Heikki Mykrä

DEFINING MACROINVERTEBRATE ASSEMBLAGE TYPES OF HEADWATER STREAMS: IMPLICATIONS FOR BIOASSESSMENT AND CONSERVATION

We used data from 235 boreal headwater streams in Finland to examine whether macroinvertebrate assemblages constitute clearly definable types, and how well biologically defined assemblage types can be predicted using environmental variables. Two- way indicator species analysis produced 10 assemblage types, which differed significantly from each other (multiresponse permutation procedure, MRPP). However, based on MRPP and nonmetric multidimensional scaling, there was wide variation among sites within each assemblage type, and high degrees of overlap among assemblage types. Such continuous variation was also evidenced by the low number of effective indicator taxa (indicator value method) for each assemblage type. Furthermore, discriminant function analysis based on environmental variables showed a moderate yet variable prediction success (59.6% of sites correctly predicted; range 0-96% per group). Canonical correspondence analysis indicated that variation in assemblage structure was primarily related to latitude and water chemistry, especially pH and water color. The absence of discrete macroinvertebrate assemblage types in boreal headwater streams may stem from a number of reasons: (1) macroinvertebrate taxa inhabiting boreal streams tend to exhibit individualistic, taxon-specific responses to environmental gradients, (2) they have wide environmental tolerances and geographic dis- tributions, and (3) boreal streams are characterized by frequent, unpredictable disturbances. Our results suggest that local filters in headwater streams are relatively weak, resulting in poorly distinguishable assemblage types. By contrast, the major latitudinal gradients in macroinvertebrate assemblage structure suggest that regional-scale filters, e.g., temperature, exert strong control over taxon distributions in headwater streams. We suggest that a tiered approach combining regional stratification and subsequent prediction of assemblage struc- ture could provide a suitable framework for the bioassessment and conservation of boreal headwater streams.

Twenty years of stream restoration in Finland: little response by benthic macroinvertebrate communities

The primary focus of many in-stream restoration projects is to enhance habitat diversity for salmonid fishes, yet the lack of properly designed monitoring studies, particularly ones with pre-restoration data, limits any attempts to assess whether restoration has succeeded in improving salmonid habitat. Even less is known about the impacts of fisheries-related restoration on other, non-target biota. We examined how restoration aiming at the enhancement of juvenile brown trout (Salmo trutta L.) affects benthic macroinvertebrates, using two separate data sets: (1) a before-after-control-impact (BACI) design with three years before and three after restoration in differently restored and control reaches of six streams; and (2) a space-time substitution design including channelized, restored, and near-natural streams with an almost 20-year perspective on the recovery of invertebrate communities. In the BACI design, total macroinvertebrate density differed significantly from before to after restoration. Following restoration, densities decreased in all treatments, but less so in the controls than in restored sections. Taxonomic richness also decreased from before to after restoration, but this happened similarly in all treatments. In the long-term comparative study, macroinvertebrate species richness showed no difference between the channel types. Community composition differed significantly between the restored and natural streams, but not between restored and channelized streams. Overall, the in-stream restoration measures used increased stream habitat diversity but did not enhance benthic biodiversity. While many macroinvertebrates may be dispersal limited, our study sites should not have been too distant to reach within almost two decades. A key explanation for the weak responses by macroinvertebrate communities may have been historical. When Fennoscandian streams were channelized for log floating, the loss of habitat heterogeneity was only partial. Therefore, habitat may not have been limiting the macroinvertebrate communities to begin with. Stream restoration to support trout fisheries has strong public acceptance in Finland and will likely continue to increase in the near future. Therefore, more effort should be placed on assessing restoration success from a biodiversity perspective using multiple organism groups in both stream and riparian ecosystems.

Control of stream insect assemblages: roles of spatial configuration and local environmental factors

Abstract 1. Current views in ecology emphasise that community structure is the sum of multiple processes, with imprints of both regional and local drivers. However, the degree to which stream insect assemblages are structured by spatial configuration (complying with the dispersal‐based neutral hypothesis) and local environmental features (complying with the niche‐based species sorting hypothesis) has not been rigorously examined based on surveys in multiple years.

The effect of sample duration on the efficiency of kick-sampling in two streams with contrasting substratum heterogeneity

Representative sampling is a common problem in studies comparing taxonomic rielmess or composition of ecologieal assemblages. In broad-scale surveys and biomonitoring programs using lotic macroinvertebrates, kick-sampling has been widely used as a standard sampling teclmique. Kick-sampling can be used in a variety ofhabitats (BRADLEY & ÜRMEROD 2002) and is considered more cost-effective than alternative, more quantitative sampling methods (STOREY et al. 1991 ). The efficiency ofkieksampling may be affeeted, however, by substratum structure (STOREY et al. 1991) or other in-stream habitat eharaeteristies (PARSONS & NORRIS 1996). These factors often vary between sites, thus potentially obscuring among-site eomparisons. Such variation is generally controlled for by stratifying sampling to riffie habitats where several subsamples are then taken to maximize the diversity o f microhabitats sampled (RESH & McELRAVY 1993). Riffies, however, vary in habitat eomplexity, and eomplex habitats often harbour more speeies than do strueturally simpler ones (DOWNES et al. 1998, 2000). Therefore, differenees in maeoinvertebrate community composition among streams with differing habitat characteristies may be underestimated i f the effieiency o f sampling is affeeted by substratum heterogeneity, or if the area sampled is too small to adequately cover variation in microhabitats within a site. Sample size also bears a close relationship to the detection o f rare and endangered taxa, which are often the primary targets of broad-seale biodiversity surveys. Although several studies have eompared kiek-sampling to other stream invertebrate sampling methods, few have directly assessed the effeets of sample size and/or substratum structure on the effieieney of kiek-sampling (but see BRADLEY & ÜRMEROD 2002).

Weak relationships between landscape characteristics and multiple facets of stream macroinvertebrate biodiversity in a boreal drainage basin

Variability in biodiversity is often assessed based on species richness, and this adherence to a single index has been typical in studies of ecology, biogeography, and conservation in the past two decades. More recent studies have suggested that species richness alone is insufficient as a measure of biodiversity, mainly because it is not necessarily correlated with other measures of biodiversity. We examined (1) if nine indices embracing species diversity, functional diversity, and taxonomic distinctness of stream macroinvertebrate assemblages show congruent patterns, and (2) if these indices show similar relationships to landscape characteristics. Not all indices varied similarly and were thus not significantly correlated. There were three principal components that effectively described variation in the correlation structure of the nine indices. These three components were: (1) diversity and evenness indices, (2) two indices of taxonomic distinctness, and (3) species richness and functional richness. Four of the nine biodiversity indices examined showed no significant relationships to landscape-catchment characteristics, and even the significant correlations between the remaining five indices and explanatory variables were rather weak. However, species richness showed a rather strong quadratic relationship to catchment area. Our study provided a number of suggestions for future biodiversity studies at the landscape scale. First, given that different indices describe different components of biodiversity and are not strongly correlated, multiple indices should be considered in any study describing stream biodiversity. Second, despite the study was restricted to near-pristine streams, all indices showed considerable variation. Thus, this natural variability should be accounted for prior to the examination of anthropogenic effects on stream biodiversity. Third, landscape-catchment variables may have only limited value in explaining variability in biodiversity indices, at least in regions with no strong anthropogenic gradients in land-use.

Assessing stream condition using macroinvertebrates and macrophytes: concordance of community responses to human impact

Freshwater bioassessment is typically based on well known taxonomic groups, such as fish and macroinvertebrates, while less studied groups are frequently excluded from bioassessment programs. We examined the community variation of stream macroinvertebrates and macrophytes (mainly aquatic bryophytes) in near-pristine reference sites and sites impacted by agriculture, forestry, or in-stream habitat alteration in western and central Finland. We specifically examined whether these taxonomic groups exhibited concordant variation in community structure and whether they could be used as surrogates for each other in stream bioassessment. The effects of human disturbance on taxonomic composition and community structure were investigated using canonical correspondence analysis and RIVPACS-type predictive models. The community concordance between macroinvertebrates and macrophytes was higher when test sites were included, suggesting that environmental degradation may increase community concordance between different biotic groups. However, despite increased concordance, macroinvertebrates and macrophytes responded to different stressors. The sensitivity of the two groups to human disturbance also differed markedly. The macroinvertebrate model determined 13 (with P t = 0+ as the threshold for species inclusion) and 12 (with P t = 0.4) out of 35 test sites as altered, whereas the macrophyte model detected impairment for only 7 sites (both probability levels), likely because of the low species richness of macrophytes in our study streams. Low richness of macrophytes may thus hinder their use in bioassessment based on taxonomic completeness, suggesting that measures that do not rely solely on species loss might perform better in at least high-gradient boreal streams.

Decomposer communities in human‐impacted streams: species dominance rather than richness affects leaf decomposition

Summary There is compelling evidence that anthropogenic disturbance can decrease biodiversity and impair ecological functioning. A major challenge to biodiversity–ecosystem function research is to disentangle the effects of biodiversity loss on ecosystem functions from the direct effects of human disturbance. We studied the influence of human disturbance (acidification and eutrophication) and a natural stressor (low pH due to bedrock geology) on leaf-shredding macroinvertebrates, fungal decomposers and leaf decomposition rates in boreal streams. We used pyrosequencing techniques to determine fungal richness and assemblage structure. Decomposition rates were higher in anthropogenically disturbed than in circumneutral reference or naturally acidic sites, but did not differ between the latter two groups. Macroinvertebrate richness was higher in circumneutral than in human-impacted or naturally acidic sites, and shredder evenness was highest in circumneutral sites. Fungal evenness was also lower in human-disturbed than in reference sites, whereas fungal richness did not vary among site groups. Decomposition rate in fine-mesh bags was related positively to current velocity and fungal dominance, while in coarse-mesh bags, it was related positively to total phosphorus. In anthropogenically disturbed streams, the effects of low pH were overridden by eutrophication, and increased decomposition rates resulted from disturbance-induced increase in species dominance rather than richness. Furthermore, decomposition rates were positively correlated with abundances of dominant taxa, suggesting that ecosystem processes may be driven by a few key species. Synthesis and applications. Our results suggest that leaf decomposition rates are insensitive to natural background variation, supporting the use of decomposition assays, preferably accompanied by molecular analysis of fungal assemblages, to assess stream ecosystem health. Instead of focusing solely on diversity, however, more emphasis should be placed at changes in dominance patterns, particularly if management aims are to improve stream ecosystem functioning.

Predicting the stream macroinvertebrate fauna across regional scales: influence of geographical extent on model performance

Abstract Predictive modeling is used increasingly often in freshwater bioassessment. These models are routinely applied across broad geographical areas, yet the influence of spatial extent on model performance has not been assessed directly. We examined the influence of changing geographical extent on the performance of River InVertebrate Prediction and Classification System (RIVPACS)–type predictive models of benthic macroinvertebrates in boreal headwater streams representative of reference conditions. Separate models were constructed for 2 hierarchical extents: within ecoregions (middle boreal and north boreal) and across ecoregions. We assessed the accuracy and precision of the models using root mean squared error (RMSE) of the observed/expected (O/E) ratio of the number of predicted taxa. We compared the RMSE of the within-ecoregion models to that of the across-ecoregions models. We tested the models using 2 thresholds for probability of species occurrence (Pt): Pt = 0+ (all predicted taxa) and Pt = 0.5 (only common taxa). We also assessed the importance of catchment vs local habitat variables as predictors of macroinvertebrate fauna. Within the 2 ecoregions, region-specific models performed better than the across-ecoregions model with Pt = 0+, but the difference in performance of models with different regional extents decreased when Pt = 0.5. Catchment area and stream slope were the only consistently significant predictor variables, but overall, only minor differences were found in the relative importance of catchment vs local attributes. Models with Pt = 0.5 performed better than those with Pt = 0+; this result suggests that models based only on common taxa might perform more consistently than models based on all taxa in the assessment of headwater streams. However, the expected number of taxa would then become very low (mean: 8.0–12.6 with Pt = 0.5), which might impede detection of human impact. Alternative approaches that do not rely solely on taxon occurrences but also incorporate aspects of assemblage composition might be needed for assessment of headwater streams.

Responses of spring macroinvertebrate and bryophyte communities to habitat modification: community composition, species richness, and red-listed species

Abstract.  Like all headwater systems, springs have been increasingly modified by multiple land uses, but the effects of these modifications on species diversity and community composition are poorly understood. We assessed the consequences of forestry-related disturbance (mainly draining) on benthic macroinvertebrate and bryophyte diversity and community composition in boreal springs. We used predictive modeling (BEnthic Assessment of SedimenT [BEAST]), indicator species analysis, and descriptive statistics on data from 55 near-pristine reference springs and 20 modified (test) springs spanning 3 ecoregions (hemiboreal to middle boreal) in Finland. Invertebrate and bryophyte communities were relatively similar between the reference and test springs. BEAST identified deviation from the reference condition in 9 and 10 sites based on benthic macroinvertebrates and bryophytes, respectively. These sites were identified mostly as possibly different from reference, with the exception of a few seriously degraded springs with low levels of groundwater flow. Indicator species for both reference-condition and test springs included spring-preferring and ubiquitous taxa. Bryophyte richness was lower in test than in reference springs, but no significant differences were detected for macroinvertebrate richness. Red-listed species were more common in reference than in test springs. Our results suggest that, despite only moderate effects on community composition, anthropogenic disturbance impoverishes spring fauna and flora and causes a decline of spring-preferring red-listed species. Restoration will be necessary to preserve biodiversity in springs, but benign methods should be used to avoid further disturbance to biota, particularly red-listed spring specialists.

Temporal variability in taxonomic completeness of stream macroinvertebrate assemblages

Abstract.  Natural temporal variation in biological assemblages is generally acknowledged, but rarely controlled for in existing bioassessment approaches. Many bioassessment methods ignore the variation of assemblage structure through time and, thus, lack a process to evaluate the effects of natural temporal variation on their outcomes. We used an 11-y data set from 24 near-pristine reference streams (validation sites) and a 5-y data set from 10 streams exposed to human disturbance (test sites) in northern Finland to examine: 1) the amount of interannual variation in macroinvertebrate communities in both types of streams measured as variation in taxonomic completeness (i.e., ratio of observed/expected taxa; O/E ratio), and 2) how temporal variability affects bioassessment outcomes. Most of the variation in O/E values was explained by stream type (validation or test). Interannual variation in O/E was low at both validation and test sites, a result supporting the assumption of high constancy of biological assemblages through time. Temporal variation in O/E at validation sites was related to climatic factors (regional precipitation) and to some local environmental variables, e.g., substratum heterogeneity. Communities in validation and test sites responded differentially to climatic variation. The low interannual variation in O/E suggests that 1-y data from reference sites can be extrapolated across years relatively safely, but climatically extreme years may be problematic. Temporal variability in O/E was low at both types of sites, but it was still high enough to result in variable ecological status assessments across years, mainly at test sites with lower O/E ratios. Thus, at test sites, use of only 1-y data may cause erroneous management decisions, and accurate assessment of the biological quality of test sites might require repeated sampling over several years.