Spatiotemporal segregation regulates species richness along biomass gradients in estuarine fish assemblages

Abstract

The number of fish species in subtropical and tropical estuaries is much greater than in temperate regions and is greater in large estuaries than in small ones. Thus, large estuaries of the tropics are of primary interest for biodiversity conservation, and the processes underpinning the distribution of fish diversity in these ecosystems deserve attention. Factors controlling biodiversity patterns have been the focus of numerous recent investigations and include species interactions, speciation, environmental gradients and heterogeneity, with all of these operating differently at each observation scale. Today, the question is no longer whether particular biodiversity patterns occur, but why they occur and whether they are consistent across scales and organisms under study. In this study we combined the results of 2 classical ecological patterns, namely diversity-biomass relationships and species co-occurrences, into a common framework. Fish sampling was carried out in the coastal Terminos Lagoon (southern Gulf of Mexico) during 2 periods (1980-1981 and 1998-1999) using 408 tows. A total of 22 834 individuals representing 106 fish species were collected. Our study revealed that both diversity-biomass relationships and species co-occurrences show inconsistent patterns across years, seasons and zones. However, taken together, our results show that C-score values, which measure spatiotemporal segregation among species, were closely related to the shape of observed richness-biomass relationships. When fish assemblages showed positive C-score values (species segregation), the relationship between species richness and fish biomass was either hump-shaped or non-significant. Conversely, negative C-score values (species aggregation) were associated with an increase in fish species richness along the total biomass gradient. Our results support the idea that species segregation due to competition or spatiotemporal partitioning can be an important determinant of biodiversity patterns in species-rich tropical fish assemblages, but only during some periods and for some spatial strata. © Inter-Research 2007.