Resistance and Resilience of Macroinvertebrate Assemblages to Drying and Flood in a Tallgrass Prairie Stream System

Intermittent streams are common worldwide, and the ability of invertebrates to recover from floods and drought is a key feature of communities from these highly disturbed ecosystems. The macroinvertebrate assemblages of Kings Creek in northeastern Kansas were sampled regularly from four intermittent and two perennial sites over 2 years (1995–1996) to investigate the response and recovery to seasonal drying and floods. A 9mo drying period reduced taxa richness and density to 14% and 3% of pre-drying assemblages, respectively, in 1995–1996, whereas a 2mo drying period reduced richness by half and density to 4% of pre-drying assemblages in 1996. Floods at intermittent sites reduced densities and richness by 95% and 50%, respectively. A >50 y-flood reduced macroinvertebrate richness by 97% and density by >99% at a downstream perennial site. Resistance and resilience of total macroinvertebrate density was typically greater to floods than to drying, whereas resilience of taxa richness did not differ between disturbance types. The time required for recovery to pre-flood conditions (richness and density) was half as long (27 vs. 76 day) for intermittent sites compared to perennial sites. Colonization of intermittent sites was a function of distance from upstream refugia. Floods were a more important disturbance on assemblages in a downstream reach as compared to upstream reaches. In contrast, upstream reaches were more likely to dry. Recovery following flood and drought was dominated by colonization as opposed to tolerance, thus resilience is more important than resistance in regulating macroinvertebrate communities in these streams, and relative position in the landscape affects disturbance type, intensity, and ability of communities to recover from disturbance.     

Land-use influences macroinvertebrate community response following a pulse disturbance

1. We tested the hypothesis that interactions between disturbance types can influence invertebrate community response and recovery in two streams draining pasture (press‐pulse disturbance) and native forest (pulse disturbance) catchments before and after a one‐in‐28‐year flood. We also sampled drift and adult insects to gain insights into the relative importance of these two postdisturbance recolonisation pathways. 2. Taxa numbers and total density declined markedly at the forested site after the flood, but there was a delayed response at the pasture site, reflecting greater initial resistance to this pulse disturbance among taxa adapted to the underlying press disturbance. 3. Community composition was less stable at the pasture site where per cent abundance of taxa was highly variable prior to the flood and over the 2‐year postflood sampling period. After the flood, the pasture stream fauna was more heavily dominated by vagile taxa, including several chironomid species and hydroptilid caddisflies. 4. Taxa numbers and densities recovered to preflood levels within 5–7 months at both sites, but a range of taxa‐specific responses was observed that took up to 18 months to recover to preflood densities. Community stability at the pasture site had not returned to preflood composition by 2 years postflood. 5. Changes in drift densities of several common stream invertebrates at the pasture site reflected postflood changes in benthic densities and seasonally low drift in winter. Terrestrial invertebrates dominated drift at the pasture site for 3 months postflood whereas Ephemeroptera were most common at the native forest site. 6. Flight patterns of selected adult aquatic insects showed a strongly seasonal pattern. Abundance of adults at the pasture site in the second year following the flood increased in line with the recovery of the non‐Dipteran benthic fauna. Significant upstream flight occurred for several caddisfly species at the native forest site, and weakly directional or downstream flight was evident for most common Plecoptera and Ephemeroptera. 7. This study indicates that the magnitude and duration of responses to major pulse disturbances can depend on the presence or absence of an underlying press disturbance. This finding has implications for monitoring, and suggests that a knowledge of disturbance history beyond 2 years may be required to interpret mechanisms contributing to observed land‐use impacts.     

Short-term recovery of benthos following disturbance from stream habitat rehabilitation

The recovery of benthic macroinvertebrates after disturbance from stream rehabilitation was studied in the River Livojoki, northern Finland. The stream that had been channelized for log transport was rehabilitated on 1 July 1992 by digging holes and inserting boulders. We measured habitat characteristics and sampled benthic animals before and after rehabilitation, including an unrehabilitated control site. The immediate effect of rehabilitation was a slight decrease in the abundances of benthic insects. Recolonization occurred rapidly, within 10 days. Disturbance of the rehabilitation did not have a detectable effect on the macroinvertebrate community. Most species-level changes and community patterns reflected seasonal life history events. Timing of such rehabilitation work can be critical for the recovery rate, which depends on the colonization abilities of the species present after disturbance. We suggest that many disturbances (including minor floods and moderate rehabilitation procedures) may have only small, short-term effects on benthic communities. We emphasize the importance of considering seasonality in studies of disturbance in streams.     

Severe drought drives novel community trajectories in desert stream pools

1. Ecological communities can be relatively stable for long periods of time, and then, often as a result of disturbance, transition rapidly to a novel state. When communities fail to recover to pre‐disturbance configurations, they are said to have experienced a regime shift or to be in an alternative stable state. 2. In this 8‐year study, we quantified the effects of complete water loss and subsequent altered disturbance regime on aquatic insect communities inhabiting a formerly perennial desert stream. We monitored two study pools seasonally for 4 years before and 4 years after the transition from perennial to intermittent flow to evaluate pre‐drying community dynamics and post‐drying recovery trajectories. 3. Mean species richness was not affected by the transition to intermittent flow, though seasonal patterns of richness did change. Sample densities were much higher in post‐drying samples. 4. The stream pool communities underwent a catastrophic regime shift after transition to intermittent flow, moving to an alternative stable state with novel seasonal trajectories, and did not recover to pre‐drying configurations after 4 years. Six invertebrate species were extirpated by the initial drying event, while other species were as much as 40 times more abundant in post‐drying samples. In general, large‐bodied top predators were extirpated from the system and replaced with high abundances of smaller‐bodied mesopredators. 5. Our results suggest that the loss of perennial flow caused by intensified droughts and water withdrawals could lead to significant changes in community structure and species composition at local and regional scales.     

Interactive community responses to disturbance in streams: disturbance history moderates the influence of disturbance types

Both disturbance history and disturbance type act to structure communities through selecting for particular species traits but they may also interact. For example, flooding selects for species with flood‐resistant traits in streams, but those traits could make communities susceptible to other disturbances and so could cause shifts in community composition due to anthropogenic climate change. To better understand the interactive influences of disturbance history and type on community composition, we investigated the response of macroinvertebrate communities to disturbance using in‐stream channels. Using a split‐plot design, individual channels in five ‘stable’ streams and five ‘frequently disturbed’ streams (disturbance history) were subject to different disturbance type treatments (flooding, drying and a control). Disturbance type independently drove effects on species diversity, but all other effects of disturbance type depended on disturbance history. In particular, the interaction of disturbance type and history determined overall community response. Both disturbance types tested produced similar community responses in frequently disturbed streams, including changes in community composition and alterations to the abundance of less mobile taxa, but low‐flow had a significantly greater effect in stable streams. Macroinvertebrate drift was greatest in the rock‐rolling treatments and significantly less in the low‐flow treatment for both disturbance histories. Therefore, disturbance history moderated the effects of disturbance type and determined the mechanism of community response by determining how well species were adapted to disturbance. This outcome suggests that previous disturbances strongly influence how vulnerable communities are to changes in disturbance, and so should be considered when predicting how changes in disturbance regimes will affect future community composition.     

Headwater streams and forest management: Does ecoregional context influence logging effects on benthic communities?

Effects of forest management on stream communities have been widely documented, but the role that climate plays in the disturbance outcomes is not understood. In order to determine whether the effect of disturbance from forest management on headwater stream communities varies by climate, we evaluated benthic macroinvertebrate communities in 24 headwater streams that differed in forest management (logged-roaded vs. unlogged-unroaded, hereafter logged and unlogged) within two ecological sub-regions (wet versus dry) within the eastern Cascade Range, Washington, USA. In both ecoregions, total macroinvertebrate density was highest at logged sites (P = 0.001) with gathering-collectors and shredders dominating. Total taxonomic richness and diversity did not differ between ecoregions or forest management types. Shredder densities were positively correlated with total deciduous and Sitka alder (Alnus sinuata) riparian cover. Further, differences in shredder density between logged and unlogged sites were greater in the wet ecoregion (logging × ecoregion interaction; P = 0.006) suggesting that differences in post-logging forest succession between ecoregions were responsible for differences in shredder abundance. Headwater stream benthic community structure was influenced by logging and regional differences in climate. Future development of ecoregional classification models at the subbasin scale, and use of functional metrics in addition to structural metrics, may allow for more accurate assessments of anthropogenic disturbances in mountainous regions where mosaics of localized differences in climate are common.     

Relationships between stream macroinvertebrate communities and new flood‐based indices of glacial influence

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Macroinvertebrate communities in streams with contrasting water sources in the Japanese Alps

Alpine streams are typically fed from a range of water sources including glacial meltwater, snowmelt, groundwater flow, and surface rainfall runoff. These contributions are projected to shift with climate change, particularly in the Japanese Alps where snow is expected to decrease, but rainfall events increase. The overarching aim of the study was to understand the key variables driving macroinvertebrate community composition in groundwater and snowmelt‐fed streams (n = 6) in the Kamikochi region of the northern Japanese Alps (April–December 2017). Macroinvertebrate abundance, species richness, and diversity were not significantly different between the two stream types. Community structure, however, was different between groundwater and snowmelt‐fed streams with macroinvertebrate taxa specialized for the environmental conditions present in each system. Temporal variation in the abundance, species richness, and diversity of macroinvertebrate communities was also significantly different between groundwater and snowmelt streams over the study period, with snowmelt streams exhibiting far higher levels of variation. Two snowmelt streams considered perennial proved to be intermittent with periodic drying of the streambed, but the macroinvertebrates in these systems rebounded rapidly after flows resumed with no reduction in taxonomic diversity. These same streams, nevertheless, showed a major reduction in diversity and abundance following periods of high flow, indicating floods rather than periodic drying was a major driver of community structure. This conclusion was also supported from functional analyses, which showed that the more variable snowmelt streams were characterized by taxa with resistant, rather than resilient, life‐history traits. The findings demonstrate the potential for significant turnover in species composition with changing environmental conditions in Japanese alpine stream systems, with groundwater‐fed streams potentially more resilient to future changes in comparison to snowmelt‐fed streams.     

Flow‐related disturbances in forested and agricultural rivers: influences on benthic macroinvertebrates

Flow disturbances and conversions of land‐use types are two major factors that influence river ecosystems. However, few studies have considered their interactions and separated their individual effects on aquatic organisms. Using monthly monitoring data from two streams with different land‐use types (i.e. forest and agriculture) in the subtropical Central China over three years, we accurately predicted the changes of macroinvertebrate communities under flood disturbances and land‐use type conversions. The dominant taxa and main community metrics significantly declined following flash floods. Several mayflies and chironomid had rapid rates of recovery, which could reach high abundance in three months after floods. And most of the community metrics recovered more rapidly in the forested river than that in the agricultural river. Stepwise multiple regression (SMR) models were used to investigate the relationships between biotic metrics and hydrological and temporal variables. For example, SMR revealed that floods reduced the stability of benthic communities, and the length of low flow period was of considerable importance to the recovery of the fauna. Two‐way ANOVA indicated that intra‐annual fluctuation had more (e.g. the total abundance and wet biomass), equal (e.g. total richness, EPT richness, percent EPT abundance, and Margalef index), or less (e.g. tolerant value) influence on macroinvertebrate communities than land‐use types. Consequently, the effects of floods on macroinvertebrates should be taken into account when macroinvertebrates are used as indicators for assessing river ecosystem. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Resistance and resilience of stream insect communities to repeated hydrologic disturbances after a wildfire

1. Wildfires are often followed by severe, sediment‐laden floods in burned catchments. In this study, we documented resistance and resilience of stream insect communities to repeated postfire flash floods in a ‘burned stream’. We employed a before‐after‐control‐impact (BACI) design, where communities in comparable reaches of a burned stream and a reference stream were sampled from 2 years before, to 6 years after, a crown wildfire in north‐central New Mexico. 2. The first 100‐year flood following the 1996 Dome wildfire reduced total insect density and taxon richness to near zero in the burned stream. Despite showing low resistance, density returned rapidly to prefire levels because of colonisation by simuliids, chironomids and the mayfly Baetis tricaudatus. In general, taxa that were generalist feeders (collectors) with strong larval dispersal dominated communities in early postfire years with repeated, moderate flash floods. 3. Taxon richness and community composition were less resilient to postfire hydrologic disturbances. Taxon richness did not recover until floods dampened 4 years after the fire. Despite hydrologic recovery, composition in the burned stream still differed from prefire and reference stream compositions after 6 years postfire. A unique assemblage, dominated by taxa with strong larval or adult dispersal, was established after flash floods abated. Specialist feeders (shredders and grazers) that were common in prefire years were reduced or absent in the postfire assemblage. 4. Community succession in the burned stream was explained by the interaction between species traits, geographic barriers to colonisation and hydrologic conditions after the fire. Comparable changes in insect density, taxon richness, community composition and trait representation were not found in the reference stream, providing strong evidence that repeated postfire flash floods shaped community responses in the burned stream.     

Drought ecohydrology alters the structure and function of benthic invertebrate communities in mountain streams

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Taxonomic and biological trait differences of stream macroinvertebrate communities between mediterranean and temperate regions: implications for future climatic scenarios

Streams in mediterranean regions have highly seasonal discharge patterns, with predictable torrential floods and severe droughts. In contrast, discharge is less variable in temperate regions and intermittent flow conditions are uncommon. Hydroclimatic models predict that climate change would increase frequency and severity of floods and droughts across Europe, thus increasing the proportion of streams with mediterranean characteristics in actually temperate areas. Correspondingly, understanding actual ecological differences between mediterranean and temperate streams may help to anticipate large‐scale ecological impacts of climate change. Given that large‐scale factors determine local community composition, we hypothesized that climatic differences between mediterranean and temperate regions should affect the taxonomic and biological trait composition in streams. We assembled the abundance of stream macroinvertebrate genera of 265 sites each from the Mediterranean Basin and from temperate Europe and linked these abundances to published information on 61 categories of 11 biological traits reflecting the potential of resilience from and resistance to disturbances. Although regional taxonomic richness was higher in the mediterranean than in the temperate region, local taxonomic richness and diversity did not significantly differ between regions. Local trait richness and diversity were significantly higher in the mediterranean region. Both local taxonomic and trait‐community composition differed between regions, but the former varied much more than the latter, highlighting that climate change could produce large changes in the taxonomic but rather weak changes in the trait composition. The mediterranean region was characterized by macroinvertebrates with higher dispersion and colonization capabilities, suggesting that species loss in the temperate region, by extinction or northward emigration of taxa, would be compensated for by immigration of southern mediterranean taxa. Thus, climate change would likely have stronger implications for the local conservation of taxa than for the trait composition of stream macroinvertebrate communities.     

Permafrost thaw and intense thermokarst activity decreases abundance of stream benthic macroinvertebrates

Intensification of permafrost thaw has increased the frequency and magnitude of large permafrost slope disturbances (mega slumps) in glaciated terrain of northwestern Canada. Individual thermokarst disturbances up to 40 ha in area have made large volumes of previously frozen sediments available for leaching and transport to adjacent streams, significantly increasing sediment and solute loads in these systems. To test the effects of this climate‐sensitive disturbance regime on the ecology of Arctic streams, we explored the relationship between physical and chemical variables and benthic macroinvertebrate communities in disturbed and undisturbed stream reaches in the Peel Plateau, Northwest Territories, Canada. Highly disturbed and undisturbed stream reaches differed with respect to taxonomic composition and invertebrate abundance. Minimally disturbed reaches were not differentiated by these variables but rather were distributed along a disturbance gradient between highly disturbed and undisturbed sites. In particular, there was evidence of a strong negative relationship between macroinvertebrate abundance and total suspended solids, and a positive relationship between abundance and the distance from the disturbance. Increases in both sediments and nutrients appear to be the proximate cause of community differences in highly disturbed streams. Declines in macroinvertebrate abundance in response to slump activity have implications for the food webs of these systems, potentially leading to negative impacts on higher trophic levels, such as fish. Furthermore, the disturbance impacts on stream health can be expected to intensify as climate change increases the frequency and magnitude of thermokarst.     

Testing for temporal coherence across spatial extents: the roles of climate and local factors in regulating stream macroinvertebrate community dynamics

Temporal coherence or spatial synchrony refers to the tendency of population, community or ecosystem dynamics to behave similarly among locations through time as a result of spatially‐correlated environmental stochasticity (Moran effect), dispersal or trophic interactions. While terrestrial studies have treated synchrony mainly as a population‐level concept, the majority of freshwater studies have focused on community‐level patterns, particularly in lake planktonic communities. We used spatially and temporally hierarchical data on benthic stream invertebrates across six years, with three seasonal samples a year, in 11 boreal streams to assess temporal coherence at three spatial extents: 1) among regions (watersheds), 2) among streams within a region, and 3) among riffles within a stream, using the average of correlation coefficients for stream/riffle pairs across years. Our results revealed the primacy of strongly synchronized climatic factors (precipitation, air temperature) in inducing temporal coherence of macroinvertebrate assemblages across geographically distinct sites (i.e. Moran effect). Coherence tended to decrease with increasing spatial extent, but positive coherence was detected for most biological variables even at the largest extent (about 350 km). The generally high level of coherence reflected the strong seasonality of boreal freshwater communities. A hydrologically exceptional year enhanced the synchrony of biological variables, particularly total macroinvertebrate abundance. Regionally low precipitation in that year led to a substantial decrease in benthic densities across a broad spatial extent, followed by a rapid post‐drought recovery. Coherence at the among‐riffle (within‐stream) extent was lower than expected, implying that local‐scale habitat filters determine community dynamics at smaller spatial extents. Thus, temporal coherence of stream benthic communities appears to be controlled by partly different processes at different spatial scales.     

The influences of climatic variation and vegetation on stream biota: lessons from the Big Dry in southeastern Australia

Aquatic biodiversity faces increasing threats from climate change, escalating exploitation of water and land use intensification. Loss of vegetation in catchments (= watersheds) has been identified as a substantial problem for many river basins, and there is an urgent need to better understand how climate change may interact with changes in catchment vegetation to influence the ecological condition of freshwater ecosystems. We used 20 years of biological monitoring data from Victoria, southeastern Australia, to explore the influences of catchment vegetation and climate on stream macroinvertebrate assemblages. Southeastern Australia experienced a severe drought from 1997 to 2009, with reductions of stream flows >50% in some areas. The prolonged drying substantially altered macroinvertebrate assemblages, with reduced prevalence of many flow‐dependent taxa and increased prevalence of taxa that are tolerant of low‐flow conditions and poor water quality. Stream condition, as assessed by several commonly used macroinvertebrate indices, was consistently better in reaches with extensive native tree cover in upstream catchments. Prolonged drought apparently caused similar absolute declines in macroinvertebrate condition indices regardless of vegetation cover, but streams with intact catchment and riparian vegetation started in better condition and remained so throughout the drought. The largest positive effects of catchment tree cover on both water quality and macroinvertebrate assemblages occurred above a threshold of ca. 60% areal tree cover in upstream catchments and in higher rainfall areas. Riparian tree cover also had positive effects on macroinvertebrate assemblages, especially in warmer catchments. Our results suggest that the benefits of extensive tree cover via improved water quality and in‐channel habitat persist during drought and show the potential for vegetation management to reduce negative impacts of climatic extremes for aquatic ecosystems.     

Assessing stream channel restoration: the phased recovery framework

Channel reconfiguration is one of the most common and costly stream restoration techniques, though its effectiveness is frequently questioned. Project monitoring often tracks changes in macroinvertebrate communities and other responses for a 5‐year period. However, channel reconfiguration is a documented disturbance to stream ecosystems, suggesting that this form of restoration initiates succession over longer time frames than monitoring typically captures. To address the role of succession in stream ecosystem recovery, we developed the Phased Recovery Framework (PRF) which proposes benchmarks represented by predictable habitat structure and community composition based on project age. The PRF was tested across nine stream restoration projects in western Montana, ranging in age from 1 to 18 years, each paired with an established reference system. We tested for differences in channel form, habitat character, and macroinvertebrate community composition. While restoration established desired channel form, most biotic variables had not recovered to reference condition even for the oldest projects. Across all sites, phases of the PRF were poor predictors of response. However, analyzing responses to reconfiguration independently for sites in watersheds with unimpaired water quality versus those experiencing excessive nutrient enrichment (i.e. impaired sites) indicated that biological variables converged on reference conditions at unimpaired sites, but diverged across impaired reaches. These large‐scale anthropogenic influences may play a stronger role in recovery than do changes to channel form and need to be incorporated into project design and success criteria. Assessment of the PRF suggests that short‐term monitoring is not likely to produce reliable indicators of effectiveness without incorporating locally appropriate change associated with watershed impairment and successional progression.     

Habitat heterogeneity and disturbance influence patterns of community temporal variability in a small temperate stream

Both habitat heterogeneity and disturbance can profoundly influence ecological systems at many levels of biological and ecological organization. However, the joint influences of heterogeneity and disturbance on temporal variability in communities have received little attention despite the intense homogenizing influence of human activity. I performed a field manipulation of substrate heterogeneity in a small New England stream, and measured changes in benthic macroinvertebrate communities for 100 days—a period that included both a severe drought and a flood. Generally, community variability decreased with increasing substrate heterogeneity. However, within sampling intervals, this relationship tended to fluctuate through time, apparently tracking changes in hydrology. At the beginning of the experiment, community temporal variability clearly decreased along a gradient of increasing substrate heterogeneity—a result consistent with an observational study performed the previous year. During the subsequent weeks, droughts and flooding created exceptionally high variability in both hydrology and benthic macroinvertebrate community structure resulting in the disappearance of this relationship. However, during the last weeks of the experiment when hydrologic conditions were relatively more stable, the negatively sloped relationship between community temporal variability and habitat heterogeneity reemerged and mimicked relationships observed both early in the experiment and in the previous year’s study. High habitat heterogeneity may promote temporal stability through several mechanisms including stabilization of resources and increased refugia from minor disturbances or predation. However, the results of this experiment suggest that severe disturbance events can create large-scale environmental variability that effectively swamps the influence of habitat heterogeneity, illustrating that a thorough understanding of community temporal variability in natural systems will necessarily consider sources of environmental variability at multiple spatial and temporal scales. Handling editor: L. M. Bini     

Long-term seasonal variation in the biological traits of benthic-macroinvertebrates in two Mediterranean-climate streams in California, U.S.A.

1. We investigated the seasonal variation of biological traits and the influence of interannual rainfall variability on this pattern. Using long‐term survey data (6–19 years) from an intermittent and a perennial stream in the Mediterranean‐climate region of northern California, we examined 16 fuzzy‐coded biological traits (e.g. maximum size, life cycle duration, and mode of respiration). 2. Seasonal habitat variability is higher in the intermittent stream than in the perennial stream. During the winter and spring wet‐season both streams flood; however, during the summer dry‐season, the intermittent stream forms isolated pools in (occasionally drying completely). 3. Seasonal habitat variability influenced both taxonomic and biological trait composition. Distinct taxonomic communities were present in each season, particularly in the intermittent stream. The intermittent stream also exhibited more seasonal variation in biological traits than the perennial stream. 4. Despite statistically significant seasonal variation, trait composition was relatively stable among seasons in comparison with taxonomic composition and abundance. Taxonomic composition varied considerably between seasons, because of high seasonal and interannual replacement of taxa resulting from seasonal habitat changes. 5. The seasonality of taxonomic composition and abundance was sensitive to interannual rainfall variability. In dry years, the taxonomic composition of communities was more similar between seasons than in wet years, while trait composition was relatively insensitive to rainfall variability. 6. Despite high seasonal variation in abundance and taxonomic composition, biological traits of aquatic macroinvertebrates varied less and exhibited seasonal stability, which may be a result of the unpredictability and harshness of stream environments.     

Temporal response of stream benthic macroinvertebrate communities to the synergistic effects of anthropogenic acidification and natural drought events

1. We evaluated whether the surprisingly weak biological recovery associated with declines in acid deposition were related to drought‐induced re‐acidification of streams. We used test site analysis (TSA) to characterise temporal changes (1995–2003) in the degree and nature of impairments to stream benthic macroinvertebrate (BMI) communities influenced by acid deposition and drought. 2. The BMI communities in four historically impacted test streams were compared with communities in 30 minimally impacted reference streams. Six multivariate (e.g. ordination axes scores) and four traditional (e.g. % Diptera) summary metrics were used to describe BMI communities. Using all metrics simultaneously (i.e. Mahalanobis or generalised distance), the TSA provided a single probability that a test community was impaired. If a test community was significantly impaired, a further analysis was done to identify the metric(s) important in distinguishing the test community from reference condition. 3. Results of the TSAs indicated that the generalised distances between test communities and the reference condition were inversely related to stream water pH (n = 36). The TSAs also indicated ordination metrics based on BMI abundance were important in distinguishing significantly impaired communities from reference conditions. Temporal trends indicated that there has been short‐term recovery of these BMI communities, but that overall improvements have been hampered by acid or metal toxicity associated with drought‐induced re‐acidification of the streams. 4. Our use of a variety of summary metrics to obtain a single statistical test of significance within the context of the reference‐condition approach provided a simple and unambiguous framework for evaluating the biological condition of test sites.     

Context‐dependent resistance of freshwater invertebrate communities to drying

More freshwater ecosystems are drying in response to global change thereby posing serious threat to freshwater biota and functions. The production of desiccation‐resistant forms is an important adaptation that helps maintain biodiversity in temporary freshwaters by buffering communities from drying, but its potential to mitigate the negative effects of drying in freshwater ecosystems could vary greatly across regions and ecosystem types. We explored this context dependency by quantifying the potential contribution of desiccation‐resistance forms to invertebrate community recovery across levels of regional drying prevalence (defined as the occurrence of drying events in freshwaters in a given region) and ecosystem types (lentic, lotic) in temporary neotropical freshwaters. We first predicted that regional drying prevalence influences the selection of species with desiccation‐resistant forms from the regional species pools and thus increases the ability of communities to recover from drying. Second, we predicted lentic freshwaters harbor higher proportions of species with desiccation‐resistant forms compared to lotic, in response to contrasted hydrologic connectivity. To test these predictions, we used natural experiments to quantify the contribution of desiccation‐resistant forms to benthic invertebrate community recovery in nine intermittent streams and six geographically isolated temporary wetlands from three Bolivian regions differing in drying prevalence. The contribution of desiccation‐resistant forms to community recovery was highest where regional drying prevalence was high, suggesting the species pool was adapted to regional disturbance regimes. The contribution of desiccation‐resistant forms to community recovery was lower in streams than in wetlands, emphasizing the importance of hydrologic connectivity and associated recolonization processes from in‐stream refuges to recovery in lotic systems. In all regions, the majority of functional traits were present in desiccation‐resistant taxa indicating this adaptation may help maintain ecosystem functions by buffering communities from the loss of functional traits. Accounting for regional context and hydrologic connectivity in community recovery processes following drying can help refine predictions of freshwater biodiversity response to global change.