Aim Freshwater biodiversity is declining world‐wide as a result of human‐caused degradation of freshwater ecosystems. Declines in species richness are well documented for many taxonomic groups, but less is known about how and why species composition and beta diversity (β) change in response to environmental degradation in freshwater environments. Location The Mid‐Atlantic Highlands region of the USA (36–42° N, 74–82° E) and Finland in northern Europe (60–68° N, 20–32° E). Methods We used null models, occupancy analyses, ordinations and multitaxon niche models to assess how β of stream macroinvertebrate assemblages in two biogeographically distinct regions responded to environmental alteration and what mechanisms were most likely to have caused the observed differences in β. Results Macroinvertebrate assemblages at anthropogenically disturbed sites were significantly more dissimilar from one another than were reference sites, a result contrary to those reported in several other studies. In this study, anthropogenic disturbances decreased the regional prevalence of most common taxa and increased the prevalence of several less common taxa. These differences in taxon occupancies and β between the reference condition and disturbed streams are unlikely to be the result of a single mechanism, although environmental filtering was probably of primary importance. Main conclusions The effects of anthropogenic disturbance on assemblage composition can be variable and probably depend on: initial ecological conditions; the magnitude, type and uniformity of environmental alteration occurring in a region; and the sensitivity of individual taxa. It is unlikely that β is controlled by a single mechanism, and these variable mechanisms may have diverse effects on taxon occupancies and hence β. Restoration ecologists will need to consider how β varies naturally across different landscape settings and in response to different types of disturbance when developing region‐wide restoration strategies.

We used field-derived data from streams in Nevada, USA, to quantify relationships between stream biological condition, in-stream stressors, and potential sources of stress (land use). We used 2 freshwater macroinvertebrate-based indices to measure biological condition: a multimetric index (MMI) and an observed to expected (O/E) index of taxonomic completeness. We considered 4 categories of potential stressors: dissolved metals, total dissolved solids, nutrients, and flow alteration. For physicochemical factors that varied predictably across natural environmental gradients, we quantified potential stress as the site-specific difference between observed (O) and expected (E) levels of each factor (O-E stress). We then used 2 sets of Random Forest models to quantify relationships between: 1) biological condition and potential stressors, and 2) stressor values and land uses. The 2 indices of biological condition were differentially responsive to stressors, indicating that no single measure of biological condition could fully characterize assemblage response to stress. Total dissolved solids (as measured by electrical conductivity [EC]) and metal contamination were the stressors most strongly associated with biological degradation. The most likely sources of these stressors were agriculture, urban development, and mining. Our findings highlight the need to develop EC criteria for streams. Measures of biological condition and stress that account for natural variability should reduce errors of inference and increase confidence in causal analyses. This approach will require development of robust models capable of predicting physical and chemical reference conditions. Causal analyses for individual sites require appropriate hypotheses about which stressors and what levels of stress can cause biological degradation. Our study demonstrates the usefulness of field data collected from multiple sites within a region for developing these hypotheses.

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