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Examines the sources of ecological theories that match species traits with environmental constraints. Contributors to ecological theory; Relationship between biological traits and environmental constraints; Problems relating to biological traits and environmental constraints.

Field experiments in a fishless stream were carried out on an abundant caddisfly with a predatory, net-spinning larva, Plectrocnemia conspersa (Curtis), to assess whether net site availability affects their microdistribution. 2. Net sites were supplemented by adding nought, one or four artificial structures to replicated patches on the stream bed. In each of three experiments at different seasons (summer, autumn and late winter), caddis densities increased significantly in patches with extra net sites. 3. The response of caddis to supplemented net sites could be affected by the subsidiary effects of food and of fish. These potential interactions were assessed in each experiment by varying net site density in two additional treatment stretches in which (1) prey abundance was increased by releasing Daphnia, and (2) brown trout (Salmo trutta L.) were enclosed. The responses of caddis in these two treatments were compared to that in the reference stretch, where only net site density varied. 4. Increased food abundance enhanced the response of caddis to net site supplementation in winter, when natural prey was least abundant, but not in summer or autumn. We suggest that extra food affects the mechanism determining net building only when prey availability is below some threshold. 5. The presence of fish precluded any effect of extra net sites in summer, but had no effect in autumn (the winter fish treatment was lost). We suggest fish predation reduced the densities of caddis in summer, so that net sites no longer limited local densities. In autumn, fallen leaves provided refugia from fish, which consequently were less effective predators of P. conspersa. [ABSTRACT FROM AUTHOR]

1. In each of twenty-six week-long experiments, the colonization by macroinvertebrates of boxes of natural sediment in a stony stream was measured. The experiments took place between February and November 1992 and environmental conditions prevailing during the weeks (particularly discharge and temperature) differed widely. 2. Colonization rate also varied widely between the weekly experiments and was sensitive to discharge, temperature and background benthic density, depending on the taxa considered. 3. A 'mobility index' measured colonization rate independently of background benthic density. This index was most strongly and positively correlated with discharge for the abundant stonefly Leuctra nigra, the net-spinning caddis Plectrocnemia conspersa and the Chironomidae, perhaps indicating that their mobility is physically driven by the influence of flow on drift. 4. Mobility in a second stonefly (Nemurella pictetii) was greatest in summer, when flows were low but temperature was high. Peak mobility in this species occurred during an ochreous bloom on the stream bed. It is likely the mobility of N. pictetii is more active than the other taxa. 5. There were thus differences among taxa in their mobility at baseflow and in their susceptibility to flow fluctuations. The population consequences of differences in mobility among taxa are discussed. [ABSTRACT FROM AUTHOR]

Methane oxidation produces biomass that is a potential source of particulate carbon for consumers, and is in addition to photosynthetic production. We assessed methanotrophy and photosynthetic production under differing conditions of light and methane concentration. We measured methane oxidation and photosynthesis in gravel sediments from adjacent shaded and unshaded stretches of 15 chalk rivers in southern England, and also in 30 artificial channels in which we manipulated light and methane experimentally. The capacity for methane oxidation was 78% higher in the shade than unshaded areas, indicating a denser, or more active, methanotrophic assemblage on shaded riverbeds, and the difference was most pronounced when methane concentration was high. Across the 15 rivers, methanotrophic production ranged from 16 to 650 nmol C cm−2 d−1 and net photosynthetic production from 256 to 35,750 nmol C cm−2 d−1. The relative importance of methanotrophy to their total production (i.e., photosynthetic and methanotrophic) increased with methane concentration and ranged from 0.1-2.4% and 0.2-13% in unshaded and shaded areas, respectively. Over an annual cycle in one river, the response of the methanotrophs in the shade to a high summer methane concentration was ∼ five times greater than in the open; in winter, there was no effect of shading on methane oxidation. The response of methanotrophy to shading and methane concentration in the artificial channels resembled that found in the rivers. Methanotrophy makes a non-negligible (here up to ∼ 13%) contribution to particulate carbon production in these streams, is disproportionately greater in the shade, and constitutes a distinct carbon pathway available for their food webs. [ABSTRACT FROM AUTHOR]

We measured the δ13C values of dominant primary consumers and their potential food sources in a groundwater-fed lowland river. The δ13C of most consumers, such as Gammarus and Simulium, reflected that of the dominant forms of photosynthetic production, whereas the cased larvae of two caddis flies (Agapetus and Silo) were consistently 13C depleted (mean δ13C: -41.2% and -40.4%, respectively) throughout the year. The river water was supersaturated (approximately 50 times atmospheric) with methane, reflecting both supersaturation in the groundwater and local production in fine sediments. We measured appreciable rates of methane oxidation, relative to water only controls, in the biofilms on gravel, on the caddis fly cases, and on the bottom of larger rocks. In addition, there was a marked difference in the ratio of methane-oxidizing potential to chlorophyll across those substrata. This ratio was below detection in the biofilm (i.e., no methane oxidation) on the tops of rocks, greater on the bottom of rocks, and maximal for the gravels and the caddis cases. If the caddis larvae acquire most of their carbon by grazing the tops of such rocks (where they are normally found), then they must acquire their depleted δ13C values by occasionally grazing biofilm where the ratio of methane oxidation to chlorophyll was much greater, and the most likely candidate is from their own or conspecific cases. Grazing methane-oxidizing bacteria could provide the caddis larvae with up to 30% of their carbon, which could represent a true subsidy from an ancient groundwater source. [ABSTRACT FROM AUTHOR]

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 stream. 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 inDargall 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 theirdistribution among patches is altered by active or passive means remains unclear. [ABSTRACT FROM AUTHOR]

The benthic communities of streams contain invertebrates of a wide range of body size and from many taxa. Owing mainly to methodological problems, however, the contribution of smaller and more obscure metazoans to community structure and dynamics, including production, is poorly known compared with that of larger size fractions and, particularly, insects and macrocrustaceans. Based on a monthly survey of a first-order, acidic English stream, we used the size-frequency method to estimate annual production of the whole metazoan benthos (down to organisms retained on a 42-µm mesh and being as taxonomically inclusive as possible). Mean total secondary production (5.22 g dry weight m-2 yr-1) was low, presumably mainly due to the stream's acidity. About 15% (0.76 g m-2 yr-1) of this total production, however, was contributed by the permanent meiofauna (species always small enough to pass through a 500-µm mesh, and mainly made up of ostracods and copepods but also including rotifers, microturbellarians, and others). By estimating separately production from the macrofaunal and meiofaunal net fractions (500/xm and 42-500 µm, respectively), we found that about 51% of total production could be accounted for by the permanent and temporary meiofauna together, the latter being defined as organisms that potentially grow into the macrofaunal size class and here consisting primarily of oligochaetes, chironomids, and plecopterans. This study points to the potentially substantial underestimation of production arising from the problems of assessing the meiofauna, including the former use of coarse-meshed (e.g., 500 µm) sampling devices, the requirement for live sorting of many soft-bodied taxa, and other difficulties of counting and identifying less well known groups. [ABSTRACT FROM AUTHOR]

1. We investigated abundance–size spectra and body size–density allometry using an exceptionally detailed data set from a stony stream. The body size distribution of the whole metazoan community was expressed in terms of both density and biomass over a 14-month survey, and the relationships of body size with population density and taxon richness were also evaluated.2. The distribution of density among size classes varied considerably between sampling occasions, although that of biomass was rather more consistent. This suggests that the body size distribution is dynamic and does not always fall into any single pattern or category, as has been assumed hitherto.3. Analysis of the modes in the density–body size distribution in individual sample-units revealed small-scale spatial variation, with little consistency in the number or position of modes in any one month.4. This stream community did not conform to the−0·75 (or indeed to any other significant regression) relationship between body size and population density, although such patterns are very sensitive to methodological details.5. In data aggregated over the whole study, most metazoan taxa fell into the size range<10–100 µg. The annual average density– and biomass–body size spectra appeared tri- and bimodal, respectively (on log–log scales), with minor troughs in the density spectrum at about 100 and 0·1 µg body mass. This could indicate, at this scale of analysis, discontinuities in the way in which benthic species exploit their environment.6. Spatial and temporal heterogeneity in size spectra, and the lack of clear density–body size allometry, could be due to biotic or physicochemical fluctuations and environmental change (including seasonal and longer-term changes in acidity) that cause rapid species turnover and changes in density and distribution, particularly among the meiofauna. Small and rare species may thus be in the process of colonization or decline and the pattern would indicate a lack of equilibrium, particularly among organisms in the smallest size classes, at this temporal scale of the study.Journal of Animal Ecology(2005)doi: 10.1111/j.1365-2656.2005.00943.x [ABSTRACT FROM AUTHOR]

Summary 1. The properties of food webs are important both in theoretical ecology and environmental management, yet remain elusive. Here, we examined 12 new stream food webs of higher taxonomic resolution and completeness than any previously published data set and combined them with other 10 published stream webs. 2. Compared with most previously published food webs, these stream communities (containing between 22 and 212 species) had more feeding links per species, a higher fraction of intermediate species (0·78–0·88) and lower fractions of top (0·04–0·09) and ‘basal’ species (0·07–0·15). 3. The exponent of the log link-log species relationship (S 1·3 ) differed significantly (ancova: P < 0·001) from the link-species ‘law’ (S 1 ) and the constant connectance hypothesis (S 2 ). Based on these results, the link scaling law and the constant connectance hypothesis must be rejected for food webs in stream systems. 4. Connectance was markedly lower than previously reported and decreased with web size. We attribute this to the body-size disparity of organisms at the top and the bottom of the web, flow disturbance and to the high physical heterogeneity and complexity of the sedimentary habitat. All these may reduce the fraction of possible feeding links that are realized. [ABSTRACT FROM AUTHOR]