Field experiments on flow refugia in streams

1. Field experiments were carried out to determine whether animals move into areas of low shear stress during periods of peak flow and therefore avoid hydraulic disturbance.
2. Flow at the scale of 0.05 m² patches was reduced experimentally by creating artificial 'refugia'. Invertebrate colonization of cages with 1.1 mm mesh sides, which provided such potential refugia by reducing hydraulic forces within the cage, was compared with colonization of cages with coarser (15 mm) mesh which did not restrict the flow.
3. Colonization of these cages was tested over a series of weekly periods in two different streams. Nine trials were completed in a stream with abundant natural flow refugia (Broadstone Stream, SE England), and during three of these trials strong spates occurred. Six trials were completed in a stream with comparatively few natural flow refugia (Dargall Lane, SW Scotland), and peak flows were relatively less.
4. In Broadstone Stream, the relative colonization of refugium cages was greatest during periods of peak flow, suggesting animals had used these low-flow areas as refugia during spates. Use of the artificial refugia did not occur in Dargall Lane, at least at the flows achieved during the trials.
5. Our results are consistent with the hypothesis that stream invertebrates accumulate in refugia during high flow disturbances. Whether their distribution among patches is altered by active or passive means remains unclear.     

Responses of tropical cladocerans to a gradient of resource quality

1. The response of three tropical cladocerans to a gradient of resource quality was compared in a series of growth bioassays using seston collected from five lakes of different depth and trophic structure in Michigan, U.S.A. To assess the food quality in terms of digestibility, assimilation experiments were performed with ³²P‐labelled seston from the same lakes. Animals were also analysed for P‐content in their tissues at the end of these assays. 2. In general, assimilation efficiency was higher when animals fed on seston from shallow compared to deep lakes, and was significantly correlated with growth rates, suggesting that shallow lakes have the best food resources in terms of digestibility and P availability. 3. Results also showed that all cladoceran species responded similarly to the resource gradient, with lower growth rates in deep lakes and higher growth rates in shallow lakes, although the strength of response (sensitivity) was different among the species tested. 4. The cladoceran Moina micrura was the most sensitive species, and also displayed the highest P‐content and maximal growth rate, a pattern consistent with the growth rate hypothesis. 5. However, seston C : P ratio and growth rates in the different resources did not correlate with the animals’ P‐contents, showing an uncoupling between RNA‐phosphorus demands for growth and seston food quality. 6. In conclusion, our results support the idea that digestion resistance in algae is a major constraint to cladocerans in natural plankton communities.     

Secondary production,longevity and resource consumption rates of freshwater shrimps in two tropical streams with contrasting geomorphology and food web structure

1. Freshwater shrimps often dominate the biomass of tropical island streams and are known to have strong effects on stream ecosystem structure and function, but little effort has been dedicated toward quantifying basic energetic and life history attributes such as growth, production and longevity. Such information is critical for understanding both the role of shrimps in ecosystem dynamics and the gravity of threats to shrimp populations posed by human activities such as shrimp harvesting, dam construction and water withdrawal. 2. We quantified growth rates and secondary production of dominant freshwater shrimps for 3 years in two Puerto Rican headwater streams that differ in food web structure because of the presence or absence of predatory fishes that are excluded from reaches above waterfalls. Using growth data, we constructed a minimum longevity model to explore the likely minimum life spans of the two dominant taxa (Atya spp. and Xiphocaris elongata). Finally, we used a bioenergetics model to quantify annual consumption rates of major basal resources by the two taxa. 3. Daily growth rates ranged from ?0.001 to 0.011 day^?1, were inversely related to body size, and were higher for small individuals of X. elongata than Atya spp. Mean annual shrimp biomass and secondary production were an order of magnitude higher in the stream that lacked predatory fishes (biomass: 4.34 g AFDM m^?2; production: 0.89 g AFDM m^?2 year^?1) than in the stream with predatory fishes (biomass: 0.12 g AFDM m^?2; production: 0.02 g AFDM m^?2 year^?1). Production : biomass ratios ranged from 0.01 to 0.38. 4. Our longevity model predicted a minimum life span of 8 years for Atya spp. and 5 years for X. elongata in the stream lacking predatory fishes. In contrast, due to a larger average size of X. elongata in the stream with predatory fishes, our model predicted a minimum life span of 11 years. Actual life spans of these taxa are likely to be much longer based on long‐term observations of marked individuals. 5. Estimated consumption rates from the bioenergetics model indicated that Atya spp. and X. elongata are important processors of organic matter resources in streams where they occur at high densities. Atya spp. and X. elongata appeared capable of consuming a large proportion of algal and insect production and the proportion of direct leaf litter inputs consumed was also appreciable (c. 40–60%). However, the consumption of suspended fine particulate organic matter (SFPOM) by Atya spp. is probably only a minor proportion of total SFPOM flux in these streams. 6. Our study suggests that geomorphic features such as waterfalls may play an important role in controlling the distribution and production of freshwater shrimps through their effects on predatory fish movement. Spatial differences in shrimp densities result in landscape‐scale variation in the significance to ecosystem processes of these long‐lived organisms, particularly as processors of major organic matter resources.     

Localization of electrical current flow across the chorionated unfertilized egg of the locust,Locusta migratoria.

Summary

Extracellular currents near the surface of the electrically excitable egg of Locusta migratoria have been studied through the combined use of the 2-dimensional vibrating probe and an intracellular microelectrode. Intracellular current injection increased extracellular current densities to levels readily detectable by the vibrating probe, and it has been possible to show that injected current traverses the egg in the posterior end containing the micropylar zone. Extracellular currents could not be detected during the rising, plateau or falling phases of the long duration (>100s) action potential in the egg. Increases in current density during the hyperpolarizing after potential which lasts for 6–10 min after termination of the action potential were also restricted to the posterior end of the egg. The results suggest that the primary region at which current traverses the chorionated egg is near or at the micropylar zone.     

Impacts of grazing and burning on spider assemblages in dry eucalypt forests of north‐eastern New South Wales,Australia

Abstract In the dry eucalypt forests of north‐eastern New South Wales, Australia, cattle grazing occurs at low intensities and is accompanied by frequent low‐intensity burning. This study investigated the combined effects of this management practice on the ground‐dwelling and arboreal (low vegetation) spider assemblages. Spiders were sampled at 49 sites representing a range of grazing intensities, using pitfall trapping, litter extraction and sweep sampling. A total of 237 spider morphospecies from 37 families were collected using this composite sampling strategy. The abundance, richness, composition and structure of spider assemblages in grazed and ungrazed forest sites were compared and related to a range of environmental variables. Spider assemblages responded to a range of environmental factors at the landscape, habitat and microhabitat scales. Forest type, spatial relationships and habitat variability at the site scale were more important in determining spider assemblages than localized low‐intensity grazing and burning. However, it is possible that a threshold intensity of grazing may exist, above which spiders respond to grazing and burning. Although low‐intensity grazing and burning may not affect spider assemblages below a threshold stocking rate, that stocking rate has yet to be established.     

A route to recovery: The early Silurian shallow-water shelly fauna in the northern Oslo basin

The shelly fauna of the lowest part of the Sælabonn Formation in the Hadeland district of the Oslo Region provides a rare insight into the shallow-water biota of the earliest Silurian and hence the start of biotic recovery after the end Ordovician extinction event. It is dominated by the brachiopods Dalmanella cf. pectinoides Bergström, Coolinia cf. columbana (Reed), Leptaena cf. haverfordensis Bancroft and Zygospiraella scotica (Salter) together with the trilobite Acernaspis elliptifrons (Esmark). The first three species in this unique association belong to eurytopic Ordovician survivor genera which were also present in the underlying unit in Hadeland and continued to thrive during the Silurian. Significantly, Zygospiraella and Acernaspis have no unequivocal Ordovician record but diversified rapidly and became common during the early Silurian (Rhuddanian) in many parts of the world. Moreover, Acernaspis and Zygospiraella were highly eurytopic, pioneer taxa that were among the first elements of the Rhuddanian shelly fauna to appear in many parts of the world especially around the margins of the remnant Iapetus Ocean, following the drowning of areas previously emergent during the late Ordovician sea-level lowstand. The Hadeland fauna provides evidence of a previously undocumented route (among several) by which life ultimately returned to the global diversity plateau established in the mid-Ordovician.     

Probing the deep shelf: a Lagerstätte from the Upper Ordovician of Girvan, southwestern Scotland

A deep‐water Konservat Lagerstätte from the lower Caradoc (Sandbian) at Girvan is dominated by the trilobite Diacanthaspis trippi, the carpoids Anatifopsis n. sp.? and a new genus of ctenocystoid together with the polyplacophoran Solenocaris solenoides and the brachiopod Onniella williamsi. Most of these are multi‐element organisms, with many specimens preserved in an articulated state in finely laminated rocks, indicating minimal disturbance and suggesting that the fauna is largely an in situ association. It contains few of the species known from other deep‐water sites of similar age at Girvan which contain diverse assemblages of trilobites and brachiopods absent from the Lagerstätte. The taphonomy of the site indicates preservation by rapid burial followed by early diagenesis under dysaerobic conditions. It provides a ‘taphonomic window’ on otherwise unknown faunas from distal shelf facies on the Ordovician Laurentian margin, and, moreover, is an important reminder of the hidden biodiversity that resided in thin‐shelled, multi‐element organisms.     

Miocene hawk converges on Secretarybird

A falconiform Miocene fossil bird from North America, represented by a nearly complete tarsometatarsus with phalanges in place, is that of a hawk that converged on the living Secretarybird Sagittarius serpentarius. It is described as a new genus and species, Apatosagittarius terrenus. The palaeoenvironment indicates a Miocene setting similar to the present day habitat of the living Secretarybird, characterized by extensive grasslands and savannas.