PARTITIONING AMONG NICHE AND NEUTRAL EXPLANATIONS FOR METACOMMUNITY PATTERNS IN CERRADO STREAM FISH COMMUNITIES

The Species Sorting concept, one of the models developed to explain patterns in metacommunity structure, suggests that relationships between biological communities and environmental conditions is the basic means of the species selection processes. A second concept is neutral theory, and the idea of neutral dynamics underpinning metacommunity structure, cannot be overlooked. The third mechanism is the Mass Effect concept, that focuses on the interaction between environmental condition and neutral effects. In the present study, we partitioned fish communities in streams between niche and neutral theory concepts, identifying the best representation of metacommunity structure, and assessed if linear and hydrographic distance were equivalent in the representation of neutral processes. The result points to the importance of species sorting mechanisms in structuring fish communities with neutral processes best represented by the linear distances. On the other hand, the best representation of species’ niches was achieved with average values and variance of the local conditions.

linear distance between all streams (Appendix 2); ii) Local W (Appendix 1 -S1 -LW), 136 defined by the linear distance between streams present in the same hydrographic unit. 137 Streams in different units had no interaction and connectivity values equal to zero 138 (Appendix 2); iii) Water W (Appendix 1 -S1 -WW), defined by the hydrographic 139 distance. The same way as in Local W, streams in different units had no interaction and 140 connectivity values equal to zero (Appendix 2); and iv) W Hydalt (Appendix 1 -S1 -141 HW), defined by the hydrographic distance between the points weighted by the slope 142 (Appendix 2). 143 We measured ecological diversity using two distinct metrics: (i) species richness and 144 (ii) beta diversity. Species richness was defined as the number of species present at the 145 site of interest. The beta diversity was calculated according to the procedure described 146 by Baselga [38], which defines beta diversity as the Sorensen dissimilarity index.

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Diversity indices were calculated for both the total fish community and for different 148 feeding guilds (i.e. detritivores, insectivores and omnivores).We used generalized linear 149 models (GLM) to identify the best way to represent environmental conditions for 150 streams fish communities. We compared models of community diversity indices and the 151 average of the environmental conditions with a second model including the average 152 together with the standard deviations of environmental conditions as predictor variables.

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This procedure was adopted to identify the best way to describe environment 154 conditions, the average or variance. The GLM models was performed using all W 155 matrices (Table 1) to identify the best means to represent neutral processes (linear or 156 hydrographic distances). To avoid multicollinearity, a Principal Component Analysis of explanation of the PCA being used as predictor variables on the models. After 160 identifying the best way to represent environmental conditions (mean or mean with 161 standard deviation), GLMs were performed for each trophic guild and all W matrices.

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This procedure was used to identify if results found for the whole community were 163 equivalent to those found for individual feeding guilds.
164 the geographic barriers into the linear distance procedure (called Local W here), the 247 linear distance was able to provide a good representation of the geographic patterns of 248 this fish community. The performance of Local W is better than the dendritic distance 249 (W Water), shown previously as the best means to represent spatial processes in aquatic 250 systems. Using Local W, we were able to identify a simple and robust way of 251 representing spatial processes considering the physical barriers separating communities.

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This result was consistent across all components of the community (i.e. overall 253 community or discrete feeding guilds) and ecological descriptors (i.e. species richness 254 or beta diversity.

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A second feature of our results is related to the dispersion of detritivorous fish, 256 which cannot be represented using simple euclidian distances between points, 257 considering or not considering geographic barriers, or hydrological distances. The best 258 way to represent space process in this guild was with the application of Hydalt W.

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These models consider the dendritic distance between points (and barriers), and the 260 direction of the flow. The application of connectivity models associated with flow 261 direction is advocated by some authors as being the most suitable for aquatic organisms 262 [43,44]. However, with the analysis of our data, we observed that this approach, is more