Bilateral and unilateral antennal lesions alter orientation abilities of the crayfish, Orconectes rusticus.

Numerous animals use chemical cues within their environments to execute various behaviors. One of these behaviors is orientation to an odor source. Crayfish, in particular, can orient to food sources under a number of different conditions. It has not been determined, however, what kind of search strategy these animals employ to successfully locate a food source. To determine the role of antennae and antennules in this behavior and to investigate different modes of orientation behavior, the orientation patterns of crayfish with complete and partial antennal lesions were examined. Detailed analysis of orientation paths confirmed that crayfish could not locate odor sources with either bilateral or unilateral lesions. This suggests that crayfish are using the spatial information obtained from these appendages to successfully orient. Animals using information from the bilaterally paired appendages in the control group exhibited increased walking speed, increased speed to source and decreased heading angles towards the source compared to these measurements taken from lesioned groups. There was no significant difference in any parameters between animals with unilateral or bilateral lesions. This strongly suggests that these animals are reliant on the spatial comparison of differences between bilaterally paired olfactory appendages for successful orientation.     

Invasions and niche width: does niche width of an introduced crayfish differ from a native crayfish?

1. Human activities have promoted the spread of species worldwide. Several crayfish species have been introduced into new areas, posing a threat to native crayfish and other biota. Invader success may depend on the ability to utilise a wide variety of habitats and resources. Successful invaders are generally expected to have broader niches and to be more plastic than non-invasive species.
2. Using stable isotope ratios of carbon and nitrogen we compared the niche widths of native noble crayfish and introduced signal crayfish, a successful invader of Swedish streams. The calculation of niche width took account of between-site differences in basal resource isotope signature ranges. We also assessed whether population density, prey biomass or prey diversity affected niche width.
3. At the species level, signal crayfish had twice the niche width of noble crayfish. However, individual populations of noble crayfish and signal crayfish in Swedish streams had similar niche widths. This suggests that signal crayfish has greater plasticity with respect to habitat utilisation and feeding than noble crayfish. Niche width in both species correlated positively with benthic invertebrate biomass and diversity, indicating that animal food sources are important for crayfish.
4. We find that assessing niche width in relation to invader success can be a useful tool trying to predict the impact of invasions on different scales. The findings in this study suggest that invaders and natives will have a similar impact on the stream scale whereas the invader will have a larger impact on the regional scale due to the ability to utilise a wider range of streams.     

The orientation inducer pheromone of the fire ant Solenopsis invicta

Abstract Foraging ants recruit nestmate workers to food sources by a variety of mechanisms. We report that one behavioural subcategory of the recruitment pheromone complex of Solenopsis invicta Buren involves orientation induction. The orientation inducer pheromone exerts its effects by changing the physiological state of the recipient rather than by releasing a measurable behaviour. Some ant species use a physical 'waggle' behaviour to motivate (change physiological state) nestmate workers to follow their chemical trail. The orientation inducer pheromone can be interpreted as a chemical analogue of the physical 'waggle' inducing effects. This behaviour is not elicited by the recruitment pheromone components responsible for orientation and/or attraction. Each of these behavioural categories is mediated by a different blend of chemicals from the Dufour's gland. Activity-concentration thresholds indicate that the attraction and inducer part of the recruitment pheromone require about 250 times more worker equivalents for a response than the orientation pheromone. Therefore, the recruitment sub-categories are differentially activated by the amount of Dufour's gland material released.     

Laboratory conditioning to still or flowing water does not affect the responses to a food stimulus by red swamp crayfish (Procambarus clarkii) in flowing water habitats

We tested the role of hydrodynamics in the use of chemical signals by the red swamp crayfish (Procambarus clarkii). We conditioned the crayfish in either a still water or flowing water habitat for 2 weeks. We then observed the crayfish in control (no stimulus) and experimental (food stimulus) trials in one of two flowing water habitats-laminar or turbulent. Habitat use was significantly different between control and experimental observations, but there was no effect of conditioning habitat or experimental habitat on the responses to the food stimulus. This study adds to a growing body of literature consistent with the idea that gross responses to chemical stimuli in crayfish are not affected by the hydrodynamic context in which they are encountered.     

New Zealand stream crayfish: functional omnivores but trophic predators?

1. The feeding ecology of the crayfish Paranephrops planifrons in streams draining catchments in native forest and pastoral land was investigated using analyses of both stomach contents and stable isotopes of carbon and nitrogen. We aimed to (1) identify the energy sources of crayfish, (2) determine whether these were affected by ontogeny or land use change, and (3) assess the functional and trophic roles of crayfish in New Zealand hill-country streams.
2. In native forest streams, crayfish stomach contents were dominated volumetrically by leaf detritus (>60%), but in pasture streams aquatic invertebrates constituted more than 40% of stomach volumes and leaf detritus <30%. Stable isotope analyses revealed that crayfish from both native forest and pasture streams incorporated energy from aquatic invertebrates into their body tissue but did not appear to utilize detritus for growth. Therefore, deforestation had little impact on crayfish energy sources.
3. In native forest streams, adult crayfish (≥20 mm orbit-carapace length (OCL)) consumed greater amounts of leaf detritus than juvenile crayfish, but a corresponding change in stable isotope signatures was not detected. Ontogenetic shifts in diet were not consistent between land use suggesting that change in local habitat and food resources, as a result of land use change, affect crayfish food choice more than factors related solely to age or size.
4. Crayfish appear to occupy the trophic position of a predator, but by functioning as omnivores, they have dual roles as both predators and processors of organic matter. The use of gut content analysis in conjunction with stable isotope analyses revealed that the functional and trophic roles of P. planifrons differed, with implications for the interpretation of diet studies and understanding of the role of omnivores in food webs.     

Growth and population size of crayfish in headwater streams: individual- and higher-level consequences of acidification

1. Environmental stress may have indirect positive effects on population size through modification of food‐web interactions, despite having negative effects on individuals. Here we evaluate the individual‐ and population‐level effects of acidification on crayfish (Cambarus bartonii) in headwater streams of the Allegheny Plateau (PA, U.S.A.) with field experiments and survey data. Median baseflow pH of 24 study reaches in nine streams varied from 4.4 to 7.4, with substantial variation found both among and within streams. 2. Two bioassays were conducted to evaluate the relationship between stream pH and crayfish growth rates. Growth rates were always higher in circumneutral reaches than in acidic reaches. Crayfish originating in acidic water grew less when transplanted into neutral water than did crayfish originating in neutral water, providing some evidence for a cost of acclimation to acidity. 3. Stream surveys showed that fish were less abundant and crayfish more abundant in acidified streams than in circumneutral streams. Crayfish density was sixfold higher in reaches with the lowest pH relative to circumneutral reaches. Large crayfish made up a higher proportion of crayfish populations at sites with high fish biomass, consistent with the hypothesis that fish predation on small individuals may be limiting crayfish population size at these sites. 4. Although individual crayfish suffered lower growth in acidified streams, increased acidity appeared to cause an increase in crayfish population size and shifts in size structure, possibly by relieving predation pressure by fish.     

Re-Meandering of Lowland Streams: Will Disobeying the Laws of Geomorphology Have Ecological Consequences?

We evaluated the restoration of physical habitats and its influence on macroinvertebrate community structure in 18 Danish lowland streams comprising six restored streams, six streams with little physical alteration and six channelized streams. We hypothesized that physical habitats and macroinvertebrate communities of restored streams would resemble those of natural streams, while those of the channelized streams would differ from both restored and near-natural streams. Physical habitats were surveyed for substrate composition, depth, width and current velocity. Macroinvertebrates were sampled along 100 m reaches in each stream, in edge habitats and in riffle/run habitats located in the center of the stream. Restoration significantly altered the physical conditions and affected the interactions between stream habitat heterogeneity and macroinvertebrate diversity. The substrate in the restored streams was dominated by pebble, whereas the substrate in the channelized and natural streams was dominated by sand. In the natural streams a relationship was identified between slope and pebble/gravel coverage, indicating a coupling of energy and substrate characteristics. Such a relationship did not occur in the channelized or in the restored streams where placement of large amounts of pebble/gravel distorted the natural relationship. The analyses revealed, a direct link between substrate heterogeneity and macroinvertebrate diversity in the natural streams. A similar relationship was not found in either the channelized or the restored streams, which we attribute to a de-coupling of the natural relationship between benthic community diversity and physical habitat diversity. Our study results suggest that restoration schemes should aim at restoring the natural physical structural complexity in the streams and at the same time enhance the possibility of re-generating the natural geomorphological processes sustaining the habitats in streams and rivers. Documentation of restoration efforts should be intensified with continuous monitoring of geomorphological and ecological changes including surveys of reference river systems.     

Does substrate coarseness matter for foraging ants? An experiment with Lasius niger (Hymenoptera; Formicidae)

We investigated whether workers of the ant species Lasius niger are able to sense and discriminate the coarseness of the substrate on which they walk. First, we studied the way in which substrate coarseness affects the ants' locomotory behaviour. Second, we investigated the spontaneous preference of ants for substrates of different coarseness. And third, we tested with a differential conditioning procedure the ants' capacity to learn to associate a given coarseness with a food reward. The locomotory behaviour of ants differed according to substrate coarseness: ants moved significantly faster and had more sinuous trajectories on a fine than on a coarse substrate. No spontaneous preference for a substrate of a given coarseness was observed and, even after 20 successive conditioning trials, there was little evidence of the effect of experience on substrate coarseness discrimination. Overall however, ants trained on fine sand made significantly more correct choice than those trained on coarse sand. We discuss these results and argue that in L. niger substrate coarseness may be more important at the collective level, by interacting with the chemical properties of the pheromone trail used in mass recruitment to food source, than at the individual level.     

Changes in Odour Intermittency Influence the Success and Search Behaviour During Orientation in the Crayfish (Orconectes Rusticus)

Aquatic organisms may use different mechanisms to locate an odour source, including odour-gated rheotaxis and chemotaxis. However, the effects of spatial and temporal information contained within odour signals in guiding orientation are under debate. In nature, turbulence and plume meandering give rise to signal intermittency, which organisms must overcome to locate an odour source successfully. In this study, crayfish were presented with odour plumes formed by a continuously released jet or a pulsed jet ranging from 0.5 to 3 Hz. Crayfish oriented significantly differently in pulsed odour plumes differed significantly from than in a continuously released jet plume. In a continuous odour plumes, crayfish walked faster, had more accurate heading and turning angles, and stopped less often. The results suggest that crayfish are able to orient in a pulsed odour plumes, but that the spatial and temporal complexity of a continuous released odour plume allowed them to be more successful to find the odour source.     

Influence of peat substrate on the distribution and behaviour patterns of sand shrimp, Crangon septemspinosa, under experimental conditions

Peat harvesting is a lucrative industry in New Brunswick, Canada, and is for the most part located along the coast. Current methods of mining peatlands are such that high levels of peat fibers are transported by runoff into fresh and marine waters. To understand better the influence of peat substrate on aquatic organisms living in peat‐impacted habitats, the sand shrimp (Crangon septemspinosa) was used as a bioindicator. A series of laboratory experiments was conducted in which shrimp were provided with a choice between sand and peat. Long‐term studies on starved shrimp confirmed that whereas foraging activities took place on both substrates, shrimp display an overall preference for sand (73% of all shrimp observed were on sand, n=60). When food was deposited on each of the substrates, shrimp generally opted to feed on sand (75% of all shrimp observed were on sand, n=60). After all food on sand was consumed, shrimp tended to move to peat. This was followed by a short period of foraging on both sand and peat with an eventual return to sand in most instances. The presence of food on both substrates did not influence their preference for sand. However, shrimp will move onto a peat substrate to forage if food is present only there. This observation suggests that, although there is a marked preference for sand, peat is not completely repellent to them. Finally, histological investigations of the shrimp digestive system revealed that peat was ingested with food particles and processed in the gastric mill. There was, however, no discernible discrimination between the two substances during ingestion. Finally, starved shrimp on peat substrate did not ingest peat fibers, suggesting that peat is not perceived as an alternative food source.     

Interaction of Press and Pulse Disturbance on Crayfish Populations: Flood Impacts in Pasture and Forest Streams

Interactions between press and pulse disturbances can significantly impact benthic macroinvertebrate (crayfish) populations. Press disturbances such as land-use change can make crayfish more vulnerable to the detrimental effects of pulse disturbances, by changing the habitats available to crayfish. The impact of a pulse disturbance, a major flood (1 in 28 years return period), on crayfish (Paranephrops planifrons) was significantly greater in pasture than native forest streams. Population densities of crayfish in three forest and three pasture streams had been measured seasonally for 2 years prior to the flood and many crayfish had been marked with individually identifiable tags. Low numbers of marked crayfish were recaptured after the flood in the forest streams, but no marked crayfish were identified in any pasture stream. Crayfish densities in one pasture stream declined from an average of 5 m^–2 prior to the flood to <1 m^–2 soon after the flood and it took 3 years for the population to show evidence of recovery. Macrophytes and cobbles, the dominant habitats of crayfish in pasture streams, did not appear to provide stable refugia during the flood. Habitat stability was linked to the riparian zone in forest streams where undercut banks, tree roots, and pools were important habitats for crayfish. Frequent pulse disturbance could affect population persistence of refugia-dependent species when the pressure of land-use change affects the stability of habitats, but this may only be evident over long time scales.     

Do introduced crayfish affect benthic fish in stony littoral habitats of large boreal lakes?

Invasive crayfish are spreading rapidly across Europe, where they are replacing the native crayfish species and impacting negatively on some other biota. Freshwater crayfish and many benthic fishes share similar habitat and food requirements and hence potentially compete for resources. In this study, we investigated impacts of the introduced signal crayfish (Pacifastacus leniusculus) on fish in stony littoral habitats of two large boreal lakes. We compared the littoral fish community composition and the densities of two common benthic fish species between sites with and without crayfish. To evaluate whether signal crayfish share the same food resources as benthic littoral fish or change their feeding habits, we used mixing models and trophic niche estimates based on analyses of stable isotopes of carbon and nitrogen. Both the community composition of littoral fish and the densities of benthic fish species were similar at sites with and without signal crayfish. Even though stable isotope signatures indicated strong dietary overlap between crayfish and benthic fish, the use of food sources and trophic niche widths of fish were not noticeably different between crayfish sites and non-crayfish sites. Our results suggest that, at current densities, the non-native signal crayfish does not have significant impacts on benthic fish in the stony littoral habitats of large boreal lakes.     

Habitat characteristics of crayfish (<Emphasis Type="Italic">Paranephrops planifrons</Emphasis>) in New Zealand streams using generalised additive models (GAMs)

Quantitative data on the habitat characteristics of stream crayfish have been generally lacking and competing demands on water resources has created a need to address this knowledge gap. We investigated day-time habitat relationships of stream crayfish (Paranephrops planifrons White) from 793 quadrats at 30 rivers and streams in the North Island, New Zealand to develop models of koura presence–absence and abundance. The model (stepwise GAM) included width, cover, median substrate size, edge location, velocity and depth, and correctly predicted presence–absence of crayfish (8–39 mm OCL) at 73.4% of quadrats and of young-of-the-year (YOY) ≤8 mm OCL at 83.4% of quadrats. Streams ranged from 1.6 to 11.5 m in width and the probability of finding both crayfish size classes reduced sharply as streams became wider than 6 m and as the substrate became large (i.e., boulder > 256 mm). Crayfish, particularly YOY, were most likely to be found in association with cover and at the stream edge. YOY were associated with shallow depths and fine substrates, whereas larger crayfish showed a preference for cobble substrate. Undercut banks, leaf litter, tree roots, and woody debris were strongly related to the presence–absence of crayfish. The model for crayfish abundance (log-linear Poisson GAM) explained 50% of the variation between quadrats with cover, velocity, edge location, depth, and the overall crayfish abundance at each particular stream being significant variables. Highest crayfish numbers were recorded in still or slow flowing water, with the majority occurring where velocities were below 0.4 m/s. Water depths up to 0.7 m were sampled, but highest numbers were found in depths of 0.2–0.3 m. Our presence–absence model determined variables that were significant over all streams, whereas our abundance model determined variables that were significant within streams. Use of the GAMs models enabled us to untangle the multiple factors contributing to habitat selection. Cover, velocity, and locations at the stream edge were important determinants of both presence–absence and abundance. Generally, substrate was important when comparing between streams, but not within streams, whereas depth was a significant determinant of abundance within streams, but not presence–absence between streams. Handling editor: K. Martens     

Crayfish as geomorphic agents and ecosystem engineers: effect of a biomass gradient on baseflow and flood-induced transport of gravel and sand in experimental streams

SUMMARY 1. Using experimental streams, we studied the impact of the crayfish Orconectes limosus on (i) the transport of gravel and sand at baseflow; (ii) the sediment surface (bedform, particle consolidation, proportion of sand, algal and gravel cover); and (iii) the critical shear stress (τ_C) causing incipient gravel and sand motion during simulated floods. We examined (i) and (ii) in experimental outdoor flumes that replicated riffle‐pool sequences and (iii) in a larger laboratory flume, in which we exposed sediments retrieved from the outdoor flumes to a progressively increasing discharge. 2. Habitat changes induced by crayfish, such as bedform alterations in riffles (downstream displacement of riffle heads) and the increase of gravel on sand dunes in pools, had major impacts on the spatial and temporal patterns of the baseflow transport of gravel and sand. 3. In addition to their impact on bedform in riffles and on gravel cover in pools, crayfish prevented the physical consolidation of particles in riffles and reduced the algal cover and the proportion of sand in the surface layer in both riffles and pools. These crayfish impacts on sediment surface variables had complex, interacting effects on the mobilisation of gravel and sand during subsequent flood simulations. For sand, crayfish progressively decreased the τ_C (i.e. the sum of bedform drag and skin friction) by about 50% along the entire biomass gradient in pools, whereas the presence of crayfish abruptly decreased the τ_C by about 75% in riffles. For gravel, the discharge causing motion in riffles produced a shear stress (in terms of skin friction) on an even bedform that was about 75% lower in all flumes with crayfish compared with the flumes without crayfish. Crayfish had no impact on τ_C for gravel in pools. 4. Scaling‐up these experimental results to real streams suggests that crayfish could affect the patch dynamics of major sediment transport events and habitat suitability for other organisms that, at larger spatial scales, could increase the overall spatio‐temporal habitat diversity and thus the overall structural and functional biodiversity of lotic communities.     

Non‐lethal predator effects on the performance of a native and an exotic crayfish species

1. I tested the hypothesis that the potential for non‐lethal effects of predators are more important for overall performance of the fast‐growing exotic signal crayfish (Pacifastacus leniusculus Dana) than for the slower growing native noble crayfish (Astacus astacus L.). I further tested if omnivorous crayfish switched to feed on less risky food sources in the presence of predators, a behaviour that could reduce the feeding costs associated with predator avoidance. 2. In a 2 month long outdoor pool experiment, I measured behaviour, survival, cheliped loss, growth, and food consumption in juvenile noble or signal crayfish in pools with either a caged predatory dragonfly larvae (Aeshna sp.), a planktivorous fish that do not feed on crayfish (sunbleak, Leucaspius delineatus Heckel), or predator‐free controls. Crayfish had access to multiple food sources: live zooplankton, detritus and periphyton. Frozen chironomid larvae were also supplied ad libitum outside crayfish refuges, simulating food in a risky habitat. 3. Crayfish were mainly active during hours of darkness, with signal crayfish spending significantly more time outside refuges than noble crayfish. The proportion of crayfish outside refuges varied between crayfish species, time and predator treatment, with signal crayfish spending more time in refuges at night in the presence of fish. 4. Survival in noble crayfish was higher than in signal crayfish, and signal crayfish had a higher frequency of lost chelipeds, indicating a high level of intraspecific interactions. Crayfish survival was not affected by the presence of predators. 5. Gut‐contents analysis and stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) indicated that the two crayfish species had similar food preferences, and that crayfish received most of their energy from feeding on invertebrates (e.g. chironomid larvae), although detritus was the most frequent food item in their guts. Signal crayfish guts were more full than those of noble crayfish, but signal crayfish in pools with fish contained significantly less food and fewer had consumed chironomids compared with predator‐free controls. Length increase of signal crayfish (35%) was significantly higher than of noble crayfish (20%), but signal crayfish in pools with fish grew less than in control pools. 6. This short‐term study indicates that fish species that do not pose a lethal threat to an organism may indirectly cause reductions in growth by affecting behaviour and feeding. This may occur even though prey are omnivorous and have access to and consume multiple food sources. These non‐lethal effects of predators are expected to be particularly important in exotic crayfish species that show a general response to fish, have high individual growth rates, and when their feeding on the most profitable food source is reduced.     

Habitat use by endangered Japanese crayfish (<Emphasis Type="Italic">Cambaroides japonicus</Emphasis>) in low-gradient streams of southern Hokkaido,Japan: reach and microhabitat-scale analysis

The Japanese crayfish (Cambaroides japonicus), the only native crayfish in Japan, is endangered and has experienced rapid population declines. We surveyed the habitat requirements of Japanese crayfish at the reach and microhabitat scales in semi-natural low-gradient streams. Habitat use by Japanese crayfish differed between the spatial scales. Reach-scale analysis revealed that the bed slope was the only positive predictor of crayfish density. This finding indicates that relatively high-gradient reaches, such as headwater reaches, are an important habitat for the conservation of Japanese crayfish in low-gradient streams. Microhabitat-scale analysis showed that crayfish density was positively affected by substrate coarseness and the presence of instream vegetation cover (bank vegetation, woody debris, and leaf patches), whereas it was negatively affected by distance from the stream edge. Coarse substrates and vegetation cover may function as shelters from water flows and predators during low flow periods. The use of stream-edge areas may allow quick access to refugia and enable the crayfish to avoid unexpected flood disturbance and predation. These habitat characteristics should therefore be preserved for the conservation of Japanese crayfish, and scale-dependent habitat characteristics should be considered in future conservation plans.     

The Use of Artificial Benthic Collectors for Assessment of Spatial Patterns of Settlement of Megalopae of Carcinus maenas (L.) (Brachyura: Portunidae) in the Lower Mira Estuary, Portugal

Artificial benthic collectors have been widely used for the assessment of settlement rates of decapod crustaceans. However, to date no consistent works have addressed spatial patterns of settlement in different estuarine habitats, and no specific studies targeted the interaction of artificial surfaces with the surrounding natural substrate. It may be expected that the artificial surface may produce a different thigmotactic response when compared to the natural substrate, which may limit the use of this technique for assessment of natural settlement rates. In this study the settlement rates of megalopae of the estuarine crab Carcinus maenas were addressed, specifically deploying artificial benthic collectors in different habitats both intertidal and subtidal in the lower Mira estuary. A number of experiments were performed concerning stratification and temporal fluctuations of settlement. Further, the interaction of collector surface with the surrounding substrate was investigated, by comparing settlement rates in natural and artificial substrates in different habitats. Results have shown significant differences in settlement between different estuarine habitats, both in spatially replicated experiments and in a high-resolution temporal experiment. However, comparison between settlement rates in artificial and natural substrates has shown that there is a strong interference between collectors and surrounding substrate, limiting interpretation of results concerning settlement rates in artificial substrate alone.     

Chemical cues modify species interactions: the ecological consequences of predator avoidance by freshwater snails

Chemical signals released by predators or injured prey often induce shifts in the traits of prey species, which may in turn affect species interactions. Here we investigate the role that chemical cues play in mediating species interactions in the littoral food web of lakes. Previous studies have shown that predators induce shifts in the morphology, life history, and behavior of the freshwater snail Physella, but the ecological consequences of developing these inducible defenses are not well documented. We observed habitat use of the freshwater snail Physella gyrina along a depth gradient in a natural lake, and found they increased their use of covered habitats with increasing depth. We hypothesized that this habitat shift was due to changes in the level and type of predation risk, and that the habitat shift would affect periphyton standing crops. These hypotheses were tested in a mesocosm experiment in which we manipulated the presence of molluscivorous fish and crayfish. Predators were confined to cages and snail density was identical in all treatments, so any effects of predators were mediated through trait shifts induced by chemical cues. In the presence of fish, Physella moved under cover, but in the presence of crayfish, Physella avoided cover and moved to the water surface. These non‐lethal effects of predators on snail habitat use influenced the interaction between snails and their periphyton resources. In the presence of fish, periphyton standing crop in covered habitats was reduced to just 8% of periphyton in the absence of fish. Crayfish had no significant effect on periphyton in covered habitats, but they reduced periphyton in near‐surface habitats to 39% of the standing crop in the absence of crayfish. The combined effects of fish and crayfish were generally intermediate to their individual effects. We conclude that because chemical cues often have strong effects on individual traits and trophic interactions are sensitive to trait values, chemical cues may play an important role in shaping the structure and dynamics of food webs.     

VARIATION IN DIATOM COMMUNITY STRUCTURE AMONG HABITATS IN SANDY STREAMS1

Community structure of benthic diatoms in two sandy streams was studied for two summers to assess differences in abundances and species composition among habitats and between years and streams. The greatest differences in abundances (cells·cm^?2) were lower abundances on rocks than on sand, whether the sand was clean or covered with flocculent organic material. Relatively little variation in abundances occurred between years and streams. Species composition of diatom communities varied more among habitats and between streams than from year to year. Species composition was most unique in floc-covered sand when communities in clean and floc-covered sand, rock, plant, and plankton were compared. Diatom species composition in these sandy streams was most similar on sand and rocks.     

The influence of productivity and width of littoral zone on the trophic position of a large-bodied omnivore

Omnivory is common in many food webs. Omnivores in different habitats can potentially change their feeding behaviour and alter their trophic position and role according to habitat conditions. Here we examine the trophic level and diet of the omnivorous signal crayfish (Pacifastacus leniusculus) in gradients of trophic status and lake size, both of which have been previously suggested to affect trophic position of predators separately or combined as productive space. We found the trophic position of omnivorous crayfish to be positively correlated with lake trophic status, but found no evidence for any influence of lake size or productive space on crayfish trophic position. The higher trophic position of crayfish in eutrophic lakes was largely caused by a shift in crayfish diet and not by an increase in trophic links in basal parts of the food web. Hence, our results support the “productivity hypothesis,” suggesting that food chains can be longer in more productive systems. Furthermore, stable isotope data indicated that larger crayfish are more predatory than smaller crayfish in lakes with wider littoral zones. Wider littoral zones promoted the development of intrapopulation differences in trophic position whereas narrow littoral zones did not. Hence, differences in habitat quality between and within lakes seem to influence the trophic positions of omnivorous crayfish. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.