We assessed the relationship between habitat heterogeneity and bird species richness and composition within wetlands of the floodplain of the Middle Paraná River, Argentina. Given the high habitat heterogeneity in these wetland systems, we sought to determine whether (i) there was a positive relationship between bird species richness and habitat heterogeneity; (ii) whether bird species richness was associated with certain types of individual habitat types; (iii) whether there was a pattern of species nestedness and turnover between sites as a function of habitat heterogeneity and composition, respectively; and (iv) whether individual species exhibited associations with habitat heterogeneity. Point counts were used to survey birds at 60 sites. We estimated the area of eight habitat types found within a 200-m radius from the centre of each site and calculated number and Pielou's evenness of habitat types. These indices, together with area proportion of each habitat type, were used as explanatory factors of bird species richness in linear regression models. Habitat heterogeneity per se rather than area of individual habitat types was a more important predictor of species richness in these fluvial wetlands. Sites with more habitat types supported more bird species. Results showed that individual bird species were associated with different habitat types and, therefore, sites that contained more habitat types contained more species. Number of habitat types accounted for species nestedness between sites whereas composition of habitat types accounted for species turnover between sites. Results suggest that selection of heterogeneous sites by individual species could help explain the positive heterogeneity-species richness relationship. Our findings highlight the importance of habitat heterogeneity per se resulting from flood disturbances in maintaining bird richness in fluvial systems.

Responses of waterbirds to habitat variation could account for their responses to fluctuations in river levels because hydrological fluctuations influence habitat availability across floodplains.Relationships between water level and waterbird occurrence were examined in floodplain wetlands of the Middle Paraná River to assess (a) whether occurrences of waterbird species were associated with water-level fluctuations of the river, (b) which habitats were associated with species that showed a relationship with water level, (c) the influence of water level on these habitats, and (d) whether influence of water level on these species was related to water-level influence on habitats. Through the use of regressions and structural equation models, we assessed whether direct relationships between each species and water level remained important after considering the influence of habitat variation. Of 21 species analysed, occurrences of 12 species showed an association with water-level fluctuations. Indirect effects of water level through habitats fully accounted for this association in 5 species. Variation in habitat conditions did not, however, fully account for responses of 6 species, suggesting that although habitat variation can be an important factor, other variables are necessary to explain responses of some species to water level. One species was not associated with any habitat and therefore was not included in this analysis. Our results agreed with the idea that an important fraction of waterbird responses to hydrological fluctuations is related to the effects of these fluctuations on the availability of habitats across the floodplain. Our results provide data that help delimit groups of waterbird species that respond in similar ways to hydrological fluctuations.

Study of temporal variation of bird assemblages may be particularly important in highly dynamic ecosystems as fluvial wetlands to identify the factors that influence and contribute to the maintaining of regional diversity. We study temporal variation in regional bird species composition and richness and the factors that drive it in the fluvial system of the Paraná River in Argentina. We sampled birds on a regional scale by performing surveys during 2 years (2011-2013; 16 total surveys) at 60 sites located along the fluvial system. We recorded a total of 162 species and 12,738 individuals, with a mean of 72.7 ± 1.9 species and 796.1 ± 41 individuals per survey. Temporal beta diversity accounted for 57% of regional species richness. Bird composition varied seasonally because of the turnover (and not nestedness) of species between autumn-winter and spring-summer periods; migrant species contributed to but did not account for all these differences. Difference in bird composition between years was caused primarily by turnover of species within autumn-winter periods; that turnover was related to changes in water levels. Temporal variation in availability of habitats across the regional extent of the fluvial system accounted for the highest proportion of temporal beta diversity. Species richness did not vary seasonally nor was it related to fluctuations in water level or habitats. Our results showed that climatic seasonality and environmental dynamics of the fluvial system, as a function of water-level fluctuations and of variations in habitat availability, jointly influence regional composition of birds through temporal turnover of species. These variations in regional composition were not related to variations in regional species richness. These results show the importance of the natural dynamic of the fluvial system to conserve the processes that maintain the regional assemblage of birds.