In nature, two populations may interact in di¡erent ways during their lifetime, and even undergo transitions from one type of interaction to another. A model for the dynamics of these transitions has been developed in this study. The interaction coe¤cients ij in the Lotka^Volterra equations are re-interpreted as nonlinear functions of population densities N i , N j , modulated by environmental parameters, which o¡ers the possibility of a change in sign. Transitions can take place owing to variations in population density (endogenous e¡ect), or in the environmental parameters (exogenous e¡ect). Models for both facultative and obligate associations are examined. Graphical stability analyses show that multiple density equilibria are possible, accounting for the occurrence of the transitions.

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Biological invasions have a great impact on biodiversity and ecosystem functioning worldwide.

The main trophic interrelations were analysed in 12 species which represent 93% of the total catch from a beach seine fishery developed in Celestún, Yucatán. The diet was determined based on monthly samples over 4 yr. Five different trophic guilds were found on the basis of feeding preferences: microcrustacean and polychaete feeders, piscivores, omnivores and herbivores. For microcrustacean feeders Orthopristis chrysoptera, Eucinostomus gula, E. argenteus and Opisthonema oglinum the diet is narrow with a large overlap between. For polychaete feeders, Arius felis, A. melanopus and Haemulon plumieri diet is broad with the higher overlap between the catfishes. Among piscivores, Bardiella chrysura and Harengula jaguana have a high overlap between them, but a lower one with other species from the community. Among omnivores Lagodon rhomboides has the broadest diet and is considered to be a generalist while Acanthostraceon quadricornis is a specialist having a very narrow diet and a lower value of overlap with the other species. Finally, Archosargus rhomboidalis is a herbivore with the narrowest niche breadth and lowest overlap. In addition to a description of diets and trophic inter‐relations, the results emphasize the importance to the beach seine fishery of the benthic food resources.

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