of maturity; Hypostomus spp.; Brazil

Functional diversity of fish communities has been measured according to (a) interspecific variability, assuming that intraspecific variability is negligible, or (b) morphological differences, as good descriptors of complex functions, such as diet. These two assumptions have been scarcely tested on the individual level, especially in species‐rich tropical ecosystems. Here, we adapted intraspecific specialisation (ISpe) and intraspecific originality indices (IOri) to assess complementary components of intraspecific variability. Next, we applied these indices to evaluate the intra‐ and interspecific variability of morphological and diet traits in two contrasting Brazilian stream‐dwelling fish assemblages (rainforest and savannah). We also compared correlations between morphology and diet at the individual and species level to test whether accounting for intraspecific variability increases the predictability of diet due to morphological differences. Significant contributions of intraspecific variability to differences between fish were revealed for morphology and diet. Intraspecific variability in the diet was higher than that in morphology in both assemblages. The ISpe was positively correlated to IOri in the diet of both ecosystems. The morphological–dietary relationships were significant but weak at both individual and species levels. Our findings highlight the importance of measuring individual variability and accounting for complementary components of the intraspecific variability (ISpe and IOri). Importantly, we showed that the variability in morphology does not predict diet variability at both intra‐ and interspecific levels. Thus, high intraspecific variability in morphology and diet challenges the use of functional traits measured at the species level to describe the functional diversity of different fish assemblages.

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