Disentangling natural and anthropogenic effects on benthic macroinvertebrate assemblages in western US streams

Stream macroinvertebrate assemblages are shaped by natural and human-related factors that operate through complex hierarchical pathways. Quantifying these relationships can support stream ecological assessment. We applied a structural equation modeling framework to evaluate hypothesized pathways by which watershed, riparian, and in-stream factors affect benthic macroinvertebrate condition in the Western Mountains (WMT) and Xeric (XER) ecoregions in the US. We developed a conceptual model grounded in theory, empirical evidence, and expert opinion to evaluate the following hypotheses: 1) macroinvertebrate assemblages are primarily driven by proximal, in-stream factors (e.g., water quality and physical habitat), 2) anthropogenic land use affect macroinvertebrates indirectly by altering in-stream characteristics, and 3) riparian vegetation cover attenuates land use effects. We tested our model separately on three measures of benthic macroinvertebrate assemblage condition: ratio of observed-to-expected taxonomic composition (O/E), a multi-metric index (MMI), and richness of Ephemeroptera, Plecoptera, and Trichoptera taxa (EPT). Across models, macroinvertebrate condition was largely driven by reach-level characteristics related to riparian cover, in-stream physical habitat (relative bed stability), and water quality (total nitrogen). Land use disturbances in the watershed and stream reach impaired macroinvertebrate condition indirectly by altering relative bed stability, water quality, and riparian cover/complexity. Riparian cover was associated with greater macroinvertebrate condition by reducing land use impacts on stream flow, streambed substrate, and water quality, but the pathways differed among ecoregions. In the WMT, reach-level riparian cover affected macroinvertebrate assemblages in part through indirect pathways associated with greater streambed stability and reduced total nitrogen concentrations. In contrast in the XER, watershed-level riparian cover affected macroinvertebrate assemblages through greater specific stream power. Identifying the relative effects and pathways that natural and anthropogenic factors affect macroinvertebrates can support effective stream assessment by serving as a framework for prioritizing management and conservation efforts. 

Impact/Purpose

Key to developing effective environmental policy is assessing how candidate policies affect human well-being and valuation. NCEE in collaboration with OW and ORD seek to examine these relationships as mediated through aquatic ecosystems. Disentangling these complex relationships is challenging without appropriate analytic frameworks. We used a structural equation modeling (SEM) framework to evaluate and quantify the pathways in which natural and anthropogenic factors affect aquatic condition. We applied SEM to evaluate hypothesized watershed, riparian, and in-stream factors in affecting stream biotic condition using the US EPA National Rivers and Streams Assessment and StreamCat datasets. This information is a necessary component to bridge the gap linking management actions to ecosystem condition, and future work will expand to incorporate human valuation. The SEM framework is a promising analytic approach to leverage broad-scale data to develop mechanistic understanding of management effects on ecosystem condition.

Citation

Fergus, Carol, Jacqueline Brooks, P. Kaufmann, A. Herlihy, R. Hill, R. Mitchell, AND P. Ringold. Disentangling natural and anthropogenic effects on benthic macroinvertebrate assemblages in western US streams. ESA Journals, 14(11):e4688, (2023). [DOI: 10.1002/ecs2.4688]

DOI: Disentangling natural and anthropogenic effects on benthic macroinvertebrate assemblages in western US streams