Low Functional β-Diversity Despite High Taxonomic β-Diversity among Tropical Estuarine Fish Communities

The concept of beta-diversity, defined as dissimilarity among communities, has been widely used to investigate biodiversity patterns and community assembly rules. However, in ecosystems with high taxonomic beta-diversity, due to marked environmental gradients, the level of functional beta-diversity among communities is largely overlooked while it may reveal processes shaping community structure. Here, decomposing biodiversity indices into alpha (local) and gamma (regional) components, we estimated taxonomic and functional beta-diversity among tropical estuarine fish communities, through space and time. We found extremely low functional beta-diversity values among fish communities (<1.5%) despite high dissimilarity in species composition and species dominance. Additionally, in contrast to the high alpha and gamma taxonomic diversities, alpha and gamma functional diversities were very close to the minimal value. These patterns were caused by two dominant functional groups which maintained a similar functional structure over space and time, despite the strong dissimilarity in taxonomic structure along environmental gradients. Our findings suggest that taxonomic and functional beta-diversity deserve to be quantified simultaneously since these two facets can show contrasting patterns and the differences can in turn shed light on community assembly rules.