Flight activity of insects along a mountain stream: is directional flight adaptive?

1. Flight activity of Trichoptera, Plecoptera and Ephemeroptera was studied by sticky trapping for 12 months at five sites along a New Zealand mountain stream. Over 19 000 insects were captured by the traps, which were located in forest and grassland reaches, including a reach with intermittent flow.
2. Most species occurred predominantly in forest or grassland, although some were trapped throughout the stream. Longitudinal distributions of adults and their larvae were strongly correlated.
3. Flight periods of 24 caddisflies, three mayflies and four stoneflies ranged from 2 to 12 months. Six species were trapped in all months and 17 (55%) in more than 5 months.
4. The most abundant forest-dwelling caddisfly species were over-represented on the downstream sides of sticky traps located in, and immediately below, forest indicating a majority was flying upstream. Upstream flight compensates for downstream drift of larvae and should maximize the likelihood that forest-dwelling species will locate preferred habitat for egg, larval and/or adult development. Unlike the caddisflies, the stonefly Spaniocerca zelandica was over-represented on the upstream sides of traps, suggesting that some adults may float or fly downstream following emergence.
5. In contrast to forest-dwelling species, only one common caddisfly ( Oxyethira albiceps ) was over-represented on the downstream sides of traps at grassland sites. Unlike the forest-dwelling species, most species taken at the downstream sites probably came from a variety of sources, including a nearby stream.     

Nutrient Subsidies from Iteroparous Fish Migrations Can Enhance Stream Productivity

Migratory animals often transfer nutrients between ecosystems, enhancing productivity in the subsidized system. Most research on nutrient subsidies by migratory fishes has focused on Pacific salmon, whose semelparous life history is unusual among migratory fishes. To test whether iteroparous species can provide ecologically important nutrient inputs to stream ecosystems, we experimentally blocked the migration of suckers (Catostomidae) midway up an oligotrophic tributary of Lake Michigan. Comparing reaches upstream of the barrier to downstream reaches containing thousands of breeding fish, we found that suckers elevated phosphorus and nitrogen concentrations three- to five-fold. Algal accrual was doubled and caddisflies grew 12% larger in subsidized reaches relative to reference reaches. An enclosure experiment demonstrated that caddisflies with access to a fish carcass rapidly became enriched in ¹⁵N and ¹³C, and experimental carcass additions were rapidly colonized by high densities of caddisflies. However, under natural conditions below the experimental barrier, caddisflies became enriched in ¹⁵N but not ¹³C, indicating that fish-derived nutrients entered the stream food web primarily through indirect pathways rather than direct consumption of carcasses or gametes. At pupation, an average of 18% of caddisfly tissue N below the barrier was sucker-derived. In comparison to our focal stream, a reference stream with few suckers showed no seasonal or longitudinal patterns in nutrients and stable isotopes. These results demonstrate that iteroparous fish migrations can spur productivity via nutrient subsidies, despite low mortality rates. Thus, concerns about negative ecosystem-level consequences of blocking migrations of semelparous fishes are also applicable to iteroparous species when migrations are large.     

Tufa Barriers from a Caddisfly's Point of View: Streams or Lake Outlets?

The caddisfly community composition in different microhabitats at tufa barriers was studied in the Plitvice Lakes NP, Croatia. Three tufa barriers were investigated and six emergence traps were installed at each site covering various microhabitats. Sampling was conducted monthly during the year 2008. Tufa barriers are lake outlet habitats, but they feature a variety of microhabitats similar to streams, which is reflected in the composition of caddisfly communities regarding longitudinal distribution preferences. The caddisfly communities at all three sites were dominated by species typical for the rhithral zone, but there was a shift in species composition along a longitudinal gradient, from the epirhithral to the metarhithral zone. Analysis of functional feeding guilds showed considerable differences between the caddisfly community at the Labudovac barrier and both downstream barriers, shifting from one with a quite diverse composition, to one completely dominated by passive filter‐feeders. Passive filter feeders were not represented by the same taxa at up‐ and downstream barriers (i.e., by Hydropsyche species and Wormaldia species, at the Labudovac barrier and at both downstream barriers, respectively). Due to high complexity and habitat diversity, the highest diversity and equitability of caddisfly communities were recorded at microhabitats with particulate tufa and medium current velocity (10–20 cm/s). The lowest diversity and species richness were recorded for silt with low current velocity (0–10 cm/s). Abundance of caddisflies was positively correlated with current velocity due to a very high proportion of rheophilic passive filter feeders in the communities. However, community composition and structure is only to some degree influenced by microhabitat characteristics, but rather by their general position within the barrage‐lake system, i.e., longitudinal distribution and thereby availability of different food resources. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Restoring Stream Ecosystems: Lessons from a Midwestern State

Reach‐scale stream restorations are becoming a common approach to repair degraded streams, but the effectiveness of these projects is rarely evaluated or reported. We surveyed governmental, private, and nonprofit organizations in the state of Indiana to determine the frequency and nature of reach‐scale stream restorations in this midwestern U.S. state. For 10 attempted restorations in Indiana, questionnaires and on‐site assessments were used to better evaluate current designs for restoring stream ecosystems. At each restoration site, habitat and water quality were evaluated in restored and unrestored reaches. Our surveys identified commonalities across all restorations, including the type of restoration, project goals, structures installed, and level of monitoring conducted. In general, most restorations were described as stream‐relocation projects that combined riparian and in‐stream enhancements. Fewer than half of the restorations conducted pre‐ or post‐restoration monitoring, and most monitoring involved evaluations of riparian vegetation rather than aquatic variables. On‐site assessments revealed that restored reaches had significantly lower stream widths and greater depths than did upstream unrestored reaches, but riparian canopy cover often was lower in restored than in unrestored reaches. This study provides basic information on midwestern restoration strategies, which is needed to identify strengths and weaknesses in current practices and to better inform future stream restorations.     

Dispersal of adult caddisflies (Trichoptera) into forests alongside three New Zealand streams

The abundance and taxonomic richness of adult caddisfly faunas were determined at varing distances (up to 200 m) away from three North Island, New Zealand, streams to help define appropriate forested riparian zone widths for adult aquatic insects. Adults were collected using sticky traps and ultraviolet light traps on four occasions over summer. Light traps were more effective at catching caddisflies than sticky traps, but both methods gave similar patterns of declining abundance and taxonomic richness with distance from the stream edge. Abundances of total caddisflies at 20 m were <21% of those caught at the stream edge for both trapping techniques. The same trends were evident for abundances of most common species in light traps, whereas bimodal peaks were evident with distance from the channel edge for percentage females of three common species. More than 30% of species caught at the stream edge was found in light traps at least 70 m into forest at all sites. A faster rate of decline for abundance than richness with distance away from the stream reflected the relatively large distances travelled by representatives of many species. Similar results from the three sites for numbers and species richness indicate that the main area of activity for adult Trichoptera in forested riparian zones was within 30 m of the stream edge at these sites.     

Relating larval distributions to patterns of oviposition: evidence from lotic hydrobiosid caddisflies

1. To assess the influence of oviposition patterns on distributions of hydrobiosid caddisfly larvae, abundances of three hydrobiosid caddisfly species were estimated in whole reaches with and without suitable oviposition substrata along an upland temperate Australian stream. In addition, within‐reach spatial patterns were examined in relation to known oviposition locations as well as flow characteristics that corresponded to oviposition preferences. 2. Larval abundances in all samples were low relative to previous estimates of egg inputs into reaches. The presence of suitable oviposition substrata at a reach did not influence the abundances of larvae. For one species, benthic samples taken proximate to oviposition substrata revealed a sharp decline in abundance between first and later instars. Larvae of two taxa exhibited flow‐specific habitat preferences mirroring those described as important as cues for oviposition site selection. Previous estimates of egg mass aggregation were also reflected in similarly high levels of larval clumping; however, larval aggregation did not differ between reaches with and without oviposition sites. 3. Collectively, our results suggest that a large difference in the supply of potential recruits does not translate into marked differences in larval abundances of hydrobiosids at the reach level but may account for some variation in larval distribution within a reach and between instars. This evidence is consistent with the notion that (i) posthatching dispersal between reaches is substantial and (ii) mortality of larvae, particularly early instars, is high and (iii) within reach habitat preferences change with larval growth. 4. If adults are unable to lay eggs at reaches without suitable oviposition substrata, then reaches with oviposition substrata may be crucial as a source of recruits elsewhere. Furthermore, high mortality and/or dispersal among first instars signal this as an important part of the life history. Further data on the mortality and dispersal rates of newly hatched larvae would greatly benefit our understanding of the importance of local births in structuring patterns of abundance in stream invertebrates.     

Effects of environmental factors on the phytoplankton community in the Gulf of Finland – unattended high frequency measurements and multivariate analyses

We studied life history and secondary production of Cheumatopsyche spp. (Trichoptera: Hydropsychidae) in StroublesCreek, a 3rd order stream near Blacksburg, Virginia to compare lifehistories and production dynamics in two reaches with differentland-uses. The upstream reach was influenced by agriculturalactivities and more annual degree-days accumulated in this sectionof the stream. The downstream reach was surrounded by forest,received less thermal input than the upstream reach, and showedlittle evidence of human disturbance. Differences between the twosites included a shift in dominant taxa and changes in voltinismand production of hydropsychids. Cheumatopsyche pettiti(Banks) was dominant at the upstream site, while C. pettitiand Cheumatopsyche oxa Ross were both abundant downstream.Cheumatopsyche larvae and other aquatic insects had higherdensities at the forested downstream site. C. pettiti wasbivoltine at the upstream site, but C. pettiti and C. oxa were partially bivoltine downstream, with a small summergeneration overlapping with continuous emergence of overwinteringlarvae. Cheumatopsyche production at the two sites inStroubles Creek was higher than in forested, low-order streams ofNorth America, but similar to some values obtained at streams inagricultural areas and large rivers. Production was higher at thedownstream site, but the superior bivoltinism at the upstream sitecounterbalanced, to some extent, the increase in larval densitythat caused higher production downstream. The adjustment in lifehistory and production dynamics, in the two reaches, showed theecological plasticity of Cheumatopsyche and the importance ofhydropsychids in the energy flow of moderately disturbedstreams.     

Ecological values of Hamilton urban streams (North Island, New Zealand): constraints and opportunities for restoration

Urban streams globally are characterised by degraded habitat conditions and low aquatic biodiversity, but are increasingly becoming the focus of restoration activities. We investigated habitat quality, ecological function, and fish and macroinvertebrate community composition of gully streams in Hamilton City, New Zealand, and compared these with a selection of periurban sites surrounded by rural land. A similar complement of fish species was found at urban and periurban sites, including two threatened species, with only one introduced fish widespread (Gambusia affinis). Stream macroinvertebrate community metrics indicated low ecological condition at most urban and periurban sites, but highlighted the presence of one high value urban site with a fauna dominated by sensitive taxa. Light-trapping around seepages in city gullies revealed the presence of several caddisfly species normally associated with native forest, suggesting that seepage habitats can provide important refugia for some aquatic insects in urban environments. Qualitative measures of stream habitat were not significantly different between urban and periurban sites, but urban streams had significantly lower hydraulic function and higher biogeochemical function than periurban streams. These functional differences are thought to reflect, respectively, (1) the combined effects of channel modification and stormwater hydrology, and (2) the influence of riparian vegetation providing shade and enhancing habitat in streams. Significant relationships between some macroinvertebrate community metrics and riparian vegetation buffering and bank protection suggest that riparian enhancement may have beneficial ecological outcomes in some urban streams. Other actions that may contribute to urban stream restoration goals include an integrated catchment approach to resolving fish passage issues, active reintroduction of wood to streams to enhance cover and habitat heterogeneity, and seeding of depauperate streams with native migratory fish to help initiate natural recolonisation.     

Culvert Replacement and Stream Habitat Restoration: Implications from Brook Trout Management in an Appalachian Watershed, U.S.A.

Large-scale culvert replacement programs could benefit migratory fish populations by reconnecting reproductive and foraging habitats in fragmented watersheds. The objectives of this study were to: (1) identify stream and culvert characteristics contributing to fish passage barriers within an Appalachian watershed, U.S.A.; (2) quantify the total amount of Brook trout ( Salvelinus fontinalis ) reproductive habitat isolated above culverts; and (3) use an ecological currency to identify culvert replacement priorities and stream mitigation credit opportunities. We surveyed 120 state-owned culverts and used a fish passage assessment filter to determine the "passability" of each culvert. We then constructed a geographic information system stream network model to quantify the amount of trout reproductive habitat isolated by culverts. Ninety-seven percent of surveyed culverts were classified as obstacles or complete barriers to trout dispersal. Culvert impassability was higher in small streams with slopes exceeding 3–5%, suggesting a direct relationship between slope and impassability. Thirty-three percent of Brook trout reproductive habitat, representing over 200 km of stream, was isolated by culverts. This is a conservative estimate, because we did not survey privately or federally owned culverts. The top 20 prioritized culverts accounted for nearly half of the habitat loss. Our results indicate that standard culvert designs placed in streams with slopes exceeding 5% consistently produce trout dispersal barriers and should be avoided during new road construction. The process developed here provides an efficient method for identifying culvert replacement priorities and may be used to maximize watershed scale benefits of stream restoration.     

Variation in local abundance and species richness of stream fishes in relation to dispersal barriers: implications for management and conservation

1. Barriers to immigration, all else being equal, should in principle depress local abundance and reduce local species richness. These issues are particularly relevant to stream‐dwelling species when improperly designed road crossings act as barriers to migration with potential impacts on the viability of upstream populations. However, because abundance and richness are highly spatially and temporally heterogeneous and the relative importance of immigration on demography is uncertain, population‐ and community‐level effects can be difficult to detect. 2. In this study, we tested the effects of potential barriers to upstream movements on the local abundance and species richness of a diverse assemblage of resident stream fishes in the Monongahela National Forest, West Virginia, U.S.A. Fishes were sampled using simple standard techniques above‐ and below road crossings that were either likely or unlikely to be barriers to upstream fish movements (based on physical dimensions of the crossing). We predicted that abundance of resident fishes would be lower in the upstream sections of streams with predicted impassable barriers, that the strength of the effect would vary among species and that variable effects on abundance would translate into lower species richness. 3. Supporting these predictions, the statistical model that best accounted for variation in abundance and species richness included a significant interaction between location (upstream or downstream of crossing) and type (passable or impassable crossing). Stream sections located above predicated impassable culverts had fewer than half the number of species and less than half the total fish abundance, while stream sections above and below passable culverts had essentially equivalent richness and abundance. 4. Our results are consistent with the importance of immigration and population connectivity to local abundance and species richness of stream fishes. In turn, these results suggest that when measured at appropriate scales (multiple streams within catchments), with simple protocols amenable to use by management agencies, differences in local abundance and species richness may serve as indicators of the extent to which road crossings are barriers to fish movement and help determine whether road‐crossing improvements have restored connectivity to stream fish populations and communities.     

Maternal behaviours may explain riffle‐scale variations in some stream insect populations

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Fishpond dams affect leaf‐litter processing and associated detritivore communities along intermittent low‐order streams

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Influence of marine-derived nutrients from spawning salmon on aquatic insect communities in southeast Alaskan streams

The objective of this study was to evaluate the influence of the nutrient transfer system between anadromous salmon and aquatic insect communities across multiple, natural stream systems. Between 2000 and 2002, we sampled seven streams in southeast Alaska, seasonally. Of the seven study streams, four received large annual salmon runs (high-run streams), and three were no-run streams. All the streams selected had a natural waterfall barrier to salmon, providing an upstream control reach for each study stream. Insect density, biomass, richness, diversity and functional feeding groups were analyzed before, during and after the fall salmon run in each stream section (i.e. above and below the barrier) of the seven study streams between 2001 and 2002. Results showed that diversity and richness were similar across stream sections and run size within each period, except for during the run when both were significantly lower in downstream sections of high-run streams. Functional feeding group patterns showed higher abundance and biomass of collector–gatherers and shredders during the post spawning, carcass decomposition period. High-run streams had upstream sections with greater abundance and biomass of mayflies (dominated by Baetidae, Heptageniidae and Ephemerellidae) during the run, and downstream sections with greater abundance and biomass of dipterans (dominated by Chironomidae). This study suggests that the often published positive relationship between MDN and stream insect abundance and biomass may only exist for certain taxa, primarily chironomid midges.     

Success and failure in a lotic crayfish invasion: the roles of hydrologic variability and habitat alteration

1. This paper examines the distribution, habitat relationships, and potential for spread of non‐native signal crayfish (Pacifastacus leniusculus) in streams of the Truckee River catchment, California, U.S.A. Crayfish associations with natural features and with impoundments and flow alteration were examined in a survey of 33 streams. Abundance changes were followed over 5 years, which included some of the highest and lowest flows on record, in three streams, two unregulated and one regulated. Movement of marked crayfish was studied in one 0.5 km stream reach just upstream from a reservoir. 2. Signal crayfish were most abundant in low‐gradient streams and were positively associated with proximity to reservoirs (both upstream and downstream). Crayfish were more likely to be found in regulated than unregulated sites, and did not occur in sites upstream of barriers, such as culverts, that separated them from reservoirs or lakes. 3. Crayfish declined in abundance in years following particularly intense and prolonged wet‐season spates, leading to a negative association between crayfish abundance and both peak discharge and duration of bankfull flows. 4. Crayfish moved distances of up to 277 m, and at rates of up to 120 m day^?1, suggesting significant dispersal ability. Larger crayfish moved greater distances and were more likely to move downstream. Female crayfish showed a pattern of upstream movement in early summer and downstream movement late in the summer, opposite the pattern found in two other studies. 5. These results suggest that natural or artificial gradient barriers and, in regulated systems, management of flow regimes to include bankfull or greater flows may help to control invasive crayfish in streams.     

Sequential fishways reconnect a coastal river reflecting restored migratory pathways for an entire fish community

River regulation infrastructure has been implicated in worldwide aquatic biodiversity loss. In‐stream barriers such as weirs prevent fish migration and the impact can be particularly severe for diadromous species. Fishways are frequently installed on in‐stream barriers to reconnect migratory pathways and rehabilitate diadromous fish populations. Sequential fishways may be necessary in rivers where multiple barriers prevent migration. We monitored a coastal fish community's response to fish passage restoration at 10 predominantly low‐level weirs in the freshwater reaches of the Nepean River in southeastern Australia. Few studies have simultaneously assessed fish community passage at successive fishways within a river system. Prior to the installation of the fishways, there was a gradient of reduced species diversity in an upstream direction including the absence of many diadromous species, despite the regular inundation frequency of most weirs. Following the installation of the fishways, species diversity was still greater in the downstream monitoring sites; however, there was evidence of a positive change in fish community structure in middle and upstream sites. Most notably, three diadromous species rapidly expanded their distribution upstream and one amphidromous species expanded its downstream distribution. This study demonstrates appropriately designed successive fishways can successfully reconnect river systems for an entire fish community, encompassing species with a broad range of swimming abilities and diverse life histories.     

The upstream/downstream movement paradox of lotic invertebrates: quantitative evidence from a Welsh mountain stream

• 1 This study attempts to determine the mechanisms by which the downstream displacement of lotic invertebrates is compensated by the upstream movements of immatures and adults. To this end, submerged and aerial traps were set up at three sites, each 100 m apart on a small mountain stream (Yr Ogof) in North Wales and operated for 12 months.
• 2 At Site 1 (the source), 19.3% of the benthic invertebrates lost as drift were replaced by the upstream movement of aquatic stages. At Site 2, the average upstream compensation was 51.1% (although there was a marked difference between the two traps at the site). At downstream Site 3 the average compensation was 55.4%. The net loss of benthos downstream over 1 year was estimated to be 261920 individuals from Site 1, 41891 from Site 2, and 40470 from Site 3.
• 3 At Sites 1 and 2, the numbers of invertebrates drifting were positively correlated with both current speed and water depth. At Sites 2 and 3, the numbers moving upstream were positively correlated with water temperature. At all three sites, the numbers of animals drifting were positively correlated with the numbers moving upstream, under water.
• 4 In none of the eight common species of stoneffy did females show a strong overall upstream flight preference—however, two species, Leuctra nigra and Nemoura erratica, showed a significant downstream preference.
• 5 As with the stonefly nymphs, most of the caddisfly larvae showed a longitudinal distribution. Agapetus fuscipes larvae were most abundant at Site 1, yet females showed no preferred flight direction—this would seem to be inappropriate for maintaining this larval distribution pattern. However, females caught moving upstream at Site 2 were bigger and contained twice as many eggs, which were also larger, than females moving upstream at Site 1. The flight patterns of the other Trichoptera also seemed inappropriate for maintaining their respective larval distributions. Immature insects moved upstream at certain sites and times, but trends were not consistent. There was no correlation between female flight direction and local wind speed and direction.
• 6 Downstream displacement of the most abundant non-insect species, Gammarus pulex, was quite considerable at all three sites. However, at several times during the year the numbers moving upstream approached or exceeded those drifting. Maintenance of benthic population densities in this species is thought to be through a combination of upstream movement (which was highly correlated with benthic density at Sites 1 and 3) and production of young over a long (10 month) reproductive period. Maintenance of the benthic populations of the immature stages of the majority of insect species in Yr Ogof is thought to be through oviposition by females in the vicinity of their emergence sites.

     

Aquatic insects in an estuarine environment: densities, distribution and salinity tolerance

1. Aquatic insects were quantitatively surveyed at five sites along the tidally influenced section of a river-dominated estuary in North Wales. Site 1 was the furthest upstream and was established as a reference site as it was never inundated by salt water. Site 5 was the furthest downstream and was inundated by all incoming tides. Numerically, insects made up 32% of the estuarine invertebrate fauna. 2. Although the densities of most insect taxa decreased towards the estuary mouth, there were significant numbers present downstream for much of the year; for example, in April at site 4 (which was inundated by 81% of all high tides), a mean of 3514 chironomid larvae were recorded per m² of estuary bed. Even at site 5, which was inundated twice daily, there were 747 larvae per m². Among the larger aquatic insects, caddisfly and elmid beetle larvae, together with stonefly nymphs, were consistently taken at site 4 (e.g. maxima of forty-eight caddisfly larvae m^–2 in December and seventy elmids m^–2 in April), although their densities were lower than upstream. 3. There were seasonal shifts in the longitudinal distribution of several taxa, most notably the extension of chironomids down the estuary in April and July, and the concentration of simuliid larvae and mayfly nymphs at site 2 in July. The total freshwater benthos showed a downstream shift between September and December, which was maintained through April and into the summer. The latter was despite peak saltwater inundation (highest tides) in October, November and April. In June and July, when saltwater intrusion was lowest, the ranges of many aquatic insects had contracted to sites 1 and 2. 4. Laboratory experiments showed that virtually all individuals of nineteen species of insects collected from site 1 (freshwater) survived a 4-h immersion in 8.75‰ saltwater (25% strength seawater). Immersion in progressively more saline solutions reduced the survivorship of first the mayflies, followed by the caddisflies Glossosoma conformis and Hydropsyche instabilis. After 4 h in full strength seawater, all specimens of the stonefly Dinocras cephalotes, over half of the Perla bipunctata, and some individuals of nine species of caddisfly were alive. Four species of caddisfly (Sericostoma personatum, Odontocerum albicorne, Potamophylax cingulatus and Adicella reducta) survived a 24-h simulated tidal cycle of immersion. With the exception of P. cingulatus, a few individuals of these caddisfly species survived immersion in full-strength seawater for 24 h. For some individual species there was good agreement between their observed longitudinal distribution in the estuary and laboratory-measured salinity tolerance; however, there was no significant correlation, overall, for the fauna.     

Homage to the Virgin of Ecology,or why an aquatic insect unadapted to desiccation may maintain populations in very small,temporary Mediterranean streams

In temporary streams, the annual constriction of drying is associated with high local extinction risk. To survive in such habitats, organisms with no specific biological traits for coping with dry periods should experience high colonisation rates from permanent reaches of the same basin or from other basins. Hydropsyche siltalai is a widespread caddisfly common in permanent and temporary headwaters reaches in the Mediterranean climate region of the Iberian Peninsula. In this study, we used genetic analyses to test if populations of H. siltalai in temporary streams are resettled from populations of the same basin or from other basins. The geographical distribution of H. siltalai was surveyed in 97 temporary and permanent reaches across four basins; larvae were found in 22 reaches (12 temporary and 10 permanent). Population genetic analyses of 11 selected reaches (6 temporary and 5 permanent) revealed low genetic diversity and no genetic population structure among and within basins. Overall, H. siltalai appeared to disperse well among basins independent of stream temporality. Permanent reaches from different basins act as a source of the individuals that recolonise temporary reaches after local extinctions, indicating a metapopulation structure at regional scale. Moreover, our results support other studies that showed that dispersal among basins is a recurrent pattern in aquatic insects.     

Habitat filtering and adult dispersal determine the taxonomic composition of stream insects in an urbanizing landscape

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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.