Disentangling the pathways of land use impacts on the functional structure of fish assemblages in Amazon streams

Abstract

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.

Land cover, riparian zones and instream habitat influence stream fish assemblages in the eastern Amazon

“…However, this finding does not consider potential time lags or legacy effects (Harding, Benfield, Bolstad, Helfman, & Jones, 1998;Leitão et al, 2018;Zeni et al, 2017) and land cover change in the basin is rapid and evolving (e.g., secondary forest and new agricultural crops, such as sugarcane, likely will expand over the coming years). However, this finding does not consider potential time lags or legacy effects (Harding, Benfield, Bolstad, Helfman, & Jones, 1998;Leitão et al, 2018;Zeni et al, 2017) and land cover change in the basin is rapid and evolving (e.g., secondary forest and new agricultural crops, such as sugarcane, likely will expand over the coming years).…”

“…Most of the remaining forests are subjected to reduced-impact logging that aims to minimise damage to soil and forest, or conventional logging that involves minimal consideration of forest resilience (Prudente, Pompeu, Juen, & Montag, 2017). Related studies from the project Sustainable Amazon Network (Rede Amazônia Sustentável) by Leitão et al (2018), Gardner et al (2013) and Leal et al (2016Leal et al ( , 2017 are from the same basin, but different sampling sites. Some of the sites sampled in this study were surveyed in previous studies (Benone et al, 2017;Ferreira, Begot, Prudente, Juen, & Montag, 2018;Juen et al, 2016;Prudente, Pompeu, & Montag, 2018;Prudente et al, 2017).…”

“…Expansion of road networks directly increases stream channel erosion and sedimentation, promotes further expansion of agriculture and access to streams and disrupts connectivity (Leal et al, 2016;Leitão et al, 2018;Wantzen & Mol, 2013). Disrupted connectivity likely affects the potential for dispersal and the importance of intersite distance on assemblage composition and structure.…”

“…Long-term monitoring studies should incorporate key indicator variables for habitat evaluation (e.g., undercut bank and SD incision height from this study, wood volume as proposed by Leal et al (2017) and Leitão et al (2018)). Baseline data that represent relatively "pristine" conditions are lacking in most instances, which can undermine the ability of studies to interpret ecological dynamics in response to land cover change .…”

Trait–environment relationships in Amazon stream fish assemblages

“…The identification of these variables allows a more reliable prediction of the response of the fish community to environmental impacts (Pease, Lez-Díaz, Rodiles-Hernández, & Winemiller, 2012). While the effects of anthropogenic impacts on the distribution and abundance of stream fish are relatively well understood, much less attention has been paid to the functional traits of species (Leal et al, 2017;Leitão et al, 2018) and even less to the relationship between the functional structure of these communities and ecosystem function (Carvalho & Tejerina-Garro, 2015;Pease et al, 2012).…”