Observations on the diatom flora of Braunton Burrows,N. Devon

Summary The diatom flora has been analysed in 31 samples from the dune area at Braunton Burrows. Samples have been examined from (a) aquatic habitats, (b) soil, plants and mosses subject to seasonal flooding and (c) sand and mosses from dry dunes. The diatom flora of all these has been discussed and compared with data from elsewhere.Department of Botany, University of Bristol     

Observations on the diet of the Red fox (Vulpes vulpes) in South Devon

The results of an 18-month survey of fox diet in the Stoke Gabriel area of South Devon are presented. Attention is drawn to the variety of prey consumed, and some features not previously recorded are noted. The probable factors influencing the diet are discussed and availability suggested to be the most important.     

Generalized population models and the nature of genetic drift

The Wright–Fisher model of allele dynamics forms the basis for most theoretical and applied research in population genetics. Our understanding of genetic drift, and its role in suppressing the deterministic forces of Darwinian selection has relied on the specific form of sampling inherent to the Wright–Fisher model and its diffusion limit. Here we introduce and analyze a broad class of forward-time population models that share the same mean and variance as the Wright–Fisher model, but may otherwise differ. The proposed class unifies and further generalizes a number of population-genetic processes of recent interest, including the Λ and Cannings processes. Even though these models all have the same variance effective population size, they encode a rich diversity of alternative forms of genetic drift, with significant consequences for allele dynamics. We characterize in detail the behavior of standard population-genetic quantities across this family of generalized models. Some quantities, such as heterozygosity, remain unchanged; but others, such as neutral absorption times and fixation probabilities under selection, deviate by orders of magnitude from the Wright–Fisher model. We show that generalized population models can produce startling phenomena that differ qualitatively from classical behavior — such as assured fixation of a new mutant despite the presence of genetic drift. We derive the forward-time continuum limits of the generalized processes, analogous to Kimura’s diffusion limit of the Wright–Fisher process, and we discuss their relationships to the Kingman and non-Kingman coalescents. Finally, we demonstrate that some non-diffusive, generalized models are more likely, in certain respects, than the Wright–Fisher model itself, given empirical data from Drosophila populations.     

Invertebrate drift in the Snake River,Wyoming

Drifting invertebrates were collected hourly during 24-hour sampling periods at two stations in the Snake River. The greatest number of invertebrates was collected on 8 and 15 July 1966 at station 1 between 9:00 p. m. and 12 midnight, then the numbers gradually decreased until the low daylight drift rate was reached at dawn. On 26 and 27 August 1966 at station 2, the diel periodicities of drifting invertebrates were different than at station 1. Many species increased their drift rates slightly during the first hour of darkness but also exhibited a higher drift peak later in the night.Drift indices for 25 taxonomic groups of invertebrates were established from the ratio of standing crop, estimated from Surber samples, to numbers drifting. There was more apparent correlation between species life cycle stage and numbers drifting than between species abundance and numbers drifting.

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Zusammenfassung Dahintreibende, wirbellose Tiere wurden während 24-stündigen Untersuchungsperioden stündlich an zwei Stationen im Snake River gesammelt. Die grössten Anzahlen von wirbellosen Tieren wurden auf Station 1 am 8. und 15. Juli 1966 zwischen 21 und 24 Uhr gefunden. Danach verminderten sich die Zahlen allmählich bis sie bei Morgendämmerung die niedrigsten Werte des Tagesdrifts erreichten. Am 26. und 27. August 1966 waren die täglichen periodischen Zyklen der treibenden Invertebraten auf Station 2 ganz anders als die auf Station 1. Viele Arten erhöhten ihre Driftgeschwindigkeit etwas in der ersten Stunde der Dunkelheit und hatten ausserdem noch einen Höchstdrift später in der Nacht.Ein Driftindex für 25 taxonomische Gruppen wirbelloser Tiere wurde nach dem Verhältnis von Standing Crop zur Anzahl dahintreibender Tiere aufgestellt, berechnet auf Grund von Surber Proben. Es bestand eine grössere Wechselbeziehung zwischen der Stufe im Lebenszyklus der Arten und der Anzahl der Treibenden, als zwischen dem Artenquantum und der Anzahl der Treibenden.

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The research was financed by the U.S. National Park Service and was conducted at Jackson Hole Biological Research Station, Moran, Wyoming. The results were presented as part of a M.S. thesis to the Department of Zoology and Physiology, University of Wyoming in June 1967. Dr. Thomas F. Waters encouraged publication of the thesis drift data.     

Invertebrate drift in the Dan River,Israel

The Dan river, a principal source of the Jordan River, Israel, is unusually constant in discharge (8 m³·s^–1) and water temperature (15–16 °C). The Jordan headwaters constitute the southernmost oasis of a palearctic north temperate fauna, and presumably the very constancy of the Dan contributes to its important role as a regional refuge. However, little is known of river ecology from this region. We report a twelve month study of drift, undertaken to assess diel, seasonal, and spatial patterns of the abundance of drifting invertebrates.Diel periodicity in drift was detectable but minimal. Baetidae nymphs showed a pronounced nocturnal increase, gammarid amphipods a modest, twofold increase, while dipteran larvae showed no diel variation. Seasonal variation likewise was minimal and due principally to the Baetidae, while gammarid amphipods showed no significant seasonality. The notably small diel and seasonal variation in aquatic drift in the Dan may be attributable to the extremely constant physical regime.Spatial variation was substantial. Two stations located 30 and 200 m below the karstic exsurgence of the Dan provided drift densities among the lowest reported anywhere, whereas two stations located 1 and 4.5 km downstream had more typical drift densities. A water diversion project completed halfway through the study resulted in a 50% reduction in flow at the most downstream stations, but had no discernible effect on drift.     

Allochthonous and autochthonous particulate organic matter in floodplains of the River Danube: the importance of hydrological connectivity

SUMMARY 1. The elemental composition, the proportion of living organic carbon and the carbon stable isotope signatures of particulate organic matter (POM) were determined in a large river floodplain system in order to elucidate the major carbon sources in relation to the hydrological conditions over a 13‐month period. 2. Two floodplain segments and the main channel of the River Danube downstream of Vienna (Austria), were compared on the basis of discharge and water age estimations. The more dynamic floodplain was connected to the main channel for 46% of the study period and drained up to 12% of total discharge at high water. 3. The mean C : N ratio and δ¹³C signature of the POM increased from the floodplain site that was more isolated from the river (6.6; ?33‰) to the main channel (8.4; ?25‰). At the dynamic floodplain site, the C : N ratio and the δ¹³C signature of the POM increased with hydrological connectivity (expressed as water age). 4. Only during flood events (4% frequency of occurrence), a considerable input of riverine POM was observed. This input was indicated by a C : N ratio of the POM pool of more than 10, the amount of detrital carbon (>80% of the total POM pool) and a δ¹³C signature of POM of more than ?25‰ in the dynamic floodplain. 5. Plankton derived carbon, indicated by C : N ratios less than eight and δ¹³C values lower than ?25‰, dominated the particulate organic carbon (POC) pool at both floodplain sites, emphasising the importance of local (autochthonous) production. Phytoplankton was the major plankton compartment at the dynamic site, with highest biomasses at medium water ages. 6. At the dynamic floodplain site, the Danube Restoration Project has enhanced the duration of upstream surface connection with the main channel from 4 to 46% frequency of occurrence. Therefore, the export of living POC to the main channel is now established during phases of maximum phytoplankton production and doubled the estimated total export of non‐refractory POM compared with prerestoration conditions.     

Effect of drift sampler exposure time and net mesh size on invertebrate drift density in the Njoro River,Kenya

Although invertebrate drift is an important ecological process in lotic ecosystems, very little is known about it in Kenyan rivers. The primary aim of this study was to investigate the effect of driftnet mesh size and exposure duration on drift density in 2017. Drift samples were dominated by Chironomidae, Baetidae, Simuliidae, Caenidae and Culicidae. The 100 µm mesh driftnet had the highest mean invertebrate density, followed by the 250 µm and 500 µm nets. Invertebrate drift densities decreased with increased exposure time. This study demonstrates that sampler mesh size and exposure time should be taken into account when characterising invertebrate drift in streams. Future studies should consider sampling different biotopes and during different seasons.     

The relative importance of refugia in determining the drift and habitat selection of predaceous stoneflies in a sandy-bottomed stream

Patch structure in sandy, compared to rocky streams, is characterized by isolated snags that can only be colonized by drifting. By measuring drift from patches (snags) of various quality we determined the factors that influence habitat selection and drift of the predaceous stoneflies Acroneuria abnormis and Paragnetina fumosa. The presence of refugia (loose bark and leaf packs) was more important than hunger level and modified the effects of increased predator densities and aggressive interactions. Stoneflies concentrated to 8x natural densities with access to refugia remained longer on snags than a single stonefly without access to refugia. During periods of activity, refugia were defended with larger stoneflies always displacing smaller nymphs. During long periods of inactivity, two and sometimes three nymphs would rest side-by-side sharing the same refuge. Hunger level (starved versus satiated stoneflies), an indirect measure of a predator's response to prey availability, had no significant effect on drift or habitat selection regardless of the presence of refugia. Stonefly predators had a uniform distribution while their prey were clumped. Drift was deliberate and almost always delayed until night, usually at dusk or dawn. An examination of previous research plus the results of this study suggest that non-predatory intra- and interspecific interactions can be an important mechanism causing drift in streams.     

Cladocerans as indicators in the importance of passive nature conservation

The aim of our study was to verify the effectiveness of passive nature conservation with regard to the preservation of species, by examining a typical mesozooplankton community. We investigated five oxbow lakes which were similar to each other in terms of size, depth and close-to-each, but which differed markedly from one to another in terms of lake management. One is under passive protection, the others are under different forms of management. Based on the NMDS (Non-metric Dimensional Scaling) analysis, we identified the variable (transparency) which is responsible for the segregation. In the case of rare species IndVal (Indicator Value) analysis was used to evaluate the occurrence and significance of indicator species. We also found marked differences in the species composition; seven species showed indicator values for the protected oxbow lake, five of which were significant (p <?0.001). In contrast, in the case of the other four oxbow lakes, a total of one species showed an indicator value. Finally, regression tree analysis was used to distinguish the oxbow lakes from each other for both common and mass-occurring species. Among the mass-occurring species, the protected oxbow lake is characterized by a large number of Acropreus harpae (Baird, 1835), while in case of the other, non-protected, oxbow lakes Alona guttata (Sars, 1862) and Alonella excisa (Fischer, 1854) are the basis for the separate classification. There is a positive effect of passive protection regarding the quantitative and qualitative occurrence of the zooplankton community. Our study highlights the importance of this kind of passive protection in the life of a particular water body and advocates that their protection should be maintained.