Influence of environmental conditions and the fragmented landscape on the co-occurrence patterns of the ichthyofauna of a stream in southwestern Brazilian Amazon

We assessed the spatiotemporal co-occurrence patterns of a fish assemblage in the watershed of a stream in the western Amazon basin, and verified the influence of abiotic factors (physicochemical variables and the hydrological phase) on these patterns. We also examined whether species that were more similar in their tolerance of physicochemical variables tended to co-occur more frequently. The structure of the assemblage was evaluated using the standardized effect size (SES) of the C-score index and null models, with the SES being used as an indicator of the organization of the assemblage. We employed linear regression models to investigate the influence of precipitation levels and physicochemical variables on the SES. We calculated the outlying mean index and ran a Mantel test on the checkerboard unit matrix of the C-score and tolerance values to assess the influence of species tolerance on co-occurrence patterns. Finally, we examined the correlations between species pairs and both physicochemical variables and adjacent land use. We observed patterns of aggregation or randomness in varying degrees at different scales. Increased precipitation influenced the establishment of random patterns, while species pairs correlated primarily with the degree of preservation of the local habitat at each study site. During the dry season, species that were more similar to each other in their environmental tolerance tended to co-occur more systematically. If the current levels of change in land use in the study area continue, the fish assemblage investigated here may suffer a significant loss of diversity, as well as destabilization of its structure, with a high possibility of local extinction of the most sensitive species.