Charles P. Hawkins

Role of Refugia in Recovery from Disturbances: Modern Fragmented and Disconnected River Systems

Habitats or environmental factors that convey spatial and temporal resistance and/or resilience to biotic communities that have been impacted by biophysical disturbances may be called refugia. Most refugia in rivers are characterized by extensive coupling of the main channel with adjacent streamside forests, floodplain features, and groundwater. These habitats operate at different spatial scales, from localized particles, to channel units such as pools and riffles, to reaches and longer sections, and at the basin level. A spatial hierarchy of different physical components of a drainage network is proposed to provide a context for different refugia. Examples of refugia operating at different spatial scales, such as pools, large woody debris, floodplains, below dams, and catchment basins are discussed. We hope that the geomorphic context proposed for examining refugia habitats will assist in the conservation of pristine areas and attributes of river systems and also allow a better understanding of rehabilitation needs in rivers that have been extensively altered.

A Hierarchical Approach to Classifying Stream Habitat Features

Abstract We propose a hierarchical system of classifying stream habitats based on three increasingly fine descriptions of the morphological and hydraulic properties of channel geomorphic units. We define channel geomorphic units as areas of relatively homogeneous depth and flow that are bounded by sharp gradients in both depth and flow. Differences among these units provide a natural basis for habitat classification that is independent of spatial scale. At the most general level of resolution, we divide channel units into fast- and slow-water categories that approximately correspond to the commonly used terms “riffle” and “pool.” Within the fast-water category, we identify two subcategories of habitats, those that are highly turbulent (falls, cascades, chutes, rapids and riffles) and those with low turbulence (sheets and runs). Slow-water habitats include pools formed by channel scour (eddy pools, trench pools, mid-channel pools, convergence pools, lateral scour pools and plunge pools) and those formed be...

DEVELOPMENT AND EVALUATION OF PREDICTIVE MODELS FOR MEASURING THE BIOLOGICAL INTEGRITY OF STREAMS

The ratio of the number of observed taxa to that expected to occur in the absence of human-caused stress (OIE) is an intuitive and ecologically meaningful measure of biological integrity. We examined how OIE ratios derived from stream invertebrate data varied among 234 unimpaired reference sites and 254 test sites potentially impaired by past logging. Data were collected from streams in three montane ecoregions in California. Two sets of River Invertebrate Prediction and Classification System (RIVPACS) predictive mod- els were built: one set of models was based on near-species taxonomic resolution; the other was based on family identifications. Two models were built for each level of taxonomic resolution: one calculated 0 and E based on all taxa with probabilities of capture (Pj) > 0; the other calculated 0 and E based on only those taxa with Pc ? 0.5. Evaluations of the performance of each model were based on three criteria: (1) how well models predicted the taxa found at unimpaired sites, (2) the degree to which OIE values differed among unimpaired reference sites and potentially impaired test sites, and (3) the degree to which test site OIE values were correlated with independent measures of watershed alteration. Predictions of species models were more accurate than those of family models, and pre- dictions of the PC ? 0.5 species model were more robust than predictions of the PC > 0 model. OIE values derived from both species models were related to land use variables, but only assessments based on the Pc > 0.5 model were insensitive to naturally occurring differences among streams, ecoregions, and years.

Effects of Canopy, Substrate Composition, and Gradient on the Structure of Macroinvertebrate Communities in Cascade Range Streams of Oregon

The relative importance of surrounding riparian vegetation and substrate composition on invertebrate community structure was investigated in six streams in Oregon, USA. We found that canopy type was more important than substrate character in influencing total abundance and guild structure. Streams without shading had higher abundances of invertebrates than did shaded streams. Most guilds were influenced by qualitative differences in food availability rather than quantity of food or substrate composition. Open streams had higher abundances in the collector-gatherer, filter feeder, herbivore shredder and piercer, and predator guilds. Contrary to expectations, shredders were no more abundant in shaded streams than in streams lacking a riparian canopy. Scraper density was inversely related to standing crop of aufwuchs, but biomass was positively correlated with quantity of aufwuchs. Examination of dominance-diversity curves showed that both canopy and substrate influenced ranked abundances of taxa, but neither canopy nor substrate strongly influenced number of taxa. Differences in community structure were not always revealed by analysis of community-level properties, although differences in both the absolute and relative abundances of individual taxa were observed. (algae, high-quality detritus). We did not develop de- tailed analyses of either food sources or invertebrates but presented only those data necessary to explain patterns of vertebrate abundance. We now present a detailed analysis of differences in invertebrate com- munities among these streams and discuss the relative importance of food and habitat as determinants of abundances of different guilds and taxa.

Interactions between Stream Herbivores and Periphyton: A Quantitative Analysis of past Experiments

This review summarizes the state of knowledge regarding herbivory in stream ecosystems by quantitatively analyzing the results of 89 experimental studies published between 1972 and 1993. Our primary objective was to determine if general patterns exist among stream ecosystems in the type and strength of interactions occurring between herbivores (grazers) and their primary food source, periphyton. We conducted two types of meta-analyses of the published literature: (1) analyses of the proportion of studies showing significant effects for three types of interactions (effects of grazers on periphyton, effects of periphyton on grazers, and effects of grazers on other grazers and benthic animals) and (2) analyses of factors influencing the magnitude of effect that grazers had on periphyton. For effects of grazers on periphyton, we also determined (1) if the likelihood of observing significant effects varied with the spatial and temporal scale at which experiments were done and (2) if the magnitude of effect by grazers on periphyton abundance varied with spatial and temporal scale, grazer taxon, grazer abundance, and periphyton accrual based on the difference in treatments with and without grazers. Grazers held at ambient densities usually reduced periphyton biomass (70% of experiments) and altered algal taxonomic or physiognomic structure (81%) relative to grazer removal treatments, whereas grazers had slightly lower effects on periphyton productivity (usually <70% of experiments, depending on productivity measure). Experiments conducted in laboratory streams and at two spatial scales in the field (few or single habitat units and stream reaches or basins) were equally likely to report significant effects of grazers. Both short-term (≤4 wk) and longterm (>4 wk) experiments also were equally likely to report significant effects of grazers on periphyton. However, the magnitude of effect grazers had on periphyton biomass varied with the amount of periphyton accrual, grazer taxon, and grazer population biomass. Grazer effects also were higher for longer studies conducted under laboratory conditions than for shorter studies conducted in the field. A high proportion of the experiments that manipulated periphyton abundance significantly affected grazer densities and growth. Reduction in periphyton abundance usually reduced grazer density and growth. Experimental manipulations of dominant grazers typically had strong and usually negative effects on densities and growth of other species of benthic animals, either from direct (e.g., interference) or indirect (e.g., resource exploitation) mechanisms. Results of these analyses suggest that stream herbivores regulate their food resources as or more frequently than herbivores in other ecosystems, and strongly contradict the view held by many ecologists that stream communities are regulated primarily by abiotic factors. Although publication bias (i.e., the tendency for journals to publish positive results) appears minimal, we cannot yet generalize from these results to the entire universe of stream ecosystems because (1) most studies were conducted during summer base flow conditions and (2) results do not adequately represent interactions during the more physically stressful conditions that occur during periods of flooding, drought, or extreme cold. If rapid progress in the development of general stream ecosystem theory is to occur, we believe (1) future studies should be explicitly designed within the context of general ecological questions, (2) as much background information as possible describing environmental conditions should be collected, and (3) journals should permit and urge inclusion of tabular data describing both experimental conditions and treatment means and variances.

Evaluation of the use of landscape classifications for the prediction of freshwater biota: synthesis and recommendations

This paper summarizes and synthesizes the collective results that emerged from the series of papers published in this issue of J-NABS, and places these results in the context of previously published literature describing variation in aquatic biota at landscape spatial scales. Classifications based on landscape spatial scales are used or are being evaluated for use in several countries for aquatic bioassessment programs. Evaluation of the strength of classification of different approaches should provide insight for refinement of existing bioassessment programs and expedite the development of new programs. The papers in this series specifically addressed the degree to which descriptions and classification of landscape features allow us to account for, and thus predict, variation in the composition of biota among individual sites. In general, we found that although landscape classifications accounted for more biotic variation than would be expected by chance, the amount of variation related to landscape features was not large. Thus, large-scale regionalizations, if used alone to specify expected biotic conditions, will likely have limited use in aquatic bioassesments, where it is critical to specify expected conditions as accurately and precisely as possible. Landscape classifications can play an important additional role, however, by providing an initial stratification of site locations to ensure that different landscape features are adequately represented in a sampling program. In general, we believe a tiered classification based on both reach-level and larger-scale landscape features is needed to accurately predict the composition of freshwater fauna. One potential approach entails use of landscape classifications as a means of refining or augmenting classifications based on local habitat features, which appear to account for substantially more biotic variation than larger-scale environmental features. These results have significant implications for how assessment and monitoring programs at local, state/province, and national levels should be designed.

Effects of Sampling Area and Subsampling Procedure on Comparisons of Taxa Richness among Streams

the biotic integrity of communities. In all cases, we make the implicit and sometimes brash assumption that we can really measure the number of taxa in a community. Although measuring taxa richness might appear straightforward, accurate measurement has been extraordinarily difficult; and despite years of effort, no universally accepted methods for its measurement have emerged. The essential problem is that we can never completely census a taxonomic assemblage or entire community; we rely instead on estimates that describe some portion of the real taxa richness of an assemblage. The problem of knowing what percent of the taxa present have been collected is exacerbated when investigators fail to explicitly define their universe of interest (i.e., the spatial bounds of the community or communities in question). Comparisons of taxa richness among studies that used different sampling and subsampling methods are especially difficult and should be viewed skeptically. The difficulty of obtaining accurate measurements of richness is due to the collectors curve

The reference condition: predicting benchmarks for ecological and water-quality assessments

Abstract Benchmarks provide context and are a critical element of all ecological assessments. Over the last 25 y, hundreds of papers have been published on various aspects of ecological assessments, and most of the analyses described in these papers depend on specifying an ecological benchmark for context. Freshwater scientists and managers usually use reference sites (typically sites in natural or least-disturbed condition) to assess the ecological conditions at other sites. Accurate and precise assessments require that assessed sites be matched with appropriate reference conditions. Two general types of approaches have been proposed to predict reference conditions: classifications based on natural environmental settings and models that use continuously variable environmental attributes as inputs. Two types of classifications have been examined: geographic-dependent regionalizations based on general landscape features and geographic-independent typologies that are typically based on combinations of regional and channel features. We examined >1000 papers that addressed some aspect of predicting the reference condition in freshwater ecosystems. We focused on 5 types of benchmarks: ecological, thermal, hydrologic, geomorphic, and chemical. Our review showed that over the last 25 y, researchers have developed increasingly sophisticated methods that can be used to predict reference conditions. Most disciplines have increasingly moved toward site-specific modeling approaches as a way to improve both accuracy and precision of predictions, although typological approaches dominate geomorphic characterizations. Papers published in J-NABS have been especially important in advancing and refining methods for predicting ecological benchmarks. Much of the progress made in the science of ecological assessment emerged from research that advanced our understanding of how the spatial and temporal distributions of freshwater biota are related to naturally occurring environmental features and how those relationships can be most accurately and precisely described and predicted. Thus, the performance of ecological assessments is critically linked to how well we characterize freshwater environments, and research in the watershed sciences that addresses predicting thermal, hydrologic, geomorphic, and chemical attributes of freshwater ecosystems has paralleled research focused on predicting biota. We anticipate that knowledge produced from future collaborations between ecologists and watershed scientists coupled with the application of modern modeling techniques will largely determine progress in characterizing and predicting biota–environment relationships and, thus, the accuracy and precision of future ecological assessments.

Channel morphology, water temperature, and assemblage structure of stream insects

Temperature is known to be an important mechanism affecting the growth and distribution of stream insects. However, little information exists that describes how variable temperatures are among streams of similar size, especially in physically heterogeneous landscapes. We measured summer daytime temperature and the structure of riffle benthic insect assemblages from 45 montane streams in California. Summer stream temperature was nearly randomly distributed across large-scale geographic gradients of latitude (6°) and elevation (2000 m). The lack of geographic trends in summertime stream temperature appeared to be caused by the strong relationship between local channel morphology and summer water temperature. Mean daytime water temperature was most strongly related to the % of the channel present as pools, which did not vary systematically with either latitude or elevation. We used multiple multivariate regression analysis, non-metric multidimensional scaling (NMDS), and graphical techniques to both quantify differences in insect assemblage structure among streams and to determine the degree to which assemblage structure was related to temperature. NMDS analyses were conducted on 3 similarity matrices based on: 1) presence and absence of all aquatic insect taxa encountered during the study, 2) densities of the 16 most numerically abundant taxa, and 3) population biomasses of the 16 most common taxa. All 3 analyses showed that variation in assemblage structure among streams was significantly related to temperature, although assemblage structure was most strongly related to sampling date--a consequence of sampling over a 98-d period. Temperature probably influenced assemblage structure in 2 ways: 1) by influencing developmental rates of individual taxa and overall assemblage phenology, thus affecting the relative abundances of taxa found on a specific sampling date, and 2) by excluding taxa unable to tolerate certain temperature ranges. Because of the strong dependency of assemblage structure on temperature and the lack of strong geographic trends in temperature among these streams, much of the measured variation in assemblage structure appeared to be unrelated to latitude or elevation. These results have important implications for both our understanding of natural biogeographic patterns of lotic organisms and our ability to detect and model the effects of climate change and other thermal alterations on stream ecosystems.

Condition of stream ecosystems in the US: an overview of the first national assessment

Abstract The Wadeable Streams Assessment (WSA) provided the first statistically sound summary of the ecological condition of streams and small rivers in the US. Information provided in the assessment filled an important gap in meeting the requirements of the US Clean Water Act. The purpose of the WSA was to: 1) report on the ecological condition of all wadeable, perennial streams and rivers within the conterminous US, 2) describe the biological condition of these systems with direct measures of aquatic life, and 3) identify and rank the relative importance of chemical and physical stressors affecting stream and river condition. The assessment included perennial wadeable streams and rivers that accounted for 95% of the length of flowing waters in the US. The US Environmental Protection Agency, states, and tribes collected chemical, physical, and biological data at 1392 randomly selected sites. Nationally, 42% of the length of US streams was in poor condition compared to best available reference sites in their ecoregions, 25% was in fair condition, and 28% was in good condition. Results were reported for 3 major regions: Eastern Highlands, Plains and Lowlands, and West. In the West, 45% of the length of wadeable flowing waters was in good condition. In the Eastern Highlands, only 18% of the length of wadeable streams and rivers was in good condition and 52% was in poor condition. In the Plains and Lowlands, almost 30% of the length of wadeable streams and rivers was in good condition and 40% was in poor condition. The most widespread stressors observed nationally and in each of the 3 major regions were N, P, riparian disturbance, and streambed sediments. Excess nutrients and excess streambed sediments had the highest impact on biological condition; streams scoring poor for these stressors were at 2 to 3× higher risk of having poor biological condition than were streams that scored in the good range for the same stressors.