Abstract
Identifying the environmental gradients that control the functional structure of biological assemblages in reference conditions is fundamental to help river management and predict the consequences of anthropogenic stressors. Fish metrics (density of ecological guilds, and species richness) from 117 least disturbed stream reaches in several western Iberia river basins were modelled with generalized linear models in order to investigate the importance of regional- and local-scale abiotic gradients to variation in functional structure of fish assemblages. Functional patterns were primarily associated with regional features, such as catchment elevation and slope, rainfall, and drainage area. Spatial variations of fish guilds were thus associated with broad geographic gradients, showing (1) pronounced latitudinal patterns, affected mainly by climatic factors and topography, or (2) at the basin level, strong upstream-downstream patterns related to stream position in the longitudinal gradient. Maximum native species richness was observed in midsize streams in accordance with the river continuum concept. The findings of our study emphasized the need to use a multi-scale approach in order to fully assess the factors that govern the functional organization of biotic assemblages in ‘natural’ streams, as well as to improve biomonitoring and restoration of fluvial ecosystems.
Highlights
Biomonitoring and stream restoration are important tools to reestablish the health of river ecosystems with the emphasis placed on the restoration of ecological processes and ecosystem functionality [1]
The question of which spatial scale reveals the strongest relationships among environmental variables and fish assemblages has attracted considerable interest, those conclusions may depend on the biological unit employed [8], the degree of anthropogenic landscape disturbance [7,11], or the geo-climatic heterogeneity of the studied area [9]
Fish Metrics To test whether the functional structure of fish assemblages responded to environmental gradients, we considered ten metrics related to the total density of fish guilds grouped into five ecological functions, following Noble et al [23]: 1) overall tolerance guilds, based on the ability of species to survive and reproduce in a wide range of natural environmental conditions (non-tolerant (NOTO), and tolerant (TOLE)); 2) trophic guilds, based on food items in the diet of adult individuals (invertivores (INVE) and omnivorous (OMNI)); 3) feeding habitat guilds, based on the preferred habitat to live and feed (benthic (BENT) and water column (WACO)); 4) reproduction guilds, based on spawning substrate (lithophilic (LITH), phytolithophilic (PHLI), and polyphilic (POLY)); 5) migratory behaviour guild (potamodromous species (POTA))
Summary
Biomonitoring and stream restoration are important tools to reestablish the health of river ecosystems with the emphasis placed on the restoration of ecological processes and ecosystem functionality [1]. In Europe, Ferreira et al [12] for Western Iberia and Pont et al [13] for France reported that both watershed and reach-scale factors were important in structuring fish species All these studies have contributed greatly to our understanding of fish-environment relations across multiple spatial scales at the taxonomic level, little attention has been paid to the factors that determine the functional organization of stream-fish assemblages in reference conditions (but see [7]). McGarvey and Hughes [32] and McGarvey and Ward [33] described longitudinal patterns in both fish guilds and species richness as a function of river discharge across large geographic areas These patterns are usually attributed to concepts that relate the gradient in physical factors that occur along river systems, to changes in assemblage structure and function. We hypothesized that the generic biological patterns would be associated with broad geographic gradients, with variations of the functional organization of fish assemblages either with latitude or along the streams
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