Dispersal,feeding and parasitism of adult stoneflies (Plecoptera) at a New Zealand forest stream

The temporal occurrence and dispersal of stoneflies were investigated at a forest stream in the South Island, New Zealand. Although 12 species were taken in Malaise traps, only Spaniocerca zelandica and Cristaperla fimbria (Notonemouridae) were abundant. Adults of S. zelandica were present throughout the year and a wide range of different-sized nymphs, including very small individuals, was also found in all months. In contrast, the emergence period of C. fimbria was limited to about four months in summer and as nymphs are mainly hyporheic, few were taken in Surber samples. Malaise trapping on a 16 m long transect away from the stream and with more distantly located traps in forest and grassland resulted in a very high proportion of males and females of both species being caught within 1 m of the stream edge. Captured females of both species included immature, gravid, and spent individuals. Males and females of both species had almost identical diets dominated by sooty mould fungi, fungal spores, pollen and fine particulate organic matter. High proportions of them were also infected by the encysted larvae of a gordian worm (Nematomorpha) and may be important vectors in its life cycle.     

Emergence and lateral dispersal of adult Plecoptera and Trichoptera from Broadstone Stream, U.K.

1. Emergence and inland dispersal of adult stoneflies (Plecoptera) and caddisflies (Trichoptera) from Broadstone Stream, an acidic and iron-rich stream in southern England, were studied over 10 months in 1996–1997. Fifteen pyramidal emergence traps were placed randomly in a 200-m stretch. Three Malaise traps were placed above the stream and six more on each side (one wooded, one open) along a transect at distances of 1, 15, 30, 45, 60 and 75 m from the channel. 2. More than 16 000 stoneflies, belonging to 11 species, and just under 400 caddisflies (22 species) were caught. Four dominant stoneflies (Leuctra fusca, Leuctra nigra, Leuctra hippopus and Nemurella pictetii) accounted for 96% and 95% of the catches in the emergence and Malaise traps, respectively. Two caddisflies (Plectrocnemia conspersa and Potamophylax cingulatus) accounted for 63% of the catch in the Malaise traps. Few caddisflies were taken in emergence traps. 3. The emergence periods of L. fusca, L. nigra and L. hippopus were well-defined and unimodal, whereas that of N. pictetii was prolonged and erratic. Overall, more females (1285) emerged than males (740). 4. Female stoneflies and caddisflies were in the majority in the Malaise traps above the stream. On land, significantly more females than males of L. fusca, L. nigra and P. cingulatus were caught. The sex ratio of the remaining species did not deviate significantly from 1:1. 5. The three Malaise traps placed above the stream caught most of the stoneflies though there was also dispersal away from the channel, the numbers caught declining with distance. Exponential models explained between 67% and 99% of the variation in numbers of individuals with distance from the channel in the four common stoneflies. Half the individuals went less than 11–16 m from the stream, while 90% travelled less than 51 m. Significantly more L. nigra and N. pictetii were caught in the woodland than on the open side, whereas L. hippopus showed no overall preference for either side.     

The effect of long pools on the drift of macro-invertebrates in a mountain stream

Macro-invertebrate drift was measured entering and leaving two pools on the Middle Fork of the Cosumnes River, a third order California stream. Drift rates for Baetis spp., Chironomidae, Simulium spp., Capniidae and total drift were calculated. Significant differences in the numbers of organisms entering the two pools were found for Baetis, Chironomidae, and Capniidae. Comparisons of drift rates at the upstream and downstream ends of each pool showed that the abundance of Chironomidae, Simulium, Capniidae and total drift changed in different directions across the pools. The numbers of organisms leaving the two pools, however, were not significantly different for Baetis, Simulium, Capniidae and total drift. These findings lead us to hypothesize that long pools act as barriers, not filters, to stream macro-invertebrate drift. The composition of drift leaving the pools in this experiment appeared to be controlled by the composition of the benthic habitat at the tail of the pool and not by the composition of upstream drift entering the pools.     

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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Sex ratio and sexual dimorphism in mountain dioecious thuriferous juniper (Juniperus thurifera L., Cupressaceae)

Thuriferous juniper ( Juniperus thurifera L), a dioecious bush or tree is only found in isolated parts of the western Mediterranean: France, Spain, Algeria and Morocco. These mountain juniper stands are seriously endangered in Morocco as a result of intensive wood removal, and in Europe as a result of recolonization of stands by pines or oaks. Field studies were conducted to investigate sex ratio and sexual dimorphism, never previously examined, in eight different populations in the Atlas mountains and, for comparison, in one of two populations in the French Pyrenees. The sex ratio was female-biased for six of the eight Moroccan stands and especially for the oldest populations. The Pyrenean population showed a similar female-biased ratio. This particular sex ratio is possibly linked to cost of reproduction, paid by both males and females. Sex ratios can also be linked to population dynamics. Males begin to flower slightly younger than females, which explains their apparent dominance in young populations in Morocco or in a recolonization zone with young trees in the Pyrenees. Studies concerning sexual dimorphism in the western High Atlas sites showed no significant difference in phytomass between males and females. Females appear to be significantly taller but with a lower radial growth. © 2002 The Linnean Society of London, Botanical Journal of the Linnean Society , 138 , 237–244.     

Interspecific interference and the functional response of four species of carnivorous stoneflies

1. A previous study compared the functional responses to their prey and intraspecific interference in mature larvae of Perlodes microcephalus, Isoperla grammatica, Dinocras cephalotes and Perla bipunctata. The present study extends this work by assessing interspecific interference between pairs of these species in equal numbers (one, two or three larvae per species) to provide total predator densities of two, four or six larvae. Baetis larvae as prey were replaced as they were eaten, and their density per predator was varied between 20 and 200 larvae. 2. The number of prey eaten by each competing species increased curvilinearly with prey density, the relationship being well described by a Type II model. Of the two constants in the model, handling time varied considerably between species, mean values being shortest for Perlodes, slightly higher for Isoperla, and much higher for Dinocras and Perla. It was not affected significantly either by predator density or the identity of the competing species. 3. Attack rate also varied between species and decreased with predator density. This decrease was slight for Perlodes, and also for Dinocras and Perla in competition with Isoperla. The decrease in Dinocras and Perla was similar to that for intraspecific interference. 4. The decrease in attack rate was described by a convex curve for Perlodes with the other three species and for Dinocras/Perla with Isoperla, but by a concave curve (negative power function) for Isoperla competing with the other three species, and for both Dinocras and Perla in competition with Perlodes. Prey consumption also decreased with predator density, the severity of competition with different species reflecting that for attack rate. 5. A comparison with previous results for intraspecific interference showed that the latter was dominant for Perlodes in all contests and for Dinocras or Perla competing with Isoperla, whilst interspecific interference dominated for Isoperla in all contests and for Dinocras and Perla competing with Perlodes. Both types of interference were applicable to competition between Dinocras and Perla. Isoperla was the least, and Perlodes the most, aggressive of the four species with Dinocras and Perla intermediate.     

The influence of habitat heterogeneity and latitude on gamma diversity of the Nearctic Simuliidae,a ubiquitous group of stream‐dwelling insects

Among the most prominent, large‐scale patterns of species richness are the increases in richness with decreasing latitude and with increasing habitat heterogeneity. Using the stream‐dwelling larval and pupal stages of North American black flies (Diptera: Simuliidae), we address 3 broad questions about species richness: (i) Does a significant latitude–richness relationship exist? (ii) How does habitat heterogeneity influence gamma diversity? (iii) What is the sign (positive or negative) of the latitude–richness and the heterogeneity–richness relationships? We found no evidence that habitat heterogeneity influences gamma diversity. The estimated peak species richness for black flies in North America was at 50–53°N, which also corresponds with peak generic richness. All plesiomorphic, extant lineages of the Simuliidae in the Western Hemisphere are found in cool mountainous environments of North America, suggesting that peak richness at 50–53°N might be a signature of this phylogenetic pattern and a reflection of underlying historical processes.     

Effects of flooding on avian top-predators and their invertebrate prey in a monsoonal Taiwan stream

1. The effects of flooding on top predators are poorly understood globally, but particularly in monsoonal streams. We therefore attempted to assess how inter-annual and intra-annual variations in flood magnitude affected an obligate riverine predator, the brown dipper ( Cinclus pallasii ), and its invertebrate prey, in the mountain Tachia River, Taiwan. Major flooding in one of the study years (2005) allowed an insight into the effects of abnormally large flows.
2. The abundance and biomass of insects, and the abundance of dippers, decreased steadily from 2003 to 2005 as flood magnitude grew, but then increased in 2006 when more typical discharge returned. Dipper abundance, insect abundance and insect biomass were all strongly positively inter-related, but negatively related to discharge. Insect biomass, rather than abundance, was more useful in predicting brown dipper abundance.
3. Aquatic insect composition fluctuated among sampling years, revealed by non-metric multidimensional scaling, and these fluctuations were also related to discharge. In turn, dipper abundance and the mean body size of aquatic insects declined with the shift in insect composition as flow increased.
4. These data illustrate how discharge fluctuations can have pronounced effects on top predators in streams, mediated in this case by fluctuating prey abundance. While contributions from bird movement, breeding performance and mortality were not clearly differentiated, our data reveal how dippers have strategies to accommodate varying discharge in river systems. We suggest that the effects of floods on dippers should be taken into account when using this group as indicators of river quality.     

Budding dispersal and the sex ratio

There is much interest in understanding how population demography impacts upon social evolution. Here, we consider the impact of rate and pattern of dispersal upon a classic social evolutionary trait--the sex ratio. We recover existing analytical results for individual dispersal, and we extend these to allow for budding dispersal. In particular, while a cancelling of relatedness and kin competition effects means that the sex ratio is unaffected by the rate of individual dispersal, we find that a decoupling of relatedness and kin competition means that budding dispersal favours increasingly female-biased sex ratios. More generally, our analysis illustrates the relative ease with which biological problems involving class structure can be solved using a kin selection approach to social evolution theory.     

Reproductive ecology of the freshwater red alga Batrachospermum boryanum Sirodot in a temperature headwater stream

The surface and hyporheic fauna of a second-order reach of a northern California Coast Range stream (Big Canyon Creek, Lake Co., CA, USA) was examined using substrate colonization samplers (i.e. hyporheic pots) during both the wet and dry seasons in the prevailing Mediterranean climate of the region. In terms of total number of macroinvertebrate taxa, the surface (0 cm to −5.0 cm within the stream substrate) level had higher richness than any level within the hyporheic (−5.1 cm to −15.0 cm, −15.1 cm to −25.0 cm, or −25.1 cm to −35.0 cm) from the beginning of the dry season (May) to the beginning of the wet season (October); during this period the surface was not subject to disturbance from wet-season storms. During the wet season, richness at the surface was similar to that observed at any of the three hyporheic levels examined. Macroinvertebrate density at the surface was substantially reduced during the wet season; however, mean surface densities always exceeded those found within the hyporheic zone (from 25% to 78% of total macroinvertebrate numbers were found at the surface). Seasonal fluctuations in abundance of total macroinvertebrates and density of many constituent populations were less within the hyporheic zone than at the surface. Apparently, early instars of abundant surface taxa do not penetrate the interstices, and substrate disturbance due to spates is less in deeper levels. Compared with many hyporheic faunas described from other temperate-zone sites, this hyporheic community shows reduced numerical dominance by the Chironomidae.     

A population and energy model of a forest insect outbreak

A model is proposed for the dynamics of a forest insect population with account of food consumption and the response of plants to damage. Equations are derived relating the propagation coefficient, female mass, pest conversion efficacy, and plant reaction. Outbreak scenarios are analyzed as dependent on steady-state female weight. The results are compared with the data of observations in nature.     

Seasonal and spatial distribution of stoneflies in the Chubut River (Patagonia, Argentina)

Longitudinal and seasonal distributions of Plecoptera species were examined along the Chubut River in the Patagonian Precordillera and Plateau, Argentina. Chubut River (>1000 km) is the largest river in the area and the hydrological regime is modified in the lower section by an artificial reservoir (Florentino Ameghino Dam). Quantitative samples were collected in 13 sites in the higher, middle, and lower sections of the river basin. Sites were visited four times during 2004, and a total of nine species and 5772 individuals were collected in the study. Plecoptera richness decreased dramatically from the headwaters to the mouth of the river system. Two species, Antarctoperla michaelseni and Potamoperla myrmidon, were able to live below the impoundment but they were not abundant. Notoperlopsis femina and A. michaelseni abundances were higher in summer than in the other seasons, while Limnoperla jaffueli peaked in spring. Species–environmental relationships were examined using Canonical Correspondence Analysis and six independent variables were identified as the major factors structuring stoneflies assemblages. First axis was highly related to environmental variables reflecting the hydro-geological and land use gradients in the basin (conductivity, total suspended solids, periphyton Chlorophyll a). Second axis was more related to variables that changed seasonally (wet width, water temperature and soluble reactive phosphate).     

Inter- and intra-specific differences in the number of larval instars in British populations of 24 species of stoneflies (Plecoptera)

1. Ontogenetic changes during the life cycle of aquatic insects are important not only in life‐history studies but also in evaluating food‐web structure. They require information on the growth and number of larval instars but such information is lacking for many species, including Plecoptera. Therefore, the chief objectives of the present study were to determine inter‐ and intra‐specific differences in the number of larval instars in British populations of 24 species of stoneflies, to test Dyar’s hypothesis that growth followed a geometric progression, and to synthesise this information with previously published values for four British species. 2. Larvae were reared at constant temperatures in the laboratory from eggs from 63 populations (one to six populations per species). First instars from each population were divided into three batches and each batch was reared at one of three constant temperatures. For each species, the rearing temperature and source population had no significant effect on the mean size of each larval instar. 3. The relationship between the geometric mean length of each instar and instar number was well described by an exponential equation (P < 0.001, r² > 0.9 for all species), thus supporting Dyar’s hypothesis. Only one species, Brachyptera risi, had the same number of instars for males and females (12–13). For the other 15 herbivorous species and the four smaller carnivorous ones, the number of instars was higher for females than males (range 11–16 for males, 12–17 for females). The larger size of the females was due to their additional instars, not a sex difference in growth rates. In contrast, there was a clear growth separation of the sexes after the 9th or 10th instar for the four largest carnivores. The number of larval instars was highest for these four species (range 16–19 for males, 18–23 for females), and females were much larger than males. 4. A multiple regression equation with data from the present and previous studies (n = 27) showed that variability in the mean length of the first instar and the maximum number of larval instars for each species accounted for 88% and 91% of the variability in the mean length of the final instar for males and females, respectively. 5. Values for Plecoptera in other countries were in general agreement with those in the present study, especially in the same families. Two old, but widely quoted, high values are doubtful. The present study and four previous ones provide a sound basis for ontogenetic studies on 28 species of Plecoptera and their role in aquatic ecosystems.     

Sex difference in natal dispersal distances of Golden Eagles Aquila chrysaetos in Scotland

Natal dispersal distance (NDD) is critical in understanding and defining populations and their conservation. It is defined as the linear distance between the natal location and first reproductive (‘effective NDD’) or potential reproductive (‘gross NDD’) location. It is a measure of gene flow and the functional connectivity across generations between individuals breeding in the same or different geographies. NDD is difficult to record in large raptors. GPS-satellite telemetry has facilitated its recording. Previous Scottish studies showed that gross and effective NDD were apparently equivalent, and an algorithm based on telemetric data could identify the territory settlement timing and location of birds originally tagged as nestlings. We analysed natal dispersal data from 39 Golden Eagles GPS-tagged in Scotland to estimate NDD. Raw median estimates were 29.8 km for males (n = 22) and 58.6 km for females (n = 17), 38.1 km averaged across sexes. Males had significantly shorter NDD, as theoretically predicted. Our NDD estimates were shorter but not grossly dissimilar to those from the USA, where sex differences in NDD had not been confirmed. Respective sample sizes may underly the latter contrast in confirmation. We also showed that in the absence of data from sexed birds, NDD estimates can be different. Natal dispersal duration was not related to NDD, suggesting that time to prospect a territory opportunity was not associated with the selected territory's distance from the natal site. The previous status of the subsequent settled territory (occupied or vacant) was also not related to NDD. We conclude that sex differences in NDD are important in application to population demography and conservation. Although we found no support for two other potential drivers of NDD (natal dispersal duration and previous territory status), identifying additional influences on NDD is in its infancy in large raptors and deserves more study.     

Landscape variables influence taxonomic and trait composition of insect assemblages in Neotropical savanna streams

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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.     

Effects of sexual differences of age at maturity and survival on population sex ratio

Summary Five demographical factors influencing the sex ratio of a population are classically considered. The influence of two of them is dependent on the longevity of individuals in the population. The effect of differential age at maturity between males and females is higher for animals with low annual survival, whereas the effect of differential annual survival between males and females is higher for animals with high annual survival. Such a conclusion applied to turtles, which are long life-span animals, allows us to retain differential survival between sexes as a major factor influencing the population sex ratio.     

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

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