Stable isotopes identify dispersal patterns of stonefly populations living along stream corridors

1. Populations in different locations can exchange individuals depending on the distribution and connectivity of suitable habitat, and the dispersal capabilities and behaviour of the organisms. We used an isotopic tracer, ¹⁵N, to label stoneflies (Leuctra ferruginea) to determine the extent of adult flight along stream corridors and between streams where their larvae live. 2. In four mass, mark‐capture experiments we added ¹⁵NH₄Cl continuously for several weeks to label specific regions of streams within the Hubbard Brook Experimental Forest, NH, U.S.A. We collected adult stoneflies along the labelled streams (up to 1.5 km of stream length), on transects through the forest away from labelled sections (up to 500 m), and along an 800‐m reach of adjacent tributary that flows into a labelled stream. 3. Of 966 individual adult stoneflies collected and analysed for ¹⁵N, 20% were labelled. Most labelled stoneflies were captured along stream corridors and had flown upstream a mean distance of 211 m; the net movement of the population (upstream + downstream) estimated from the midpoint of the labelled sections was 126 m upstream. The furthest male and female travelled approximately 730 m and approximately 663 m upstream, respectively. We also captured labelled mature females along an unlabelled tributary and along a forest transect 500 m from the labelled stream, thus demonstrating cross‐watershed dispersal. 4. We conclude that the adjacent forest was not a barrier to dispersal between catchments, and adult dispersal linked stonefly populations among streams across a landscape within one generation. Our data on the extent of adult dispersal provide a basis for a conceptual model identifying the boundaries of these populations, whose larvae are restricted to stream channels, and whose females must return to streams to oviposit.     

Sex ratio and maturity indicate the local dispersal and mortality of adult stoneflies

1. Despite a recent upsurge in interest, there remains remarkably little information about the dispersal and survival of the adults of most stream‐dwelling insects, although this is a basic requirement for understanding their long‐term population dynamics. 2. Using Malaise traps for a whole annual flight period, we captured adult stoneflies (Leuctra nigra) along transects perpendicular to three upland Welsh streams. We assessed spatial and temporal patterns in sex ratio to infer local dispersal and, using maturity as an age marker, estimated the mortality of adult females. 3. Nearly all adult stoneflies (99%) were taken in the period 26 April–23 July, and the onset of the male and female flight periods was the same. Most males (90%) had been caught by late June. Females were classified as immature (without ripe eggs) or mature, and 90% of immature females had been caught by mid‐late June (depending on catchment). As immature females declined in the catch, mature females increased, 10% having been caught by late‐May to early June and 90% by early to mid‐July. The median catches of immature and mature females were separated by 32 days in all three catchments. 4. There was a female bias in the sex ratio overall, which increased as time passed and was attributable partially to the greater longevity of females. Late in the flight period, however, female bias was also greater near the stream channel implying a return of mature females (but not males) from the riparian vegetation, presumably to oviposit. 5. The number of mature females was less than the number of immatures in two of the three channels. Over all three catchments, the overall mortality of females over the 32 days taken to mature was estimated at 29%, equivalent to an exponential daily rate of 1.1% day^?1. The apparently negative mortality rate in one catchment (i.e. more matures than immatures being caught) could be due to an influx of adult females from elsewhere along the channel to oviposit. 6. Natural markers of age and population structure, such as sex ratio and female maturity, thus enabled us to detect a return of females to the stream to oviposit, after prior limited dispersal into the riparian zone, and to infer longitudinal movements in search of suitable sites. We were also able to estimate mortality in the field. Such natural markers seem to have been underexploited in the study of adult aquatic insects.     

Flight activity of adult stoneflies in relation to weather

Abstract. 1. Dispersal of adult aquatic insects between streams may have important consequences for local and regional population dynamics, but little is known about how dispersal is affected by weather conditions.
2. The influence of meteorological variables on flight activity of adult stoneflies (Plecoptera: Leuctridae, Nemouridae, and Chloroperlidae) was investigated using Malaise traps adjacent to three upland streams in the Plynlimon area of mid Wales, U.K.
3. Numbers of adult stoneflies captured weekly in the traps were related positively to air temperature and related negatively to wind speed. Meteorological conditions during daylight showed stronger relationships with flight activity than did conditions at night.
4. There was inter-site variation in the strength of weather effects on stonefly flight. Wind speed was significant at only one site, which had higher average wind speed than the other sites.
5. Annual variation in weather conditions during adult flight periods may result in varying extent of dispersal between sites, influencing community dynamics over a wide area.     

Spatial population genetic structure reveals strong natal site fidelity in Echinocladius martini (Diptera: Chironomidae) in northeast Queensland,Australia

1. A diverse array of patterns has been reported regarding the spatial extent of population genetic structure and effective dispersal in freshwater macroinvertebrates. In river systems, the movements of many taxa can be restricted to varying degrees by the natural stream channel hierarchy. 2. In this study, we sampled populations of the non‐biting freshwater midge Echinocladius martini in the Paluma bioregion of tropical northeast Queensland to investigate fine scale patterns of within‐ and among‐stream dispersal and gene flow within a purported historical refuge. We amplified a 639‐bp fragment of mitochondrial cytochrome c oxidase subunit I and analysed genetic structure using pairwise Φ_ST, hierarchical amova , Mantel tests and a parsimony network. Genetic variation was partitioned among stream sections, using Streamtree , to investigate the effect of potential instream dispersal barriers. 3. The data revealed strong natal site fidelity and significant differentiation among neighbouring, geographically proximate streams. We found evidence for only episodic adult flight among sites on separate stream reaches. Overall, however, our data suggested that both larval and adult dispersal was largely limited to within a stream channel. 4. This may arise from a combination of the high density of riparian vegetation physically restricting dispersal and from the joint effects of habitat stability and large population sizes. Together these latter may make it more likely that upstream populations will persist, even in the absence of regular compensatory upstream flight, and will thus reduce the adaptive value of dispersal among streams. Taken together, these data suggest that dispersal of E. martini is highly restricted, to the scale of only a few kilometres, and hence occurs predominantly within the natal stream.     

Small‐scale patterns of genetic variation in a headwater specialist mayfly: No influence of selective forest harvesting on diversity

Terrestrial environments allow the adults of some aquatic insects to disperse between headwater streams, which may be important for maintaining population connectivity and persistence. Winged adult stages of aquatic insects are particularly sensitive to degradation of terrestrial habitat, relying on it for food, reproduction and dispersal. In this study we examined the genetic pattern of the Australian mayfly Ulmerophlebia sp. AV2, in north‐eastern New South Wales, and compared the genetic diversity in forested and partially deforested sub‐catchments. Our hypotheses were (i) patterns of mitochondrial DNA (mtDNA) variation in the Leptophlebiidae mayfly Ulmerophlebia sp. AV2 show a pattern of structuring that reflects low or widespread dispersal along the stream network and across catchments; and (ii) genetic diversity will be lower in partially deforested sub‐catchments compared to forested sub‐catchments. We found gene flow was not restricted among headwater streams within sub‐catchments but was restricted at distances >15 km. Genetic diversity was high (mean haplotype diversity >0.85) in both control and harvested sub‐catchments. Instead, a historical signature of population expansion was detected which is consistent with findings for other aquatic insect taxa of eastern Australia. Our results suggest that the selective harvesting management strategy, including the use of riparian buffer zones, within these sub‐catchments does not appear to restrict dispersal between streams or erode diversity within streams for Ulmerophlebia sp. AV2. Selective harvesting therefore appears to have minimal impacts on terrestrial/aquatic links in the life cycle of this insect.     

Strong land‐use effects on the dispersal patterns of adult stream insects: implications for transfers of aquatic subsidies to terrestrial consumers

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Riparian ground beetles (Coeloptera, Carabidae) preying on aquatic invertebrates: a feeding strategy in alpine floodplains

The food and feeding habits of riparian ground beetles were studied in four alpine floodplains (Bavaria, Germany): a 5th-order stream (the Isar) and three 3rd-order streams. The riparian fauna along the streams mainly consists of predaceous species. Riparian ground beetle densities were much higher along the Isar than along the small streams. Aquatic invertebrates composed 89% of the potential prey for carnivorous terrestrial insects along the Isar. Besides aquatic organisms washed ashore, stoneflies emerging on land are of considerable importance as potential prey for terrestrial predators. In contrast, only 34% of the potential prey organisms collected along the small streams were of aquatic origin. Food abundance was 9 times higher in the shore region of the Isar compared to the small streams. Surface drift in the Isar, a potentially important food source for riparian organisms, was about 10⁶ organisms and exuviae per meter stream width in 24 h. The drift density in the Isar was 59 times higher than that in a small stream. Terrestrial organisms provided only 3% of the drifting particles in the Isar, but 50% in the small stream. Gut content analysis reveals, that riparian ground beetles in the Isar floodplain mainly feed on aquatic organisms washed ashore or emerging on land. While small Bembidion species prefer chironomids (larvae and adults) the larger species Nebria picicornis feeds on emerging stoneflies, terrestrial riparian organisms and aquatic organisms accumulating along the shoreline. The prey of riparian ground beetles in the floodplain of the three small streams mainly consists of terrestrial species some of which may have been washed ashore. Received: 2 September 1996 / Accepted: 26 February 1997     

The lateral extent of the subsidy from an upland stream to riparian lycosid spiders

Adult aquatic insects emerging from streams can subsidize riparian food webs, but little is known of the spatial extent of these subsidies. Stable isotope (¹⁵N) enrichment of aquatic insects, principally a species of stonefly (Plecoptera: Leuctridae), emerging from an upland stream was used to trace the subsidy from the stream ecosystem to riparian spiders (Lycosidae). The downstream profile of spider δ¹⁵N correlated closely with that of adult stoneflies, indicating that they were deriving nutrition from aquatic sources. The contribution of adult aquatic insects to spider diets was determined using a two-source mixing model. Adult aquatic insects made up over 40% of spider diets adjacent to the stream, but <1% at 20 m from the stream. Enrichment of riparian spiders declined exponentially with distance from the stream channel. Aquatic-terrestrial subsidies were spatially restricted, but locally important, to riparian lycosid spiders at the study site.     

Predator effects on prey population dynamics in open systems

Animal population dynamics in open systems are affected not only by agents of mortality and the influence of species interactions on behavior and life histories, but also by dispersal and recruitment. We used an extensive data set to compare natural loss rates of two mayfly species that co-occur in high-elevation streams varying in predation risk, and experience different abiotic conditions during larval development. Our goals were to generate hypotheses relating predation to variation in prey population dynamics and to evaluate alternative mechanisms to explain such variation. While neither loss rates nor abundance of the species that develops during snowmelt (Baetis bicaudatus) varied systematically with fish, loss rates of the species that develops during baseflow (Baetis B) were higher in streams containing brook trout than streams without fish; and surprisingly, larvae of this species were most abundant in trout streams. This counter-intuitive pattern could not be explained by a trophic cascade, because densities of intermediate predators (stoneflies) did not differ between fish and fishless streams and predation by trout on stoneflies was negligible. A statistical model estimated that higher recruitment and accelerated development enables Baetis B to maintain larger populations in trout streams despite higher mortality from predation. Experimental estimates suggested that predation by trout potentially accounts for natural losses of Baetis B, but not Baetis bicaudatus. Predation by stoneflies on Baetis is negligible in fish streams, but could make an important contribution to observed losses of both species in fishless streams. Non-predatory sources of loss were higher for B. bicaudatus in trout streams, and for Baetis B in fishless streams. We conclude that predation alone cannot explain variation in population dynamics of either species; and the relative importance of predation is species- and environment-specific compared to non-predatory losses, such as other agents of mortality and non-consumptive effects of predators. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The influence of predatory fish on mayfly drift: extrapolating from experiments to nature

1. A knowledge of how individual behaviour affects populations in nature is needed to understand many ecologically important processes, such as the dispersal of larval insects in streams. The influence of chemical cues from drift‐feeding fish on the drift dispersal of mayflies has been documented in small experimental channels (i.e. < 3 m), but their influence on dispersal in natural systems (e.g. 30 m stream reaches) is unclear. 2. Using surveys in 10 Rocky Mountain streams in Western Colorado we examined whether the effects of predatory brook trout (Salvelinus fontinalis) on mayfly drift, that were apparent in stream‐side channels, could also be detected in natural streams. 3. In channel experiments, the drift of Baetis bicaudatus (Baetidae) was more responsive to variation in the concentration of chemical cues from brook trout than that of another mayfly, Epeorus deceptivus (Heptageniidae). The rate of brook trout predation on drifting mayflies of both species in a 2‐m long observation tank was higher during the day (60–75%) but still measurable at night (5–10%). Epeorus individuals released into the water column were more vulnerable to trout predation by both day and night than were Baetis larvae treated similarly. 4. Drift of all mayfly taxa in five fishless streams was aperiodic, whereas their drift was nocturnal in five trout streams. The propensity of mayflies to drift was decreased during the day and increased during the night in trout streams compared with fishless streams. In contrast to the channel experiments, fish biomass and density did not alter the nocturnal nature nor magnitude of mayfly drift in natural streams. 5. In combination, these results indicate that mayflies respond to subtle differences in concentration of fish cues in experimental channels. However, temporal and spatial variation in fish cues available to mayflies in natural streams may have obscured our ability to detect responses at larger scales.     

Local population genetic structure of the montane caddisfly Drusus discolor is driven by overland dispersal and spatial scaling

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Drought survival strategies,dispersal potential and persistence of invertebrate species in an intermittent stream landscape

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Loss of genetic diversity in the North American mayfly Ephemerella invaria associated with deforestation of headwater streams

1. Terrestrial dispersal by aquatic insects increases population connectivity in some stream species by allowing individuals to move outside the structure of the stream network. In addition, individual survival and reproductive success (as well as dispersal) are tightly linked to the quality of the terrestrial habitat. 2. In historically forested catchments, deforestation and altered land use have the potential to interfere with mayfly dispersal or mating behaviours by degrading the quality of the terrestrial matrix among headwater streams. We hypothesised that loss of tree cover in first‐order catchments would be associated with an increase in population substructure and a decrease in genetic diversity of mayfly populations. 3. To test this hypothesis, we investigated spatial patterns of genetic variation in the common mayfly Ephemerella invaria across a gradient of deforestation in the central piedmont region of eastern United States. Intraspecific genetic diversity and population substructure were estimated from data obtained using fluorescent amplified fragment length polymorphism (AFLP) markers. 4. We found that mayfly populations had low population substructure within headwater stream networks and that genetic diversity was strongly negatively correlated with mean deforestation of the first‐order catchments. The large‐scale pattern of population substructure followed a pattern of isolation by distance (IBD) in which genetic differentiation increases with geographical distance, but assignment tests placed a few individuals into populations 300 km away from the collection site. 5. Our results show that loss of genetic diversity in this widespread aquatic insect species is co‐occurring with deforestation of headwater streams. 6. Most arguments supporting protection of headwater streams in the United States have centred on the role of these streams as hydrological and biogeochemical conduits to downstream waters. Our work suggests that headwater stream land use, and specifically tree cover, may have a role in the maintenance of regional genetic diversity in some common aquatic insect species.     

Deforestation alters the resource base and biomass of endemic stream insects in eastern Madagascar

1. Rainforest streams in eastern Madagascar have species‐rich and diverse endemic insect communities, while streams in deforested areas have relatively depauperate assemblages dominated by collector‐gatherer taxa. We sampled a suite of benthic insects and their food resources in three primary rainforest streams within Ranomafana National Park in eastern Madagascar and three agriculture streams in the park's deforested peripheral zone. We analysed gut contents and combined biomass and stable isotope data to examine stream community responses to deforestation in the region, which is a threatened and globally important hotspot for freshwater biodiversity. 2. Gut analyses showed that most taxa depended largely on amorphous detritus, obtained either from biofilms (collector‐gatherers) or from seston (microfilterers). Despite different resource availability in forest versus agriculture streams, diets of each taxon did not differ between stream types, suggesting inflexible feeding modes. Carbon sources for forest stream insects were difficult to discern using δ¹³C. However, in agriculture streams dependence on terrestrial carbon sources was low relative to algal sources. Most insect taxa with δ¹³C similar to terrestrial carbon sources (e.g. the stonefly Madenemura, the caddisfly Chimarra sp. and Simulium blackflies) were absent or present at lower biomass in agriculture streams relative to forest streams. Conversely, collector‐gatherers (Afroptilum mayflies) relied on algal carbon sources and had much higher biomass in agriculture streams. 3. Our analyses indicate that a few collector‐gatherer species (mostly Ephemeroptera) can take advantage of increased primary production in biofilms and consequently dominate biomass in streams affected by deforestation. In contrast, many forest stream insects (especially those in the orders Plecoptera, Trichoptera and Diptera) depend on terrestrial carbon sources (i.e. seston and leaf litter), are unable to track resource availability and consequently decline in streams draining deforested landscapes. These forest‐specialists are often micro‐endemic and particularly vulnerable to deforestation. 4. The use of consumer biomass data in stable isotope research can help detect population‐level responses to shifts in basal resources caused by anthropogenic change. We also suggest that restoration of vegetated riparian zones in eastern Madagascar and elsewhere could mitigate the deleterious effects of deforestation on sensitive, endemic stream taxa that are dependent on terrestrial carbon sources.     

Differentiation of movement behaviour in an adaptively diverging salamander population

Dispersal is considered to be a species‐specific trait, but intraspecific variation can be high. However, when and how this complex trait starts to differentiate during the divergence of species/lineages is unknown. Here, we studied the differentiation of movement behaviour in a large salamander population (Salamandra salamandra), in which individual adaptations to different habitat conditions drive the genetic divergence of this population into two subpopulations. In this system, salamanders have adapted to the deposition and development of their larvae in ephemeral ponds vs. small first‐order streams. In general, the pond habitat is characterized as a spatially and temporally highly unpredictable habitat, while streams provide more stable and predictable conditions for the development of larvae. We analysed the fine‐scale genetic distribution of larvae, and explored whether the adaptation to different larval habitat conditions has in turn also affected dispersal strategies and home range size of adult salamanders. Based on the genetic assignment of adult individuals to their respective larval habitat type, we show that pond‐adapted salamanders occupied larger home ranges, displayed long‐distance dispersal and had a higher variability of movement types than the stream‐adapted individuals. We argue that the differentiation of phenotypically plastic traits such as dispersal and movement characteristics can be a crucial component in the course of adaptation to new habitat conditions, thereby promoting the genetic divergence of populations.     

Productivity and significance of headwater streams: population structure and biomass of the black-bellied salamander (Desmognathus quadramaculatus)

1. Headwater streams are a significant feature of the southern Appalachian landscape, comprising more than 70% of the total stream length in the region. Salamanders are the dominant vertebrate within headwater‐riparian forest ecosystems, but their ecological role is not clearly understood. 2. We studied a population of black‐bellied salamanders (Desmognathus quadramaculatus) at a headwater stream in the southern Appalachian Mountains using radio‐telemetry and mark‐recapture methods. The length and area of headwater streams in the region were estimated using GIS. 3. Home ranges of radio‐tracked salamanders were relatively small (mean = 1.06 m²). Adult salamanders in our telemetry study inhabited edge microhabitats significantly more often than either stream or riparian microhabitats, and the same trend was observed in the mark‐recapture study. 4. We estimated the population density at this site to be 11 294 salamanders ha^?1, amounting to 99.30 kg ha^?1 of biomass, an estimate that is six times greater than reported in previous studies. The majority of this biomass was found within the stream, but 22% was found in the surrounding riparian habitat more than 1 m from the stream. Using headwater stream length and area estimates, we extrapolated biomass estimates for black‐bellied salamanders inhabiting stream and riparian microhabitats across the study region. 5. We report one of the largest estimates of secondary consumer biomass for a headwater ecosystem, attesting to the overall productivity of headwater streams. Headwaters are known to be important for ecological and ecosystem processes and our biomass estimates suggest that salamanders are a critical component to these systems.     

Invertebrate feeding and emergence timing vary among streams along a gradient of riparian forest composition

1. The quality of allochthonous organic matter influences the transfer of energy and nutrients through recipient food webs. We investigated the effects of variation in the composition of riparian forests (deciduous, mixed, coniferous) on the elemental imbalance between basal resources and consumers in streams, on consumer feeding and on potential feedbacks to riparian systems via emergent aquatic insects. 2. We tested for differences in elemental stoichiometry (carbon/nitrogen/phosphorus; C/N/P) and stable isotopes (?¹³C and ?¹⁵N) between deciduous (red alder, Alnus rubra) and coniferous litter (western hemlock, Tsuga heterophylla) and among abundant stream invertebrates from streams draining different riparian forests (deciduous, mixed, coniferous). We then assessed shredder feeding preferences (of the trichopteran, Lepidostoma unicolor) for litter incubated in streams with these different forest types and quantified differences in emergence of aquatic and semiaquatic insects among streams. 3. Both initial (non‐incubated) and stream‐incubated A. rubra litter had lower C/N and C/P and were more depleted in ?¹³C and more enriched in ?¹⁵N, than T. heterophylla litter. The stoichiometry of invertebrate tissue did not vary significantly among taxa or with riparian forest composition. A predator (the plecopteran Chloroperlidae) and a collector‐gatherer (the ephemeropteran Paraleptophlebia gregalis) from mixed and coniferous forest streams were more enriched in ?¹³C and ?¹⁵N isotopes than those from deciduous streams, suggesting that low availability of palatable, N‐rich A. rubra litter may constrain energy flow and nutrient fluxes up through the food web in systems with little or no A. rubra. 4. Consumption of A. rubra litter by L. unicolor was most rapid when the litter had been incubated in streams draining deciduous forests, whereas consumption of T. heterophylla litter was not influenced by the composition of the riparian forest. 5. Peak insect emergence from coniferous forest streams occurred 1 month earlier and at 2–3× higher density than from mixed and deciduous‐forest streams, but total biomass of emerging insects throughout the study period was not different between forest types. Assemblages of emerging insects were different between deciduous and coniferous forest streams, and taxon richness and diversity were nearly 2× greater from deciduous than from coniferous forest streams. 6. Forest composition influences stream invertebrate feeding and could have reciprocal feedbacks onto riparian systems via altered insect emergence.     

Acquisition,retention and dispersal of soilborne plant pathogenic fungi by fungus gnats and moth flies

Adult fungus gnats and moth flies were experimentally demonstrated to function as potential above‐ground vectors for three soilborne plant pathogens: Verticillium dahliae, Fusarium acuminatum and Thielaviopsis basicola. The adult insects externally acquired the conidia of the pathogens after exposure to the cultures as confirmed by scanning electron microscope photography. The intestinal contents and frass deposits of larvae exposed to fungal cultures contained viable fungal propagules. Internally infested larvae developed into internally infested pupae; however, the emerging adults were free of fungal structures. Because of the maintenance of a high level of inoculum on the external body surface and the ability of these adult insects to fly, they can be a significant factor in the dispersal of soilborne fungi in greenhouse agriculture. The rate of dispersal of T. basicola by adult fungus gnats was 1.78 cm² h^?1 per insect and by adult moth flies was 1.17 cm² h^?1 per insect. The area over which the pathogen was dispersed by the adult insects increased with the increase in exposure time. The study demonstrated that adult insects are efficient distributors of soilborne plant pathogenic fungal propagules.     

Importance of Terrestrial Arthropods as Subsidies in Lowland Neotropical Rain Forest Stream Ecosystems

The importance of terrestrial arthropods has been documented in temperate stream ecosystems, but little is known about the magnitude of these inputs in tropical streams. Terrestrial arthropods falling from the canopy of tropical forests may be an important subsidy to tropical stream food webs and could also represent an important flux of nitrogen (N) and phosphorus (P) in nutrient‐poor headwater streams. We quantified input rates of terrestrial insects in eight streams draining lowland tropical wet forest in Costa Rica. In two focal headwater streams, we also measured capture efficiency by the fish assemblage and quantified terrestrially derived N‐ and P‐excretion relative to stream nutrient uptake rates. Average input rates of terrestrial insects ranged from 5 to 41 mg dry mass/m²/d, exceeding previous measurements of aquatic invertebrate secondary production in these study streams, and were relatively consistent year‐round, in contrast to values reported in temperate streams. Terrestrial insects accounted for half of the diet of the dominant fish species, Priapicthys annectens. Although terrestrially derived fish excretion was found to be a small flux relative to measured nutrient uptake rates in the focal streams, the efficient capture and processing of terrestrial arthropods by fish made these nutrients available to the local stream ecosystem. This aquatic‐terrestrial linkage is likely being decoupled by deforestation in many tropical regions, with largely unknown but potentially important ecological consequences.     

Apparent widespread gene flow in the predominantly flightless planthopper Tumidagena minuta

1. To determine whether dispersal biology can predict the pattern of population‐genetic variation among insect populations accurately, allozyme variation was assayed for populations of a saltmarsh planthopper, Tumidagena minuta, in which > 99% of the adults are flightless. 2. The pattern of genetic isolation by distance in T. minuta was compared with that in other insects, to determine whether it was similar to isolation by distance in other sedentary insects. 3. In contrast to predictions, the pattern of isolation by distance in T. minuta was most similar to that seen in the most mobile insects in a recent review of population‐genetic variation in insects. Furthermore, population‐genetic subdivision over a spatial scale of > 400 km was weak. 4. Possible causes of the apparent contradiction between dispersal biology and population‐genetic structure in this species are discussed. The results for T. minuta highlight the fact that although mobility is generally correlated with gene flow in insects, studies of population‐genetic variation must be combined with direct studies of dispersal to understand fully the degree to which populations exchange individuals.