Citation:

Leitao, R., J. Zuanon, D. Mouillot, C. Leal, R. Hughes, Phil Kaufmann, S. Villeger, P. Pompeu, D. Kasper, F. de Paula, S. Ferraz, AND T. Gardner. Disentangling the pathways of land use impacts on the functional structure of fish assemblages in Amazon streams. ECOGRAPHY. Blackwell Publishing, Malden, MA, 41(1):219-232, (2018).

Impact/Purpose:

The USEPA’s National Aquatic Resource Survey (NARS) designs, field methods, and approaches for assessing ecological condition have been applied not only in state and basin stream surveys throughout the U.S., but also in countries outside of the U.S. These applications are not only valuable tests of the NARS approaches, but contribute to environmental science and resource management through new understandings of natural and anthropogenic controls on biota and physical habitat in streams. Results from innovative applications in Brazil, for example, not only aid our understanding of the effects of landscape fragmentation and deforestation on biodiversity in Brazilian streams, but also refine approaches for interpreting aquatic resource surveys in the U.S. and elsewhere. In this article, the authors used NARS methods to collect fish assemblage and physical habitat data from small streams across a gradient of anthropogenic disturbances related to land-use change (LUC) in two large basins in the Amazon River drainage of Brazil. They used Structural Equation Modeling (SEM) to examine natural and anthropogenic controls on the taxa richness, species composition, and richness of functional traits in these streams. These analyses identify dominant pathways through which LUC alters physical and chemical habitat, and in turn, influences fish assemblages in these streams. For instance, local riparian deforestation increased aquatic macrophyte and grass cover in these small streams, with subsequent reductions of the functional evenness of assemblages (i.e., increased the dominance of few trait combinations). The authors also examined the effects of disturbance scales and habitat fragmentation on fish assemblages and their habitats. Landscape fragmentation upstream from sample sites and deforestation at catchment and riparian scales altered the channel morphology and the structure of stream bottoms, changing the functional identity of assemblages (e.g., species that use the benthic compartment were negatively affected). Fragmentation downstream from the sites reduced the functional richness (i.e., losing regional connectivity potentially reduces the range of niche occupation by assemblages), and on the functional evenness and divergence, suggesting a trend of functional homogenization of local assemblages. The research described in the manuscript is a demonstration of the utility of NARS field methods to assess the effects of human activities on fish assemblages and instream physical habitat. The analytical framework applied to NARS-style field data serves a model that could be applied with NARS data to make similar diagnostic assessments in the U.S.

Description:

Agricultural land use is a primary driver of environmental impacts on streams. However, the causal processes that shape these impacts operate through multiple pathways and at several spatial scales. This complexity undermines the development of more effective management approaches, and illustrates the need for more in-depth studies to assess the mechanisms that determine changes in stream biodiversity. Here we present results of the most comprehensive multi-scale assessment of the biological condition of streams in the Amazon to date, examining functional responses of fish assemblages to land use.We sampled fish assemblages from two large human-modified regions, and characterized stream conditions by physical habitat attributes and key landscape-change variables, including density of road crossings (i.e. riverscape fragmentation), deforestation, and agricultural intensification. Fish species were functionally characterized using ecomorphological traits describing feeding, locomotion, and habitat preferences, and these traits were used to derive indices that quantitatively describe the functional structure of the assemblages. Using structural equation modeling, we disentangled multiple drivers operating at different spatial scales, identifying causal pathways that significantly affect stream condition and the structure of the fish assemblages. Deforestation at catchment and riparian network scales altered the channel morphology and the stream bottom structure, changing the functional identity of assemblages. Local deforestation reduced the functional evenness of assemblages (i.e. increased dominance of specific trait combinations) mediated by expansion of aquatic vegetation cover. Riverscape fragmentation reduced functional richness, evenness and divergence, suggesting a trend toward functional homogenization and a reduced range of ecological niches within assemblages following the loss of regional connectivity. These results underscore the often-unrecognized importance of different land use changes, each of which can have marked effects on stream biodiversity. We draw on the relationships observed herein to suggest priorities for the improved management of stream systems in the multiple-use landscapes that predominate in human-modified tropical forests.

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