Total Invertebrate Density Research Articles

The influence of water quality on hyporheic invertebrate communities in agricultural catchments

Despite knowing that agricultural practices affect river ecosystems, studies on water quality and associated hyporheic invertebrate communities are rare. Yet such knowledge is essential for sustainable use of agricultural and water resources. This study, carried out in the agricultural region of south-eastern Ireland in Spring 2007, examined the effect of water pollution on hyporheic fauna. Six river sites of known water quality were assigned to two treatments, namely, clean and polluted rivers. Two hyporheic habitats, below the wetted channel (hyporheic substream) and below the exposed gravel bank (parafluvial), were sampled within each treatment. Total invertebrate densities and richness, crustacean densities and richness, and densities of Ephemeroptera, Plecoptera and Trichoptera (EPT) were significantly reduced in polluted hyporheic and parafluvial habitats. A higher recorded concentration of nitrate in the polluted hyporheic substream habitats was a significant factor in determining the invertebrate assemblages, with reductions in total and crustacean densities, and EPT richness. Similarly, invertebrate density and richness were reduced in polluted parafluvial habitats, likely being related to agricultural intensity, management practices and reduced riparian buffering. This study highlights the direct effects of agricultural practices on the invertebrate communities of hyporheic habitats and emphasises the need for holistic management and conservation measures to ensure future protection.

Chemical change of leaves during breakdown affects associated invertebrates in a subtropical stream

AIMS: Our objective was assess the effects of leaf chemical change during breakdown on the associated invertebrates. METHODS: We evaluate the chemical composition of leaves (of two tree species; Sebastiania brasiliensis and Campomanesia xanthocarpa) and the density of invertebrates during leaf breakdown (on four incubation times; 3, 7, 14 and 22 days) in a subtropical stream. Linear multiple regression analysis were performed to evaluate the relationship between invertebrate density and changes in leaf chemical during breakdown. RESULTS: Density of invertebrates was related to the chemical composition of leaves. There was a positive correlation of K (β = 3.48) and a negative of C:N (β = -0.34), polyphenols (β = -0.16), Ca (β = -2.98) and Mg (β = -2.58) with the total density of invertebrates. Density of invertebrates on leaves reached 38 ± 9 and 192 ± 31 individuals g-1 leaf DM during the first 3 and 7 days of incubation, had decreased by the 14th day (117 ± 18) and then increased after 22 days (270 ± 41). CONCLUSIONS: We conclude that changes in the chemistry of decomposing leaves affect invertebrate colonization process. This conclusion reinforce the importance of understand the dynamic of energy and nutrients and its association with the biological communities of tropical riparian ecosystems.

An invasive species may be better than none: invasive signal and native noble crayfish have similar community effects

Summary Human activities have resulted in the decline of native crayfish and promoted the spread of invasive crayfish species in European fresh waters, threatening ecosystem structure and functioning. We compared effects of native noble crayfish (Astacus astacus), invasive signal crayfish (Pacifastacus leniusculus) and of the absence of crayfish on leaf litter breakdown and invertebrate prey density and biomass in stream ecosystems. In microcosm experiments, invertebrate shredder density was significantly reduced by crayfish presence but similarly affected by the two crayfish species. While crayfish did not directly influence leaf litter breakdown, their presence reduced litter decay rates through predation on invertebrate shredders (i.e. trophic cascade). These effects were again similar between native and invasive crayfish but differed among shredder prey species. Field experiments confirmed that leaf litter breakdown was significantly reduced by crayfish. Although total invertebrate density was not affected by crayfish predation, species‐specific effects were observed (e.g. Limnephilidae [Trichoptera] densities were reduced in the presence of crayfish) but did not differ between native and invasive species. Our results show that crayfish can have strong, but species‐specific, effects on invertebrate prey and indirect effects on leaf litter breakdown. Contrary to previous findings and popular belief, results also indicate that native noble crayfish and invasive signal crayfish have similar effects on stream communities. Hence, from a management perspective, it may be preferable to have invasive crayfish than no crayfish when impairment results in unsuitable habitats for native species.

Macrobenthic invertebrates in the main channel of Lake Nasser, Egypt

Samples were collected seasonally from ten sectors of the main channel of Lake Nasser during 2013. Macrobenthic community consisted of twenty-four species belonging to three phyla, Annelida (five species), Mollusca (nine species) and Arthropoda (ten species). Annelida ranked the first position and constituted 52% and 68% of total population density (P.D.) and biomass of macrobenthic invertebrates (M.B.I.) respectively. Annual average of M.B.I. was 529organisms/m2 weighing 2.30g/m2. The abundance of M.B.I. species was closely related with type of sediments and water level in the lake. Tushka was the richest sector in the area where 1132organisms/m2 weighing 3.65g/m2 were recorded, while El-Madiq was the poorest one where 127organisms/m2 weighing 0.61g/m2 were recorded. Southern sectors were more populated with bottom fauna than northern sectors. This mostly attributed to the increase of Limnodrilus spp. Benthic fauna attained its maximum during autumn, while summer season sustained the lowest density. Correlation Coefficient revealed strong negative relationships between temperature and the density of oligochaets and arthropods. The highest diversity index (H) and species richness (SR) were calculated in Abu-Simble while, the lowest (H) and SR were calculated in El-Madiq.

Decadal changes in benthic invertebrate biomass and community structure in Lake Simcoe

Total benthic invertebrate biomass in shallow offshore waters (depths 10–15 m) increased 17-fold on average following the invasion of dreissenid mussels and implementation of P controls in Lake Simcoe, Ontario, despite a 50% overall decrease in total invertebrate density during the same time period. The increase in total invertebrate biomass at 10- and 15-m depths was primarily dreissenid biomass. Patterns of biomass with depth and dreissenid invasion for individual taxa were typically similar to those for density. Biomass of chironomid and nondreissenid bivalves declined at shallow (10–15 m) sites but increased at the deepest (20–30 m) sites, whereas biomass of oligochaetes declined at all sites. Density and biomass of Amphipoda, Isopoda, and Gastropoda increased at depths ≥10 m, and these taxa were found more frequently in deeper sites after dreissenid invasion than before. Increased habitation of deeper sites by these taxa may be mediated by increased habitat complexity caused by deposition of dreissenid shells, nutrient enrichment of substrate occupied by dreissenids, and improvements in hypolimnetic dissolved O2 and water clarity observed during dreissenid invasion. Increased length and biomass of profundal chironomids after dreissenid invasion may be the result of taxonomic changes in the chironomid community, which in turn appear to be closely associated with improvements in deep-water O2 concentrations. Changes in the benthic community described here probably have important consequences for the degree of coupling between nearshore and offshore habitats in Lake Simcoe.

Macroinvertebrate size–mass relationships: how specific should they be?

We assessed the relative magnitude of various factors (year, preservation method, continent, investigator, and taxonomic level) affecting prediction of invertebrate dry mass (DM) in light of the variability of assessments of invertebrate density. We developed 34 length (L)–DM relationships for Oligochaeta and 17 freshwater invertebrate families belonging to Mollusca, Crustacea, and Insecta. Comparison of our predicted DM for reference-size individuals with values from 120 other published equations revealed that 31% of predicted values were within our 95% CI and 73% were within a 2× DM range (i.e., between 0.5× and 2×). Interannual differences in exponent (slope) or scale factor (intercept) of L–DM relationships were detected for 6 of the 7 taxa investigated, but represented only 3% of total variance in predicted DM. Similarly, preservation methods and measured body dimension each accounted for a small (0–3%) fraction of total variance. Variation among investigators (12–50%) and continents (1–17%) were more important and might have reflected methodological or regional and latitudinal differences. Increasingly precise taxonomical levels explained progressively lower proportions of the total variance, a result indicating that family or a more precise taxonomic level provided a robust estimate of most invertebrate DM even if the equations were derived from other sites. However, overall variability induced by L–DM relationships was smaller than variability in total invertebrate density among replicate samples (coefficient of variation [CV] = 19–97%), a result indicating that more effort should be devoted to improving the accuracy of invertebrate density estimates than to developing site-specific L–DM relationships to assess benthic biomass in freshwater.

Response of stream invertebrates to short-term salinization: A mesocosm approach

Salinization is a major and growing threat to freshwater ecosystems, yet its effects on aquatic invertebrates have been poorly described at a community-level. Here we use a controlled experimental setting to evaluate short-term stream community responses to salinization, under conditions designed to replicate the duration (72 h) and intensity (up to 5 mS cm−1) of salinity pulses common to Mediterranean rivers subjected to mining pollution during runoff events. There was a significant overall effect, but differences between individual treatments and the control were only significant for the highest salinity treatment. The community response to salinization was characterized by a decline in total invertebrate density, taxon richness and diversity, an increase in invertebrate drift and loss of the most sensitive taxa. The findings indicate that short-term salinity increases have a significant impact on the stream invertebrate community, but concentrations of 5 mS cm−1 are needed to produce a significant ecological response.

Invertebrate colonisation during leaf processing of native, exotic and artificial detritus in a tropical stream

The relationship between leaf breakdown and colonisation by invertebrates in tropical aquatic ecosystems is poorly understood, especially in regard to the added problem of the potential effects of exotic species. To assess the colonisation by invertebrates during leaf breakdown in a third-order headwater stream in south-eastern Brazil, we conducted an experiment using the native species Miconia chartacea, the exotic species Eucalyptus grandis and artificial leaves. We hypothesised that the quality of the detritus and the leaf shape influence invertebrate colonisation because of the quality of the food and refuge offered by leaf detritus. Invertebrate density and richness were higher on leaves of E. grandis than on those of M. chartacea. Taxon richness did not differ among M. chartacea and the two sizes of artificial leaves offered, probably as a function of the chemical composition of E. grandis. Total invertebrate density was significantly higher in the organic detritus, suggesting that detritus provides food for the organisms. Our results indicate that the colonisation of invertebrates is probably affected by the chemical composition of detritus. Contrary to expectations, the community of invertebrates had no difficulty in colonising E. grandis, although it is an exotic species. In addition, the shredder activity did not influence leaf breakdown. These results may indicate that the invertebrates in this stream tend to behave as generalist feeders.

Contrasting effects of an invasive crayfish (Procambarus clarkii) on two temperate stream communities

Summary1. The effects of omnivorous exotic species on native communities are often difficult to predict because of the broad diets and behavioural flexibility of the omnivore, and the diverse abiotic and biotic characteristics of invaded systems. We investigated experimentally the effects of a gradient of density of the introduced, omnivorous red swamp crayfish Procambarus clarkii (Decapoda: Cambaridae) on two stream communities in southern California, U.S.A.2. The Ventura River is a clear, flowing stream with a cobble substratum, with abundant algae but low densities of large invertebrates, small herbivores and snails. The Santa Ynez River at the time of the study consisted of a series of drying pools underlain by sand, with abundant charophytes, large predatory invertebrates and herbivores, including snails.3. In the Ventura River, periphyton biomass and inorganic sediment decreased with increasing crayfish abundance, but in the Santa Ynez River, periphyton and sediment were unrelated to crayfish densities.4. In the Ventura River, the biomass and density of all benthic invertebrates combined, chironomids, micropredators, the meiofauna (chydorid cladocerans, copepods and ostracods), and specific predatory and herbivorous taxa, as well as taxon richness, were negatively related to crayfish density. In the Santa Ynez River, the biomass and average body size of benthic invertebrates, predatory invertebrates, herbivores and chironomids, but not total invertebrate density or taxon richness, were negatively related to crayfish density.5. Fewer large predatory invertebrates and snails (Physella gyrina) in both streams, and baetid mayflies in the Ventura River, were visible at night in channels where crayfish were abundant. Snails responded to crayfish by moving above the water line in the Santa Ynez River, but not in the Ventura River.6. We suggest that the same omnivore had different effects on these neighbouring streams because of crayfish predation on large invertebrates in the Santa Ynez River and the scarcity of such prey in the Ventura River, leading to increased crayfish grazing on periphyton, and reductions in periphyton‐associated invertebrates, in the Ventura River.

An assessment of cellulose filters as a standardized material for measuring litter breakdown in headwater streams

AbstractThe decay rate of cellulose filters and associated chemical and biological characteristics were compared with those of white oak (Quercus alba) leaves to determine whether cellulose filters could be a suitable standardized material for assessing deciduous leaf breakdown in headwater streams. The comparison was done across reaches draining mixed deciduous forest and post‐coal mining catchments, in natural and constructed channels, and ranged in flow duration from ephemeral to perennial. Decay rates of leaves and filters were predicted to differ at a given site, but the decay rates and associated characteristics of leaf and filter litterbags would be positively related. Filter decay rates did not differ across channel type or flow permanence class. Oak leaves decayed ca 2·5× faster than cellulose filters and there was no relationship between decay rates (R2 = 0·02). Ergosterol concentration, total invertebrate density, shredder density, total invertebrate biomass and taxa richness were significantly higher in oak litterbags than in filter litterbags across four sampling dates over 306 days. The biomass of invertebrate shredders colonizing litterbags did not differ between the substrate types. The C:N content was higher for filters than for oak leaves, but the mean difference between substrates decreased by ∼10‐fold over the 306‐day study. In contrast, mean differences in ergosterol concentration between substrates increased threefold over the study. Although characteristics associated with filter litterbags were positively related to those of oak leaf litterbags, most relationships had low explanatory power (R2⩽0·3); however, stronger relationships existed for total invertebrate density, shredder density and taxa richness (R2 = 0·78). Although a standardized material would be useful for incorporating litter breakdown in stream assessments, because of the strong differences in decay rate and associated characteristics, we cannot recommend cellulose filters as a suitable substrate to represent the natural breakdown of leaf material. Published in 2010. This article is a US Government work and is in the public domain in the USA.

Effects of cutting disturbance in Schoenoplectus californicus (C.A. Mey.) Soják on the benthic macroinvertebrates

Lagoons are considered protected areas because these systems play a key ecological role. However, the extraction of macrophyte Schoenoplectus californicus is held for manufacture of handcrafts, being an alternative income for riverbank communities. This study evaluated the impact of S. californicus experimental cutting on benthic macroinvertebrates trough a field experiment. Macroinvertebrates were sampled at 1, 12, 26 and 60 days after the macrophyte cutting in demarked plots (1 m²), as well at control plots. The families number was not statistically different (ANOVA, p > 0.05), but the total density of invertebrates, and the density of Ceratopogonidae were significant (ANOVA, p < 0.05) for interaction between sampling date and treatment. A Principal Components Analysis identified that the level of the water column was the variable that most influenced the variation between the samples gathered in the experiment. We concluded that the cutting of S. californicus, in this area, as the intensity of the cut held, did not affect considerably the aquatic macroinvertebrates. The results suggest that the small-scale extractivism in these regions carries little effect because the fauna of adjacent areas probably can quickly colonize the disturbed areas.

Longitudinal zonation of macroinvertebrates in an Ecuadorian glacier‐fed stream: do tropical glacial systems fit the temperate model?

Summary1. The ecology of glacier‐fed streams at temperate latitudes has been intensely studied in recent years, leading to the development of a well‐validated conceptual model on the longitudinal distribution of macroinvertebrate communities downstream of the glacier margin (Freshwater Biology, 2001a; 46, 1833). However, to our knowledge, the ecology of tropical glacier‐fed streams has not yet been studied.2. We sampled benthic macroinvertebrates and measured environmental variables at nine sites between 4730 and 4225 m altitude along a 4.3 km stretch of a glacier‐fed stream 40 km south of the equator in the Ecuadorian Andes. Our goal was to study the longitudinal distribution of the fauna in relation to environmental factors and to compare this with the conceptual model based on temperate–arctic glacier‐fed streams.3. Total density of invertebrates differed considerably at the two highest altitude sites; 4600 m−2 at a pro‐glacial lake outlet and only 4 m−2 at a site originating directly from the glacier snout. Otherwise, there was a downstream decrease in density to about 825 m−2 at the three lowest sites. Taxon richness increased with distance from the glacier, very similar to the pattern predicted. A total of 28 taxa were collected; two at the glacier snout, seven at the nearby pro‐glacial lake outlet, 13 at site 2 (&lt;400 m from the glacier) and 20 at the lowest sites.4. The numerical percentage of Chironomidae (Diptera) decreased downstream from 100 to 44%. The subfamily Podonominae was numerous at the highest sites but became much less important further downstream. The Orthocladiinae were important both in numbers and species at all sites, while Diamesinae were numerous only in the middle of the reach studied and were completely absent from the upper three sites. The limited importance of the Diamesinae, and its replacement by Podonominae, is different from the pattern typically observed in north‐temperate glacier‐fed streams. This could be because of the fact that the genus Diamesa is missing from the Neotropics.5. Stream temperature and channel stability explained most of the variability in faunal composition and richness, supporting the model. Stability increased systematically downstream while temperature did not. Surprisingly, no classical kryal zone (Tmax &lt; 4 °C) was found, as even the site closest to the glacier snout (50 m) had a Tmax of 15 °C and no site had Tmax &lt; 8 °C. We propose that this might be a general feature of equatorial glacial streams.

Spartina alterniflora invasions in the Yangtze River estuary, China: An overview of current status and ecosystem effects

The Yangtze River estuary is an important ecoregion. However, Spartina alterniflora, native to North America, was introduced to the estuary in the 1990s through both natural dispersal and humans and now it is a dominant species in the estuarine ecosystems, with its invasions leading to multiple consequences to the estuary. S. alterniflora had great competitive effects on native species, including Scirpus mariqueter and Phragmites australis, and could potentially exclude the natives locally. The presence of S. alterniflora had little influence on the total density of soil nematodes and macrobenthonic invertebrates, but significantly altered the structure of trophic functional groups of nematode and macrobenthonic invertebrate communities. The conversion of mudflats to Spartina meadows had significant effects on birds of Charadriidae and Scolopacidae, which might be attributable to the reduction of food resources and the physical alterations of habitats for shorebirds. S. alterniflora invasions increased the primary productivity of the invaded ecosystems, and altered carbon and nitrogen cycling processes. Our studies focused mainly on the effects of S. alterniflora invasions on the structure of native ecosystems; thus further studies are clearly needed to investigate how ecosystem functioning is affected by the modification of the structure of estuarine ecosystems by S. alterniflora invasions.

Size-dependent impacts of invasive alien crayfish on a littoral marsh community

The invasive signal crayfish ( Pacifastacus leniusculus) may undergo ontogenetic diet shifts with increasing size, but the consequences of crayfish size–structure changes for aquatic food webs are little known. We evaluated size-dependent impacts of signal crayfish on a littoral marsh food web using in-situ enclosures in a northern Japanese marsh. During the 60-day experiment, large crayfish (<30 mm orbital carapace length) rapidly eliminated submerged macrophytes through mechanical destruction while the comparable impacts of small crayfish (>30 mm) appeared after more time had passed. Benthic algal biomass was reduced in the presence of large crayfish but was affected little by small crayfish, suggesting that large crayfish were significant bioturbators. On the other hand, total invertebrate biomass was reduced while total invertebrate density was enhanced in the presence of both crayfish size classes. Potential crayfish impacts on invertebrates are: (1) predation on large vulnerable taxa such as Trichoptera and predatory invertebrates, (2) mechanical destruction of macrophytes, thereby reducing microhabitats for invertebrates, (3) heightened emigration of invertebrates in response to bioturbation and/or (4) predation or competition release on small invertebrates as a result of crayfish reducing their predators/competitors. Invertebrate taxa richness was nearly halved in the presence of large crayfish, probably reflecting reductions of rare taxa. Overall, functional roles of signal crayfish as omnivores and ecosystem engineers remained similar during the ontogenetic stage but the magnitude and rate of crayfish impacts intensified with increasing crayfish size. Thus, removal of large invasive crayfish alone may reduce detrimental impacts on aquatic biodiversity.

Disturbance history influences the distribution of stream invertebrates by altering microhabitat parameters: a field experiment

Summary1. We investigated the effects of local disturbance history and several biotic and abiotic habitat parameters on the microdistribution of benthic invertebrates after an experimental disturbance in a flood‐prone German stream.2. Bed movement patterns during a moderate flood were simulated by scouring and filling stream bed patches (area 0.49 m2) to a depth of 15–20 cm. Invertebrates were investigated using ceramic tiles as standardized substrata. After 1, 8, 22, 29, 36 and 50 days, we sampled one tile from each of 16 replicates of three bed stability treatments (scour, fill and stable controls). For each tile, we also determined water depth, near‐bed current velocity, the grain size of the substratum beneath the tile, epilithic algal biomass and standing stock of particulate organic matter (POM).3. Shortly after disturbance, total invertebrate density, taxon richness and density of the common taxa Baetis spp. and Chironomidae were highest in stable patches. Several weeks after disturbance, by contrast, Baetis spp. and Hydropsychidae were most common in fill and Leuctra spp. in scour patches. The black fly Simulium spp. was most abundant in fill patches from the first day onwards. Community evenness was highest in scour patches during the entire study.4. Local disturbance history also influenced algal biomass and POM standing stock at the beginning of the experiment, and water depth, current velocity and substratum grain size throughout the experiment. Scouring mainly exposed finer substrata and caused local depressions in the stream bed characterized by slower near‐bed current velocity. Algal biomass was higher in stable and scour patches and POM was highest in scour patches. In turn, all five common invertebrate taxa were frequently correlated with one or two of these habitat parameters.5. Our results suggest that several ‘direct’ initial effects of local disturbance history on the invertebrates were subsequently replaced by ‘indirect’ effects of disturbance history (via disturbance‐induced changes in habitat parameters such as current velocity or food).

On the influence of substrate morphology and surface area on phytofauna

The independent effects and interactions between substrate morphology and substrate surface area on invertebrate density or biomass colonizing artificial plant beds were assessed in a clear-water and a turbid playa lake in Castro County, Texas, USA. Total invertebrate density and biomass were consistently greater on filiform substrates than on laminar substrates with equivalent substrate surface areas. The relationship among treatments (substrates with different morphologies and surface areas) and response (invertebrate density or biomass) was assessed with equally spaced surface areas. Few statistically significant interactions between substrate morphology and surface area were detected, indicating that these factors were mostly independent from each other in their effect on colonizing invertebrates. Although infrequently, when substrate morphology and surface area were not independent, the effects of equally spaced changes in substrate surface area on the rate of change of phytofauna density or biomass per unit of substrate surface area were dependent upon substrate morphology. The absence of three-way interactions indicated that effects of substrate morphology and substrate area on phytofauna density or biomass were independent of environmental conditions outside and inside exclosures.

MIGRANT SHOREBIRD PREDATION ON BENTHIC INVERTEBRATES ALONG THE ILLINOIS RIVER, ILLINOIS

Abstract We evaluated the effect of shorebird predation on invertebrates at a wetland complex along the Illinois River, west-central Illinois, during spring migration. Using a new exclosure experiment design adapted to the shifting nature of foraging microhabitat of interior wetlands, we found that shorebird predation did not significantly deplete total invertebrate density or total biomass in open (no exclosure) versus exclosure treatments. Chironomids and oligochaetes were the most common invertebrates occurring in substrate samples. The density of oligochaetes was lower in open treatments, though the degree of difference varied both spatially and temporally. Shorebird density was positively correlated with the amount of invertebrate biomass removed from the substrate during the late-May sampling period. Our results suggest that shorebirds use an opportunistic foraging strategy and consume the most abundant invertebrate prey. The dynamic hydrology at our study site likely played a role in preventing inv...

Changes in Benthic Invertebrate Communities of South Bay, Lake Huron Following Invasion by Zebra Mussels ( Dreissena polymorpha), and Potential Effects on Lake Whitefish ( Coregonus clupeaformis) Diet and Growth

In this study we evaluated changes in benthic invertebrate communities of South Bay, Lake Huron following the invasion of zebra mussels ( Dreissena polymorpha) and considered the implications for diets and growth of whitefish ( Coregonus clupeaformis), a commercially important fish in the Great Lakes. Of the ten benthic invertebrate groups identified prior to invasion (1980–81), only densities of Diporeia and Oligochaeta have changed since the appearance of the zebra mussel, and only Diporeia and Chironomidae changed in relative abundance. These changes are similar to those observed in other areas of the Great Lakes, with the exception of an increase in Oligochaeta density. Post-invasion (2002–03) shallow-water communities appear to be more homogeneous, dominated by zebra mussels and Isopoda, whereas deep-water sites are more heterogeneous due to the loss of Diporeia. Additional data on Diporeia density for several years between 1959 and 2004 indicated that current low densities are not typical of South Bay. Based on changes in the benthic communities and published literature on whitefish diets, we predict that unless whitefish are able to switch to Mysis as an alternative to Diporeia, post-invasion whitefish diets will only contain a maximum of 57 to 84% of their former energy content. These predictions are likely underestimates, as they do not take into account increased energy costs associated with reductions in total invertebrate density at historical foraging depths.

The influence of habitat complexity on littoral invertebrate distributions: patterns differ in shallow prairie lakes with and without fish

Relationships between littoral habitat complexity and invertebrate distributions in fishless lakes are not well understood compared with well-documented relationships in lakes with fish. We examined littoral invertebrate distributions over fine-scale gradients of weed-bed habitat complexity and contrasted these patterns in four shallow prairie lakes — two with fish and two without. The above-sediment portion of submerged macrophytes and associated invertebrates was sampled from three littoral microhabitats: weed-bed centres (highly complex), weed-bed edges (moderately complex), and single plants that grew apart from distinct weed beds (least complex). Total invertebrate densities in fishless lakes did not differ between littoral microhabitats, nor were they correlated with macrophyte biomass. In contrast, total invertebrate densities in lakes with fish increased with microhabitat complexity and were positively correlated with macrophyte biomass. Weed-bed complexity also affected littoral invertebrate community structure; in all lakes, the proportion of filter-feeders decreased with increasing microhabitat complexity, but the proportion of predatory invertebrates was greater overall in fishless lakes than in lakes with fish. Our results demonstrate that small-scale variation in littoral microhabitat complexity can lead to specific patterns of invertebrate distribution that systematically differ between lakes with and without fish, and that these systematic differences may be mediated through top-down mechanisms.

Contrasting effects of salamanders on forest-floor macro- and mesofauna in laboratory microcosms

Summary We employed laboratory microcosms to test the hypothesis that predation by the salamander, Plethodon cinereus , alters invertebrate species composition of a terrestrial detritus-based food web and thereby influences leaf–litter decomposition rates. We comparedh litter invertebrates among samples of leaf–litter substrate samples in microcosms either with or without individual salamanders. We used mesh leaf–litter bags to quantify leaf–litter decomposition and temporal changes in invertebrate abundance and community composition during colonization of new litter by invertebrates. We report that substrates within microcosms with P. cinereus had more podomorphic Collembola, fewer enchytraeid worms and fewer macroinvertebrate detritivores than controls at the end of the 3 month experiment. Total invertebrate density, and densities of podomorphic Collembola and Acari increased more rapidly in litter bags within salamander microcosms than in controls. In contrast, entomobryid Collembola were fewer in litter bags in salamander microcosms in comparison to controls. However, salamanders had no effect on leaf–litter decomposition rates. We hypothesize that salamander predation enhanced microbi-detritivore mesofauna densities by (1) reducing numbers of larger detritivores that compete with mesofauna, and/or by (2) providing a subsidy in the form of salamander feces that promoted growth of microflora. Further, we hypothesize that increases of mesofauna in salamander microcosms compensated for effects on litter decomposition that may have resulted from reductions in detritivorous macrofauna.