Résumé
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.