Competition between two congeneric stream gobies for habitat in southwestern Shikoku, Japan

Competition between two congeneric gobies, Rhinogobius sp. LD (large-dark type) and CB (cross-band type), for habitat was studied in a tributary of the Shimanto River, southwestern Shikoku, Japan. Habitat use by CB, measured by water depth, current velocity, and substrate, was compared between before and after the removal of LD. After the removal of LD, CB shifted their habitat use to coarser substrates than previously, suggesting a release from effects of LD. This result supports a hypothesis from our previous study that habitat partitioning between CB and LD is a consequence of interference effects of LD on CB.     

Habitat use and feeding behavior of thirteen species of benthic stream fishes

Synopsis The densities, habitat use, and feeding behaviors of 13 fish species belonging to the benthic invertebrate-feeding guild were studied by snorkeling at three localities in the Little River of eastern Tennessee, U.S.A. Resource partitioning occurred by habitat, feeding behavior and time of activity. Differences were also found at the generic level.Cottus was a nocturnal feeder, whereasPercina andEtheostoma were, for the most part, diurnally active.Percina moved about rapidly and spent most of its time above the bottom. In contrast,Etheostoma varied considerably in the amount of time spent under cover, spent little time above the bottom, and exhibited low levels of swimming activity. Nearly all species sought cover at night, suggesting they may be particularly sensitive to predation at night. Species with small adult sizes (Etheostoma, Cottus andP. evides) were concentrated in shallow water habitats, whereas species with large adult sizes (Percina) were more abundant in deep water habitats. The habitat use data are consistent with the hypothesis that size-selective predation by centrarchid bass may cause smaller fish to avoid deep water areas. Large species should have a lower risk of predation due to their size and behavior.     

Patterns of food and microhabitat resource use by two benthic gobiid fishes

The mechanisms responsible for different patterns of habitat use by two benthic gobiid fishes, Acentrogobius sp. 1 and A. sp. 2, which displayed identical food use but resided in shallow and deep zones of coarse and fine sediments, respectively, in Lake Hamana, Shizuoka Prefecture, Japan, were investigated by field removal and laboratory sediment-grain size selection experimentation. Following field manipulation, involving removal of both species, the distribution patterns of each were found to be similar in both control and manipulated quadrats, suggesting their differential habitat use patterns may be the outcome not of competition but of differing preferences for habitat characteristics. Results of a sediment-grain size selection experiment in the laboratory suggested a weak preference of A. sp. 2 for fine sediment, while A. sp. 1 showed no grain size preference. The observed difference in the distribution patterns between these two species, therefore, may possibly have resulted from, at least in part, a combination of differences in their preference for sediment grain size and other habitat characteristics such as water depth.     

Comparison of current-velocity tolerance among six stream gobies of the genus Rhinogobius

Tolerance to current velocity was compared among six stream gobies of the genus Rhinogobius (cross-band, dark, large-dark, cobalt, and orange types, and R. flumineus). Each individual of the six gobies was exposed to incremental increases in current velocity within a laboratory flume. The current velocity at which each individual was dislodged was recorded and compared by species. Results from comparisons showed that the tolerance of gobies declined in the following order: cobalt, large-dark, dark, cross-band, R. flumineus, and orange type. This interspecific difference in velocity tolerance is consistent with their habitat segregation patterns as previously reported by field observations.     

Foraging behaviour in fishes: perspectives on variance

Synopsis The positive relationship between size of prey and frequency of ingestion by predators has been a focal point of investigations in foraging ecology. Field studies compare the frequency distribution of prey sizes in the predator's gut with that in the environment. Laboratory and field (enclosure) studies are based upon comparison of the frequency distributions of prey sizes in controlled environments, before and after the introduction of a predator. Optimal caloric return for foraging effort (i.e. the theory of optimal foraging) has been widely used as a guiding principle in attempts to explain what a fish consumes. There is a body of information, however, which seems to indicate that the perceptual potentialities and cognitive abilities of a predator can account for both the direction of the prey size versus ingestion frequency relationship and the variance surrounding it. Part of this variance may be evidence of systematic ambiguity, a property of cognitive skills causing predators to respond to the same stimulus in different ways and to different stimuli in the same way. More extensive examination of cognitive skills (minimally defined as learning, remembering and forgetting) in fish may permit causal interpretations (immediate and ultimate) of variance in predatory skills. In such a paradigm of foraging behaviour, environmental stimulus is not taken as the predator's object of response (percept); a cognitive representation connects mind to stimulus and this is the criterion for the act of perception. Cognition, here considered as a formal system which acts upon representations, connects mind to response and thus to adaptation. Studies of the relationships among rates of learning, long and short-term memory, rates of forgetting, prey behavior, size and population turnover rates, lateralization of brain functions, diel fluctuations in predator activity levels and sleep, experience, and critical periods in the development of the predator's nervous system should be examined in relation to foraging behaviour.     

A comparison of the benthic macroinvertebrate communities in a trout stream: Winter and spring 1973 and 1977

The benthic macroinvertebrate riffle community of the Bushkill Creek (Northampton County, PA) was sampled monthly, December through May 1972–73 and 1976–77. Total precipitation in 1972–73 was 22.3 cm more than in 1976–77 and water and air temperatures were also higher. In April and May, community composition (dominant taxa and similarity indices) was less similar than in December, January and February. In April and May there were fewer Hydropsyche (Trichoptera) and Ephemerella (Ephmeroptera) and more Chironomidae (Diptera) and Antocha (Diptera) than in 1973. Total numbers and biomass were lower in 1977 than in 1973. The reduction in Hydropsyche, Ephemerella, biomass and total numbers in Spring 1977 may be due to the scouring effect of ice break up. Inspite of fluctuating environmental conditions the macroinvertebrate community exhibited stability.     

Convergence of a cryptic saddle pattern in benthic freshwater fishes

Synopsis Many North American stream fishes have a similar color pattern of four dark saddles against a light background. An interesting feature of the pattern, in addition to its widespread taxonomic distribution, is its consistent configuration. The interval between the first and second saddle is usually the largest, and the last (third) interval is the smallest. All saddled North American freshwater fishes live on uneven, rocky substrates, and nearly all live in flowing water. It is hypothesized that these fishes achieve crypsis through disruptive coloration; the light spaces between the saddles mimic rocks and the dark saddles appear as shadows or gaps between rocks. Saddles are spaced unevenly because rocks in streams are a mixture of sizes; a fish that mimics a series of rocks of similar sizes is more conspicuous than one that mimics rocks of different sizes. The placement of saddles was measured on five North American species. In four of five North American species measured (a sculpin and three darters), the longest spaces are towards the head where the body is also the widest, this is thought to enhance crypsis because pieces of gravel tend to be round or square. In the madtom, the saddle pattern tends more towards even spacing. The madtom may not rely on camouflage to the same extent as other species examined because of decreased predation pressure associated with being nocturnal and possessing sharp spines and venom glands.     

The prevalence of non-indigenous species in southern California embayments and their effects on benthic macroinvertebrate communities

The prevalence of non-indigenous species (NIS) in southern California embayments was assessed from 123 Van Veen grab samples collected in nine bays and harbors during the summer of 1998. NIS occurred in all but two samples. They accounted for only 4.3% of the 633 taxa but contributed 27.5% of the abundance. There was no significant difference in the proportion of NIS abundance among ports harboring large vessels, small boat marinas, and areas where boats were not moored. Three species accounted for 92% of the NIS abundance: a spionid polychaete worm Pseudopolydora paucibranchiata, a mytilid bivalve Musculista senhousia, and a semelid bivalve Theora ubrica. The NIS did not appear to have a negative impact at the overall community level since NIS abundance was positively correlated with the abundance and richness of other species. This may be due to biogenic structures built by P. paucibranchiata and M. senhousia that enhance the abundances of other macrofauna.     

The effects of predation on the survival and size-distribution of estuarine fishes: an experimental approach

Synopsis The effect of southern flounder,Paralichthys lethostigma, predation on the survival and size-distribution of spot,Leiostomus xanthurus, was investigated using feeding experiments in outdoor tanks and estuarine ponds. In the outdoor tank experiments, southern flounder, 143 mm average standard length (SL), fed for 48 h on spot (25–30 mm SL), over a range of densities (4–20 m^–2). In these trials, flounder showed a Type II functional response to spot density. In the estuarine pond experiment, flounder density (2 or 4 flounders 70 m^–2) contributed significantly to spot mortality relative to predator-free controls. The effect of flounder density on spot mortality was non-linear; in the 4-flounder treatment the daily instantaneous mortality due to flounder (0.0177) was 3.5 times that in the 2-flounder treatment (0.0051). The total spot mortality rate in the 4-flounder treatment (0.028) was similar to that observed from published field observations. Flounder also altered the size distribution of spot relative to predator-free controls. Predation by flounder resulted in fewer intermediate-sized spot at the end of the experiment when compared to predator-free controls. Flounder also significantly reduced survival and mean length of two killifishes,Cyprinodon variegatus andFundulus heteroclitus, that invaded the pond. Flounder predation was a significant size-structuring force on the prey fish assemblage in the pond. Results from both the outdoor tank experiment and the pond experiment indicate that flounder may represent a significant source of mortality for juvenile estuarine fishes.     

Density-dependent predation ofChaoborus flavicans onDaphnia longispina in a small lake: the effect of prey size

Functional response curves of fourth instar larvae ofChaoborus flavicans preying on two size classes ofDaphnia longispina were examined throughout three summer seasons in a small forest lake. Data for each size class were fitted to Holling's disc equation. The parametersa (attack rate) andTh (handling time) were calculated for each prey size from these curves. Attack rate was greater and handling time was shorter for small (0.77 mm) than for large (1.82 mm)Daphnia. In 1:1 mixture of these prey size classes the predation rates ofChaoborus on smallDaphnia at prey densities above 20 l^–1 were greater than predicted from the single size-class experiments. The observed predation rates on largeDaphnia were lower than predicted at all prey densities. Since both single size-class and two size-class experiments were run during the same period of time the difference in observed and predicted predation rates could not be attributed to seasonal changes in prey preference ofChaoborus larvae. In experiments with a concentrated mixture of lake zooplankton (dominated byD. longispina)Chaoborus preference forDaphnia decreased as prey body size increased. There was no obvious correlation between selectivity coefficients and size-frequency distributions ofDaphnia. When medium-sizedDaphnia were omitted from calculations the preference of small over large prey did not differ significantly from the predictions of the single size-class model.     

Genetic variability in Plantago species in relation to their ecology

Summary Twelve Plantago major plants, good representatives of their populations, appeared to be genetically different for several characters which are important for adaptation to the respective habitat conditions. These characters are: juvenile growth, leaf morphology, production of secondary rosettes, flowering time, seed production, seed size and adult leaf production. The adaptive value of some of these characters was investigated by transplantation experiments in the field and by intraspecific competition experiments. The roadside type of ssp. major was adapted to trampling by being erect and elastic. The lawn type of ssp. major was adapted to a short, frequently cut, vegetation by being prostrate and by producing leaves with short petioles throughout the growth season. In the natural situations in which ssp. pleiosperma occurs, growth rate and first-year seed production of this subspecies were considerably higher than that of ssp. major. In a number of experiments, F₁s and F₂s were included, derived from crosses between the original plants. The F₁s were generally rather well adapted to both parental habitats, whereas the F₂s appeared to be less fit. The various alternatives in spending resources relevant for fitness optimization in different habitats are discussed.Grassland Species Research Group Publication No. 93     

Variation in plant stem density and its effects on foraging success of juvenile bluegill sunfish

Synopsis Juvenile bluegill sunfish, Lepomis macrochirus, are known to use beds of aquatic vegetation as a refuge from predators. This study examines the effects of increasing plant stem density on juvenile bluegill foraging. Three stem densities (100, 250 and 500 stems m⁻²), varying in their refuge potential for bluegills from predators, were tested. Results demonstrate that stem densities chosen as a refuge from predation (i.e. 500 stems m⁻²) significantly reduced bluegill foraging success and increased time required to capture prey. Therefore, juvenile bluegills seeking safety in vegetation may be faced with a trade-off between foraging success and effective refuge from predation when choosing among plant stem densities.     

Short-term cues used by foraging trout in a California stream

Stream salmonids choose foraging locations to maximize the energy benefit of foraging within the constraints of size-mediated dominance hierarchies and predation risk. But, because stream habitats are temporally variable, fish must use a search process to monitor changing habitat conditions as a means of locating potentially-better foraging locations. I explored the cues used by the cutthroat trout, Oncorhynchus clarki clarki, when searching for food at the pool scale by artificially increasing prey availability at different locations by using special feeders and by manipulating pool velocities. Behavior of individually marked fish was monitored from stream bank platforms under unmanipulated control conditions and under seven experimental sets of conditions involving different combinations of feeder location and velocity manipulation. Under natural conditions fish elected to forage in the deepest (>50 cm), fastest (0.10–0.25 m s⁻¹) locations and within 1 m of structure cover, but would readily move to shallower (<30 cm) water away from cover if velocities were manipulated to be highest there. Although fish did not locate feeders unless they were placed in high-velocity areas, when high velocity was provided fish would move into very shallow water (<20 cm) if prey were delivered there. Responses of individual trout to manipulations indicated that water velocity was the main physical cue used by fish to decide where to forage, and that fish could also learn about new food sources by observing conspecifics. Overall, results indicated fish were not “perfect searchers” that could quickly locate new food resources over short time scales, even when the new resources were within a few meters of the fish’s normal foraging location. When given the correct cues, however, fish could detect new food sources and defend them against subordinate fish. Movement of new fish into and out of the study pools during the ten-day observation period was common, consistent with the idea that trout used movement as a means of exploring and learning about habitat conditions at the reach scale.     

Trophic relationships and seasonal utilization of salt-marsh creeks by zooplanktivorous fishes

Synopsis In a high salinity estuary at North Inlet, South Carolina, co-occurrence and possible competition among adults of four dominant zooplanktivorous fishes were minimized by seasonal adjustments in lateral and vertical distributions as well as in dietary preferences. In winter, Atlantic silversides, Menidia menidia, occupied the entire water column while other planktivores were rare or absent from the estuary, and they consumed large prey such as mysid shrimps and fish larvae. An immigration of bay anchovies, Anchoa mitchilli, in the spring resulted in a redistribution of species with Atlantic silversides shifting to the surface waters and bay anchovies dominating the lower half of the water column. Both fishes consumed mostly copepods in the spring, but each favored a different species. There was little similarity in the large prey items consumed by the two fishes. Striped anchovies, Anchoa hepsetus, arrived in mid-summer and were most abundant at the surface while bay anchovies continued to dominate the bottom waters. Atlantic silversides were rare in all summer collections. The diets of the two anchovies were similar, but vertical separation during the period of maximum zooplankton abundance probably minimized competition. Rough silversides, Membras martinica, which were obligate surface dwellers, shared the upper water column with striped anchovies, but the two species had very different diets during their period of co-occurrence. Although seasonal changes in fish diets reflected shifts in zooplankton composition and all fishes consumed a variety of prey types, preferences for some prey taxa and total avoidance of others were indicated. Electivity indices indicated an especially strong selection for fiddler crab megalopae by all fishes in the summer and fall. All fishes, except rough silversides, which fed almost exclusively on copepods and crab zoeae, consumed large prey items when they were available. Fine scale partitioning of the food resources was apparent in the selection of different copepod and insect species by the fishes. Spatial and temporal separation in the distribution and/or dietary preferences of the zooplanktivores fishes probably reduces the potential for resource competition. Given the high abundances and selectivity of the planktivores, significant impacts on some zooplankton populations probably result.     

Habitat requirements of weasels <Emphasis Type="Italic">Mustela nivalis</Emphasis> constrain their impact on prey populations in complex ecosystems of the temperate zone

Differences in habitat use by prey and predator may lead to a shift of occupied niches and affect dynamics of their populations. The weasel Mustela nivalis specializes in hunting rodents, therefore habitat preferences of this predator may have important consequences for the population dynamics of its prey. We investigated habitat selection by weasels in the Bia?owie?a Forest in different seasons at the landscape and local scales, and evaluated possible consequences for the population dynamics of their prey. At the landscape scale, weasels preferred open habitats (both dry and wet) and avoided forest. In open areas they selected habitats with higher prey abundance, except during the low-density phase of the vole cycle, when the distribution of these predators was more uniform. Also in winter, the distribution of weasels at the landscape scale was proportional to available resources. In summer, within open dry and wet habitats, weasels preferred areas characterised by dense vegetation, but avoided poor plant cover. In winter, weasels used wet open areas proportionally to availability of habitats when hunting, but in contrast to summer, they rested only in habitats characterized by a lower water level, which offered better thermal conditions. At the local scale, the abundance of voles was a less important factor affecting the distribution of these predators. Although we were not able to provide direct evidence for the existence of refuges for voles, our results show that they may be located within habitat patches, where availability of dense plant cover and physiological constraints limit the activity of weasels. Our results indicate that in complex ecosystems of the temperate zone, characterized by a mosaic pattern of vegetation types and habitat specific dynamics of rodents, impact of weasels on prey populations might be limited.     

Pacific salmon effects on stream ecosystems: a quantitative synthesis

Pacific salmon (Oncorhynchus spp.) disturb sediments and fertilize streams with marine-derived nutrients during their annual spawning runs, leading researchers to classify these fish as ecosystem engineers and providers of resource subsidies. While these processes strongly influence the structure and function of salmon streams, the magnitude of salmon influence varies widely across studies. Here, we use meta-analysis to evaluate potential sources of variability among studies in stream ecosystem responses to salmon. Results obtained from 37 publications that collectively included 79 streams revealed positive, but highly inconsistent, overall effects of salmon on dissolved nutrients, sediment biofilm, macroinvertebrates, resident fish, and isotopic enrichment. Variation in these response variables was commonly influenced by salmon biomass, stream discharge, sediment size, and whether studies used artificial carcass treatments or observed a natural spawning run. Dissolved nutrients were positively related to salmon biomass per unit discharge, and the slope of the relationship for natural runs was five to ten times higher than for carcass additions. Mean effects on ammonium and phosphorus were also greater for natural runs than carcass additions, an effect attributable to excretion by live salmon. In contrast, we observed larger positive effects on benthic macroinvertebrates for carcass additions than for natural runs, likely because disturbance by live salmon was absent. Furthermore, benthic macroinvertebrates and biofilm associated with small sediments (<32 mm) displayed a negative response to salmon while those associated with large sediments (>32 mm) showed a positive response. This comprehensive analysis is the first to quantitatively identify environmental and methodological variables that influence the observed effects of salmon. Identifying sources of variation in salmon–stream interactions is a critical step toward understanding why engineering and subsidy effects vary so dramatically over space and time, and toward developing management strategies that will preserve the ecological integrity of salmon streams. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Feeding-patch choice by red deer in relation to foraging efficiency

We tested the idea that ruminants allocate their feeding time to habitat patches in relation to foraging efficiency. We used five tame red deer (Cervus elaphus) in an enclosure planted with four treatment of timothy grass (Phleum pratense) differing in their stage of growth. Older swards offered higher biomass but lower nutritional quality than younger swards. We observed time spent feeding in each treatment during each of seven trials. We measured goodness-of-fit between observed times and predictions from two alternative hypotheses differing in optimization strategy (maximizing versus matching), and a third, null hypothesis. We tested the hypotheses using two alternative currecies: digestible protein, and digestible dry matter or energy. Although digestible protein concentration and dry-matter digestibility were highly correlated (r=0.763, P<0.001), the wider range of digestible protein made it the much more sensitive measure of forage quality. Distributions of feeding time closely matched estimated intake rates of digestible protein (R _infPred ^sup2 =0.899) across all animals and trials. The other hypotheses were rejected. The results have important ecological implications in showing the underlying role of food in the selection of habitat by ruminants, and that simple, mechanistic models of forage intake and digestion can be scaled up to the level of animal behavioural choices.     

The effects of seaweed harvesting on fishes: a critique

Synopsis Black & Miller (1991) concluded that there was no large impact of an experimental harvest of rockweed,Ascophyllum nodosum, on fishes. A critique of their study demonstrates that this conclusion cannot be substantiated by their data because of sampling biases, errors in experimental design and low statistical power. Further, evidence is presented which supports the hypothesis that rockweed may provide an important nursery habitat for juvenile fishes.     

Searching behaviour and cereal aphid consumption by Bembidion lampros and Pterostichus cupreus, in relation to temperature and prey density

Maximum consumption rates were determined for two carabids, Bembidion lampros Herbst. and Pterostichus cupreus L., feeding on the cereal aphid Rhopalosiphum padi L. at different temperatures in the laboratory. Mean daily consumption increased with increasing temperature for both species, B. lampros consuming a maximum of 16 1–3 instar nymphs and 9 apterous adult aphids at 25°C. P. cupreus was particularly voracious and consumed 125 apterous adult R. padi per day at 20°C. The behaviour of both species was analysed by video filming starved beetles, maintained at different constant temperatures, in arenas sown with spring barley. The behavioural components (1) still; (2) run/walk; (3) search and (4) confrontation were identified and were common to both species. P. cupreus was more active over the temperature range tested; B. lampros was inactive under 10°C. The proportion of time spent searching, number of plants searched, and velocity increased with increasing temperature for both species. When observed in similar arenas seeded with R. padi colonies, individuals of P. cupreus significantly increased their time spent searching in arenas with increasing aphid density. Following discovery of an aphid colony, individuals climbed and searched the host plant and its nearest neighbours. Plants in aphid free arenas were rarely climbed. B. lampros was not observed climbing in either aphid free arenas or in arenas with increasing aphid densities, and did not significantly increase its time spent searching in response to increased prey density. The few B. lampros that found aphids caught them walking on the soil surface. The relative efficiences of these two carabids as predators of R. padi are discussed, and the results are compared with similar studies elsewhere with predators of Sitobion avenae on winter wheat.

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Résumé Les taux maximum de consommation de R. padi L. à différentes températures ont été déterminés au laboratoire chez deux carabes, B. lampros Herbstet P. cupreus L. La consommation moyenne a augmenté avec la température chez les deux espèces, B. lampros consommant un maximum de 15,9 larves des stades 1 à 3 et 9,1 pucerons adultes aptères, à 25°C. P. cupreus a été particulièrement vorace et a consommé 125,3 adultes aptères par jour à 20°C. Le comportement des deux espèces a été observé en filmant en vidéo des carabes à jeun, maintenus à différentes températures constantes, dans des enceintes semées en orge de printemps. Des éléments du comportement, communs aux deux espèces, ont été définis: 1) immabilité, 2) marche et course, 3) recherche, 4) affrontement. P. cupreus a été plus actif à toutes les températures, B. lamprosa été inactif au-dessous de 10°C. La part de temps consacrée à la recherche, le nombre de plantes prospectées, et la vitesse ont augmenté avec la température chez les deux espèces. Dans des enceintes similaires colonisées par R. padi, P. cupreus a significativement augmenté le temps consacré à la recherche dans les enceintes, parallèlement à l'augmentation de la densité des pucerons. Après la découverte d'une colonie de pucerons, P. cupreus escalade et prospecte la plante et ses voisines immédiates; tandis que les plantes des enceintes sans pucerons sont rarement escaladées. B. lampros n'a pas été observé escaladant des plantes d'enceintes avec ou sans pucerons, et il n'a pas accru son temps de prospection en fonction de la densité de pucerons. Les quelques B. lampros qui ont capturé des pucerons l'ont fait lorsque ceux-ci marchaient sur la surface du sol. La discussion a porté sur l'efficacité relative des deux carabes comme prédateurs de R. padi, et les résultats ont été comparés à ceux d'études du même type, menées ailleurs, avec des prédateurs de Sitobion avenae sur blé d'hiver.

     

Mate choice and cannibalism in a natural population of a stream goby,Rhinogobius sp.

Breeding ecology of the stream goby,Rhinogobius sp. LD (Large Dark), was investigated under natural conditions. Males selectively courted females of similar size to lead them to the nests, whereas females followed courting males preferentially when the relative male size was greater. Male-male competition for a female was relatively infrequent and not severe. Developmental stages of eggs and egg numbers in one nest indicated that males receive 1–3 clutches during one breeding cycle. Males guarding multiple clutches frequently ate some of the eggs, but those guarding single clutches rarely did so. Gravid females in the nest also frequently cannibalized eggs laid by a previous female, probably in order to extend the area available for egg deposition. Mate choice in this species is discussed in relation to paternal ability, limitation of available spawning area and the female-biased sex ratio.