摘要:

1. Substratum selection by the burrowing larvae of the dragonflyOnychogomphus uncatuswas examined in an artificial laboratory stream at different larval densities and in the presence of one of three other dragonfly species.2. The larvae ofO. uncatus, as well as those of the other species, clearly preferred gravelly sand substratum rather than gravel or stone.3. At low larval density (71.4 m–2) in the stream, 83% of theO. uncatuswere found in gravelly sand. An increase of larval abundance in the stream to 202.4 specimens m–2resulted in greater density in all substrata, but this increase was proportionally lowest in gravelly sand.4. The presence of a second species had various effects on the microdistribution ofO. uncatus. In the presence ofGomphus simillimusorCordulegaster boltoniiimmaculifronsthe distribution ofO. uncatuschanged significantly; their density increased in the normally less preferred substrata. This effect is interpreted as asymmetric interspecific interference. The presence of a third species,Onychogomphus forcipatus unguiculatus, had no effe

ISSN:0046-5070    

DOI:10.1046/j.1365-2427.1996.00491.x

出版商:Blackwell Science Ltd    

年代:1996

数据来源: WILEY

摘要:

1. This study involves an examination of two‐ and three‐trophic‐level food chains at two temperatures (18 and 25 °C) in order to determine how the addition of a carnivore to a predator–prey system can alter the dynamics of populations and how this effect may be temperature mediated. The system consisted of phytoplankton,Daphnia pulexand the flatwormMesostoma ehrenbergii.2. Although the plant–herbivore system is inherently unstable at 25 °C, the addition of the carnivore led to a further destabilization of theDaphnia–algal dynamics at the higher temperature. No destabilization effect of the carnivore was noted at 18 °C. At the lower temperature, all populations persisted and the carnivore induced changes only in the age structure of theDaphniapopulations rather than in overall biomass.3. The differential effects of the carnivore at two temperatures can be attributed to shifts in the life history, physiological rates and the reproductive strategy employed byMesostoma.4. Previous theoretical work has predicted that the addition of a third trophic level to an unstable predator–prey system should stabilize dynamics. Our results indicate that the effect of a carnivore on plant–herbivore dynamics can be significantly affected by a

ISSN:0046-5070    

DOI:10.1046/j.1365-2427.1996.00492.x

出版商:Blackwell Science Ltd    

年代:1996

数据来源: WILEY

摘要:

1. Although the crucial point of disturbance experiments in streams is the extent to which they can simulate an actual spate, this aspect has been widely neglected in the design of such studies. Similarly, the influence of the specific hydrological disturbance regime of a stream on its benthic community has received much theoretical attention in recent years, but hypotheses have rarely been tested in the field.2. Our field experiment compared the structure of the benthic invertebrate community in the prealpine River Necker in north‐eastern Switzerland with predictions of the patch dynamics concept about the faunal composition of frequently disturbed streams. We also compared the resistance and resilience of the invertebrates between two sites in the River Necker. A similar substratum composition at both sites, but higher shear stress values both at baseflow and bankfull discharge at site 2, implied a higher disturbance frequency at the latter site. Five patches of stream bed of ≈ 9 m2were disturbed by kicking and raking at each site, while five similar areas served as controls. From each plot, six Surber samples were taken: the first immediately after the disturbance, and the following five 1, 3, 6, 10 and 30 days later.3. Resilience of the total benthic invertebrate fauna was high. The total number of individuals recovered to undisturbed densities within 30 days at site 1 and 6 days at site 2. Taxon richness recovered within 3 days. In accordance with theory, taxa with high recolonization rates made up a major percentage of the total number of individuals, especially in disturbed plots. However, this percentage was lower at site 2 in spite of the higher disturbance frequency at this site.Rhithrogenaspp.,Leuctraspp. and the Simuliidae recovered faster to undisturbed densities at site 2. In contrast, absolute recolonization rates of these taxa were higher at site 1, where total invertebrate densities were more than twice as high as at site 2.4. Our results suggest that the time since the last disturbance should be considered as an important factor in studies of benthic invertebrate communities in prealpine rivers, because disturbances can alter the community structure. In frequently disturbed streams, very short sampling intervals may be needed to detect differences in taxon‐specific colonization rates. The specific hydrological disturbance regime of such streams is also important, because even within‐stream differences in the resilience of the benthic invertebrate community are p

ISSN:0046-5070    

DOI:10.1046/j.1365-2427.1996.00496.x

出版商:Blackwell Science Ltd    

年代:1996

数据来源: WILEY

摘要:

1. The blind isopodCaecidotea tridentatais the dominant invertebrate in a simple community of subterranean organisms inhabiting the local, shallow aquifer under Konza Prairie, Kansas, U.S.A.2. The ecology of this karst aquifer was explored by sieving water from a spring, collecting water samples for analysis, and manipulating sediments and invertebrates in the laboratory. The size frequency distribution was stable and gravid females were present throughout the year, suggesting continuous reproduction.3. The number of isopods collected per litre was negatively correlated with discharge and the abundance of bacteria, but positively related to the numbers of an amphipod (Bactrurus hubrichti). This may be explained if both the isopods and the amphipods track the saturated zone below the top of the water table; at lower discharge, the top of the water table is closer to the spring outlet. The suspended bacteria wash out in greater numbers with high discharge.4. When isopods and carbon were added in a pairwise experimental design to sediments collected from the aquifer, the isopods significantly stimulated numbers and activity of planktonic and sedimentary bacteria, while carbon had no effect. This suggests that invertebrates can affect the microbes locally in an aquifer.

ISSN:0046-5070    

DOI:10.1046/j.1365-2427.1996.00497.x

出版商:Blackwell Science Ltd    

年代:1996

数据来源: WILEY

摘要:

1. We examined the effects of prey abundance on patch selection by a benthic fish, the mottled sculpin (Cottus bairdi), in a fourth order, southern Appalachian stream (North Carolina, U.S.A.). This habitat is a mosaic of small (i.e.<0.5 m2), relatively discrete patches.2. Patches were characterized in terms of physical habitat variables, detritus (coarse particulate organic matter, CPOM) and macroinvertebrate abundance (number and biomass). We quantified patch selection by comparing the characteristics of patches utilized by sculpin with those of locally available patches. Locally available patches were selected using a constrained random sampling design (i.e. randomly selected within a 2 m radius from each fish). We also examined the relationship between macroinvertebrate abundance, CPOM and the physical characteristics of available patches.3. Patches selected by sculpin contained significantly higher macroinvertebrate abundances (both number and biomass) than locally available patches in five out of six seasonal samples. Sculpin also occupied patches with significantly higher amounts of CPOM in three out of five seasonal samples. Patches utilized by sculpin, however, could not be consistently differentiated from locally available patches on the basis of physical variables. In addition, macroinvertebrate abundance was not consistently related to physical habitat variables or CPOM during the course of the study.4. Our results suggest that sculpin are able to assess patch quality on the basis of prey abundance and select patches that potentially maximize energy gain. This behaviour may produce an increase in individual fitness, especially when prey distributions are not consistently related to habitat variables. Quantifying patch use in relation to prey abundance may help elucidate the causal factors determining habitat use by benthic fishes in other lotic systems.

ISSN:0046-5070    

DOI:10.1046/j.1365-2427.1996.00498.x

出版商:Blackwell Science Ltd    

年代:1996

数据来源: WILEY

摘要:

1. The feeding habitat of a river specialist, blue duck (Hymenolaimus malacorhynchos(Gmelin 1789): Anatidae), was characterized in terms of water depth and velocity on eight occasions over a 13‐month period in a river in the central North Island of New Zealand using video to record activity and relocate feeding sites.2. Of the five feeding activities identified (‘pecking’, ‘grazing’, ‘head‐dipping’,up‐ending’ and ‘diving’), adult blue duck used mostly head‐dipping (>60% of feeding events on all dates), although diving or grazing from submerged surfaces of exposed boulders comprised major proportions of feeding behaviour (up to 33%) on occasions. Variations in feeding behaviour between dates partly reflected changes in antecedent flow conditions and the annual cycle of the birds.3. Grazing and diving occurred in significantly faster water (mostly 0.3–0.45 m s–1) and at significantly different depths (mean = 0.10 and 0.55 m, respectively) than head‐dipping (0.20 m depth and 0.28 m s–1velocity). Adult feeding depths and velocities at four sites on different dates averaged 0.20 m and 0.31 m s–1, respectively. Most feeding by 3–4‐week‐old ducklings occurred over a similar distribution of water velocities to adults but over a wider range of depths.4. Adult birds fed in significantly shallower and lower velocity water than was available on the two dates that comparisons could be made. Ducklings also fed over a slower range of water velocities but were not selective in terms of water depth.5. Energetically more expensive search methods were employed at times of high apparent energy demand to access flow microhabitats where larger bodied prey were more likely to be encountered.6. These data indicate that, like other aquatic organisms, river birds can be influenced by basic hydraulic elements of river flow, but show at the same time that adult blue duck can accommodat

ISSN:0046-5070    

DOI:10.1046/j.1365-2427.1996.00499.x

出版商:Blackwell Science Ltd    

年代:1996

数据来源: WILEY

摘要:

1. Oxygen uptake and denitrification were determined in two bioturbated sediments from a eutrophic lake in southern Sweden. In laboratory mesocosms, an organic profundal sediment was incubated withChironomus plumosusL. and a sandy littoral sediment with an organic‐rich top layer was incubated withPolypedilumsp. Both species of chironomid are sediment tube‐dwelling.2. Oxygen consumption, expressed per gram of larval dry weight, was enhanced to the same extent by the larvae in both sediments. Measurements of the respiration rate of individual larvae revealed that the respiration per gram dry weight of the smallerPolypedilumsp. was more than three times higher than that ofC. plumosus.3. Denitrification was measured using the ‘nitrogen isotope pairing’ technique. In the organic sediment, denitrification of nitrate from the water phase (dw) and denitrification of nitrate from coupled nitrification (dn) were each correlated with the biomass ofC. plumosus. In the sandy sediment, dw was correlated with the biomass ofPolypedilumsp., while dn did not show any correlation withPolypedilumsp.4. Oxygen uptake in the organic sediment was increased by a factor of 2.5, dw 5‐fold and dn 2.5‐fold at a biomass of 10 g m–2dry weight of C. plumosus. The same biomass ofPolypedilumsp. in the sandy sediment resulted in a 2‐fold stimulation of oxygen uptake and a 3‐fold stimulation of dw, while dn was not affected. These differences in stimulation between oxygen uptake and denitrification by the larvae in the sediments suggest that the stimulation pattern cannot be explained by simple extension of the sediment surface. The burrows evidently reduce the distance between the nitrate source in the water column and the denitrifiers in the anoxic zones.5. This study indicates that bioturbation by macrofauna elements can have a great impact on denitrification in lake sediments, and that different organisms can influence nitrogen turnover

ISSN:0046-5070    

DOI:10.1046/j.1365-2427.1996.00500.x

出版商:Blackwell Science Ltd    

年代:1996

数据来源: WILEY

摘要:

1. Hydraulic conditions, periphyton biomass and invertebrate communities were compared on artificial substrates exposed to a range of upstream roughness conditions across an area of uniform current velocity and depth in a gravel‐bedded river. The effect of river bed roughness was simulated by installing roughness elements upstream of artificial substrates.2. Increasing upstream roughness reduced the average near‐bed velocity above the substrates and increased short‐term variability in velocity (i.e. turbulence).3. Periphyton chlorophylladensity showed a general decline with near‐bed velocity and was significantly lower on the substrates exposed to the river bed reference and 0 mm roughness treatments than the 110 mm roughness elements. Chlorophyllawas also negatively correlated with the abundance of larger collector‐browsing invertebrates. This indicates that effects of the changes in hydraulic conditions on invertebrates may have contributed to the observed treatment effects on periphyton.4. Invertebrate abundance and diversity declined with increasing upstream roughness. Filter‐feeders, collector‐browsers and predatory invertebrates all declined in abundance with increasing upstream roughness, but the effect was strongest for filter‐feeders. Eight of the nine most common taxa showed significant treatment effects. The orthoclad chironomid,Eukiefferiellasp., was not influenced strongly by upstream roughness, but its abundance was correlated significantly with per

ISSN:0046-5070    

DOI:10.1046/j.1365-2427.1996.d01-466.x

出版商:Blackwell Science Ltd    

年代:1996

数据来源: WILEY

摘要:

1. The restricted scale of most prior studies of genetic diversity in daphniid populations provides limited information on the geographical patterning of gene frequencies. The present study addresses this gap by examining allozymic divergence in populations of the most broadly distributed daphniid in the warm temperate regions of North America,Daphnia obtusa,across its range.2. Local populations of this species show the gene frequency differentiation typical of other daphniids. In contrast to other daphniids with broad distributions, however, further divergence is apparent at a larger geographical scale, with North AmericanD. obtusafragmented into three lineages showing largely allopatric distributions. The three lineages are based primarily on allele frequency shifts at three polymorphic loci and are represented by eastern, central and south‐western groupings.3. Because of this pattern of differentiation, there is no simple monotonic relationship between geographical distance and genetic divergence. Instead, local metapopulations belonging to a specific lineage show little genetic divergence over several thousand km, while marked shifts in gene frequency occur over a few hundred km in regions where different lineages are in contac

ISSN:0046-5070    

DOI:10.1046/j.1365-2427.1996.00501.x

出版商:Blackwell Science Ltd    

年代:1996

数据来源: WILEY

摘要:

1. Methods for measuring concentrations of total and individual free amino acids (TFAA, FAA), other amino compounds and humic substances (HS) in media containing the following three treatments and the control are described: (i) the non‐axenic aquatic macrophyteCeratophyllum demersumalone; (ii) the pulmonate snail,Biomphalaria glabrataalone; (iii) the snail, plant and the epiphytic bacteria and algae together as a four‐component modular system; (iv) control without organisms.2. TFAA accumulated to give asymptotic values of 2.5 μm, 10 μmand 15 μmin treatments (i), (ii) and (iii), respectively, by the end of the 30‐day incubation period. The mass‐specific accumulation rate in the treatment with the snail alone [89.4 nmol g–1(wet weight minus shell) day–1] was approximately ten times that with the plant alone [8.4 nmol g–1(wet mass) day–1]. No FAA or HS could be detected at any time in (iv).3. On the second day of incubation the concentrations of TFAA and of some individual amino acids were significantly higher in the treatment containing the snail and plant together than the sum of the concentrations in the treatments with the plant and snail alone, presumably due to an increase in the snails’ metabolic rate and ‘sloppy feeding’.4. The differences in the relative abundance of amino compounds accumulating in media conditioned by snails alone (e.g. much larger proportions of ammonia, ethanolamine and phosphoserine than in plant‐conditioned media) and plants alone (e.g. larger proportions of asparagine and glutamine than in snail‐conditioned media) suggest that snails and plants may derive mutual benefits by exchanging amino compounds.5. The accumulation patterns of the more recalcitrant HS differ from those of the amino acids in two respects. First, the HS concentrations continued to increase throughout the 30‐day incubation period in all three treatments. Second, most of the HS in the module originates from the plant as both the concentrations and mass‐specific accumulation rates were significantly higher in the treatment with the plant alone [2.6 mg l–1, 9.09 μg g–1(wet mass) day–1, respectively] than in the treatment with the snail alone [0.75 mg l–1and 7.7 μg g–1(wet mass) day–1, respectively].6. The possible reasons for the differences in the accumulation patterns of FAA and HS in the three treatments are discussed. Evidence is also given in support of the testable hypothesis that the four components of the module derive several mutual benefits, including those arising from the release of dissolved organic m

ISSN:0046-5070    

DOI:10.1046/j.1365-2427.1996.d01-467.x

出版商:Blackwell Science Ltd    

年代:1996

数据来源: WILEY