Fishless Streams Research Articles

Predator transitions in stream communities: a model and evidence from field studies

The role of predators (particularly top predators such as fish) in structuring stream communities has been debated for 2 decades. Much of the debate may have been caused by the lack of a conceptual framework for evaluating predator effects in stream communities. First, I propose a general conceptual model of the factors (abiotic, such as stream permanence and disturbance regime; biotic, such as predation) that can influence community structure, and the conditions in which these various factors would be expected to be important. Hydrologic permanence and disturbance transitions separate streams where abiotic factors are most important in determining community structure from streams with relatively benign disturbance regimes where predation may be more important. Second, I focus on the potential effect of predators in perennial streams with relatively benign disturbance regimes. Such streams are divided longitudinally into sections where different types of predators might be important in determining community structure. Large invertebrates (stoneflies, dragonflies, shrimp, and crayfish) and salamanders may be the dominant benthic predators affecting species composition in small perennial fishless streams. A transition from invertebrate- and amphibian-dominated to fish-dominated systems may occur in larger, downstream sections (predator transition 1). In addition, longitudinal transitions in fish-assemblage structure from upstream tributaries to downstream main-channel fish assemblages (predator transition 2) may affect community structure. I present evidence supporting the above model and suggest experimental approaches to test the model. This conceptual framework may help in understanding the role of specific predators in determining prey distributions in many stream communities.

Indirect effects of predatory trout on organic matter processing in detritus‐based stream food webs

Indirect effects of predators on basal resources in allochthonous‐based food webs are poorly understood. We investigated indirect effects of predatory brown trout (Salmo trutta) on detritus dynamics in southern beech (Nothofagus spp.) forest streams in New Zealand through predation on the obligate detritivore, Zelandopsyche ingens (Trichoptera, Oeconesidae). Trout presence/absence and Z. ingens density were manipulated in flow‐through tanks to investigate the lethal and sub‐lethal effects of trout on litter processing by Z. ingens. An experiment that allowed trout access to Z. ingens showed trout predation reduced densities of Z. ingens resulting in slower breakdown of coarse particulate organic matter (CPOM) and reduced production of fine particulate organic matter (FPOM). A second experiment that prevented trout access to Z. ingens, but allowed the transmission of trout cues, resulted in no change in litter processing rates in the presence of trout. Litter processing rates were higher in high Z. ingens density treatments compared to low density treatments. Thus, trout effects on litter processing were due to reduced Z. ingens densities, not trout‐induced modifications to Z. ingens feeding behaviour. Field assays of litter processing rates using artificial leaf packs in natural streams showed significant reductions in CPOM loss in trout streams compared to fishless streams. Z. ingens dominated biomass in fishless stream leaf packs, but a facultative shredder, Olinga feredayi, dominated trout stream leaf packs. Thus, the absence of Z. ingens drove differences in processing rates between trout and fishless streams and the indirect effects of trout on litter processing observed in mesocosms were evident in complex, natural food webs. Overall our study provides evidence that predators can influence resource dynamics in donor‐controlled food webs through their effects on consumers.

Top‐down and bottom‐up influences on populations of a stream detritivore

Summary1. Knowledge of the influence of predatory fish in detritus‐based stream food webs is poor. We tested whether larval abundance of the New Zealand leaf‐shredding caddisfly,Zelandopsyche ingens(family Oeconesidae), was affected by the presence of predatory brown trout,Salmo truttaand the abundance of their primary detrital resource (Nothofagusleaves).2. The density ofZ. ingensand the biomass of leaves were determined in seven fishless streams and four trout streams in the Cass region, central South Island, on four occasions spanning 5 years.3. Physicochemical conditions were similar in trout and fishless streams, butancovaindicated thatZ. ingensnumbers were positively related to leaf biomass and that caddisfly numbers were significantly greater in fishless streams than trout streams for any given biomass of leaf. The cases of trout stream larvae were also heavier per unit length than those in fishless streams.4. Our results provide evidence for both top‐down and bottom‐up influences on a detritus‐based stream food web. Although stream detritivores may benefit from a habitat that provides both food and a degree of protection from predators, top‐down effects of predators on detritivore population abundance were still important. Thus, detrital resource availability may determine maximum attainable population size, whereas predation is likely to reduce the population to a level below that.

Effects of ambient UV-radiation on the behaviour of mayfly larvae of the genus Deleatidium from trout bearing and fishless streams in New Zealand

New Zealand ground levels of UV-radiation have always been comparatively high, however, little attention has been paid to the effects of UV-radiation on biotic interactions in New Zealand freshwater systems. In two outdoor short term microcosm experiments we tested the effects of UV-radiation on the behaviour of grazing mayfly Deleatidium (Ephemeroptera) larvae. The relationship between pigmentation of the Deleatidium larvae and their UV-related behavioural response was also examined. The first experiment showed that Deleatidium larvae were able to detect and actively avoid ambient levels of UV-radiation. The results of the second experiment suggested that Deleatidium larvae spent more time grazing on exposed surfaces if the UV-radiation was attenuated. There was also an effect of fish, larvae from trout streams were more reluctant to feed on periphytic algae than larvae from fishless streams. Larval pigmentation did not affect the UV-radiation related behavioural response. Although the presence of trout has been considered the main a major force for low daytime grazing in by Deleatidium, the present results suggest that UV-radiation can also be a factor responsible for the avoidanceance of exposed surfaces of stones during daytime.

Are populations of mayflies living in adjacent fish and fishless streams genetically differentiated?

Summary1. Conspecific populations living in habitats with different risks of predation often show phenotypic variation in defensive traits. Traits of two species of mayflies (Baetidae: Baetis bicaudatus and Baetis sp. nov.) differ between populations living in fish and fishless streams in a high altitude drainage basin in western Colorado, U.S.A. We tested for genetic differentiation between mayfly populations in these two habitat types, assuming that lack of genetic differentiation would be consistent with the hypothesis that those traits are phenotypically plastic.2. Previous work has shown that larvae of both species behave differently and undergo different developmental pathways in adjacent fish and fishless streams. These phenotypic differences in behaviour and development have been induced experimentally, suggesting that populations from fishless streams have the genetic capability to respond to fish.3. During summer 2001 we collected Baetis larvae from several fish and fishless streams, and from fish and fishless sections of the same streams. We used allozymes and a fragment of the cytochrome oxidase subunit 1 mitochondrial gene to examine genetic variation of Baetis individuals within and among streams.4. Results showed that genetic variation exists among populations of the same species of Baetis from different streams, but none of that variation was associated with the presence or absence of fish. These data confirm that populations of Baetis living in fish and fishless streams are not genetically distinct, and are consistent with the hypothesis that traits associated with environments of different risk are phenotypically plastic.

PREDATOR-INDUCED RESOURCE HETEROGENEITY IN A STREAM FOOD WEB

Heterogeneous distributions of resources and organisms are characteristic of most ecosystems, but empirical understanding of the causes and consequences of hetero- geneity is limited. We investigated whether predatory fish influenced the heterogeneity (spatial variability) of resources (algae) by modifying the behavior of primary consumers (mayflies). We hypothesized that fish would indirectly increase resource heterogeneity by reducing grazer activity, and that higher densities of grazers would reduce resource het- erogeneity only in the absence of fish. We measured the effects of predator cues (brook trout odor) on grazer behavior (Baetis) and algal heterogeneity in mesocosms (; 1m 2 ) and in simple natural systems (30-m reaches of adjacent fishless streams). In addition, we measured grazer and algal distributions in complex natural streams that varied in grazer density, presence of fish, and physical conditions. Fish odor altered mayfly grazing behavior (measured using a simple behavioral bioassay) and increased algal heterogeneity (measured by Morisita's index at the scale of individual rocks) in mesocosms and manipulated streams. Furthermore, natural streams with higher grazer densities had lower algal heterogeneity, but only if they were fishless. Interestingly, the presence of brook trout decoupled the link between grazer density and algal heterogeneity in natural streams. These observations indicate that release from the threat of predation or increased densities of grazers can homogenize algal resources in fishless streams. We hypothesize that altered foraging in environments with predatory fish, independent of grazer density, led to increased resource patchiness, possibly by allowing the influence of variation in physical characteristics (e.g., flow and substratum) to predominate or by changing grazer microhabitat use. These results support theoretical predictions that factors affecting primary consumer behavior also alter resource heterogeneity.

STAGE-SPECIFIC AND INTERACTIVE EFFECTS OF SEDIMENTATION AND TROUT ON A HEADWATER STREAM SALAMANDER

In species with complex life cycles, stage-specific effects of environmental conditions combine with factors regulating stage-specific recruitment to determine population-level response to habitat disturbance. The abundance of the stream salamander Gyrinophilus porphyriticus (Plethodontidae) is negatively related to both logging-associated sedimentation and brook trout (Salvelinus fontinalis) in headwater streams throughout New Hampshire, USA. To understand the mechanisms underlying these patterns, we investigated stage-specific and interactive effects of sedimentation and brook trout on G. porphyriticus. We conducted quantitative surveys of salamanders, brook trout, and substrate embeddedness in 15 first-order streams and used a controlled experiment to test the direct and interactive effects of these factors on larval growth and survival. G. porphyriticus larvae and adults had opposite patterns of response to sediment and brook trout. Multiple regression analysis of our survey data indicated that abundance of larvae was negatively related to brook trout abundance, but unrelated to substrate embeddedness. In contrast, abundance of adults was primarily related to substrate embeddedness. Consistent with the field pattern of larval abundance, brook trout had a negative effect on growth and survival of larvae in the experiment. However, there was no effect of sediment and no interaction between brook trout and sediment. Larval and adult abundances were not significantly correlated in the study streams, indicative of the independent effects of sedimentation and brook trout on G. porphyriticus populations. These results suggest that adult resistance to fish may facilitate G. porphyriticus coexistence with brook trout, and that larval resistance to sedimentation can buffer populations from extinction in fishless streams impacted by logging. In streams with brook trout, where larval abundances are low, reductions in adult abundance caused by logging impacts may pose a risk to species persistence. Our findings underscore the value of information on species life history, demography, and community ecology in assessing sensitivity to anthropogenic perturbation.

Long-term trends in pH, aluminium and dissolved organic carbon in Scottish fresh waters; implications for brown trout ( Salmo trutta) survival

Over the past 30 years upland streams and lochs in Scotland have shown significant signs of recovery from acidification, particularly in terms of declining concentrations of non-marine sulphate (nm-SO 4). Long-term index sites in central and southwest Scotland reveal a significant decline in the concentration of biologically important components, notably acidity (H +) and labile (toxic) forms of aluminium (Al-L), whilst dissolved organic carbon (DOC) and complexed forms of aluminium (AL-NL) have increased significantly. Although these improvements should increase the probability of brown trout survival, and have resulted in increased acid neutralising capacity (ANC) in fishless streams to values close to current critical limits, there is still a relatively poor correlation between ANC and current fish status. Site to site variability appears to be linked to the relative contribution of (H +), Al-L and DOC within the critical ANC (ANC CRIT) range. It is proposed that ANC CRIT should cover a range of ANC values as a function of Al-L concentration. Based on field studies an empirical range of critical ANC values are suggested which better predict the presence of a sustainable brown trout population.

All creatures great and small: patterns in the stream benthos across a wide range of metazoan body size

SUMMARY1. The whole metazoan community (i.e. including the meiofauna) of an acidic, fishless stream in south‐east England was surveyed over 14 months between March 1999 and April 2000. Invertebrate density, biomass and taxonomic richness were assessed on each sampling occasion in relation to physico‐chemical variables.2. The meiofauna were more numerous and diverse than the macrofauna, while their total biomass occasionally equalled that of the macrofauna.3. The meiofaunal and macrofaunal assemblages appeared to respond to different environmental factors. The meiofauna showed genuine species turnover through the year, while the macrofauna varied less in taxonomic composition though there were substantial variations in density.4. These data suggest that the meiofauna and macrofauna exist at different temporal and spatial scales and perceive their environment with a different ‘grain’.

The influence of predatory fish on mayfly drift: extrapolating from experiments to nature

1. A knowledge of how individual behaviour affects populations in nature is needed to understand many ecologically important processes, such as the dispersal of larval insects in streams. The influence of chemical cues from drift‐feeding fish on the drift dispersal of mayflies has been documented in small experimental channels (i.e. < 3 m), but their influence on dispersal in natural systems (e.g. 30 m stream reaches) is unclear.2. Using surveys in 10 Rocky Mountain streams in Western Colorado we examined whether the effects of predatory brook trout (Salvelinus fontinalis) on mayfly drift, that were apparent in stream‐side channels, could also be detected in natural streams.3. In channel experiments, the drift of Baetis bicaudatus (Baetidae) was more responsive to variation in the concentration of chemical cues from brook trout than that of another mayfly, Epeorus deceptivus (Heptageniidae). The rate of brook trout predation on drifting mayflies of both species in a 2‐m long observation tank was higher during the day (60–75%) but still measurable at night (5–10%). Epeorus individuals released into the water column were more vulnerable to trout predation by both day and night than were Baetis larvae treated similarly.4. Drift of all mayfly taxa in five fishless streams was aperiodic, whereas their drift was nocturnal in five trout streams. The propensity of mayflies to drift was decreased during the day and increased during the night in trout streams compared with fishless streams. In contrast to the channel experiments, fish biomass and density did not alter the nocturnal nature nor magnitude of mayfly drift in natural streams.5. In combination, these results indicate that mayflies respond to subtle differences in concentration of fish cues in experimental channels. However, temporal and spatial variation in fish cues available to mayflies in natural streams may have obscured our ability to detect responses at larger scales.

Impact of introduced trout on Nesameletus mayfly populations measured using electrofishing

Despite the widespread introduction of salmonid fishes for sports fisheries their effects on recipient invertebrate communities are not well understood. This lack of knowledge is associated with the methodological difficulties involved in assessing the effects of predatory fish on stream invertebrate populations. A sampling technique, involving the use of backpack electrofishing equipment, was used to compare the abundance of Nesameletus mayflies in 15 streams varying in the presence of introduced brown trout (Salmo trutta) and native Canterbury galaxias (Galaxias vulgaris). Nesameletus were induced to drift into a downstream net by applying electric current to 2m 2 of stream bed using a small (12 cm diameter) anode. Density estimates for Nesameletus from electrofishing were on average five times higher than those obtained from Surber sampling. Fishless streams and streams with only Canterbury galaxias contained a biomass and density of Nesameletus that were two to three orders of magnitude higher than streams with only brown trout. When Canterbury galaxias and trout were both present, however, densities of Nesameletus were intermediate to those found in trout versus troutless streams. This could be because 'trout only' streams contained larger trout which were more effective predators. Streams with trout were wider, but were otherwise not consistently different from those without trout so comparisons were unlikely to be confounded by physical conditions. The average size of nymphs in fishless streams was significantly smaller because a new generation of nymphs had hatched in fishless streams, but was absent from streams containing fish. The effects of the introduced fish on Nesameletus populations indicate that troutless streams are important habitats for maintaining regional Nesameletus spp. populations and that further range expansions of brown trout could decrease populations of vulnerable invertebrates.

INDUCED MORPHOLOGICAL DEFENSES IN THE WILD: PREDATOR EFFECTS ON A MAYFLY, DRUNELLA COLORADENSIS

Phenotypic plasticity may enable organisms to optimize their phenotypes in environments that are heterogeneous over time or space. For example, inducible defenses are favored for prey populations faced with variable predation risk. We studied the impact of brook trout (Salvelinus fontinalis) on the development of defensive morphological characters in larvae of the mayfly, Drunella coloradensis, both in natural streams and in a series of experiments carried out in streamside, circular, flow-through chambers. Drunella larvae in natural streams with trout had longer caudal filaments per unit body size and relatively heavier exoskeletons than Drunella in fishless streams. Female larvae that developed in fish streams were also significantly smaller and less fecund at metamorphosis than those living in fishless streams, suggesting a possible fitness cost associated with development of morphological defenses. However, timing of metamorphosis of Drunella larvae to the adult stage did not differ between fish and fishless streams. Larvae originating from fish and fishless streams were reared and exposed to water with fish cues or fishless control water for three weeks (1998) or until they developed black wing pads, i.e., just before emergence (1999). In both experiments waterborne fish cues induced the development of relatively longer caudal filaments, but only in Drunella originating from sites without fish. Waterborne fish cues had no effect on Drunella growth rates, behavior, or size at emergence. Drunella originating from fish streams had significantly lower mortality when exposed to predation by trout than Drunella originating from fishless streams. Caudal filament length appears to enhance survival, as Drunella with intact caudal filaments had lower predation rates than Drunella with their tails artificially shortened. This study provides evidence of chemically induced morphological plasticity that could reduce predation rates of these mayflies in natural stream environments.

PREDATOR CHEMICALS INDUCE CHANGES IN MAYFLY LIFE HISTORY TRAITS: A WHOLE-STREAM MANIPULATION

In high-elevation streams of western Colorado, mayflies (Baetis bicaudatus) develop faster, but mature at a smaller size where trout are present compared to streams where fish are absent. These life history traits reduce the time of larval exposure to trout predation, but cost reduced fecundity. We designed a field experiment involving manipulation of whole streams to determine whether these changes were caused by the presence of brook trout, and specifically, whether they could be triggered by trout chemical cues. In 1999 and 2000, we introduced water from containers with brook trout (Salvelinus fontinalis) into five naturally fishless streams, and fishless stream water into five adjacent control streams, to determine whether these cues alone could induce the mayfly life history traits we have observed in natural trout streams. As in previous small-scale experiments, the size at which mayflies matured declined significantly in streams with added trout chemicals but did not change in streams with fishless water only. Thus, life history traits similar to those observed in the field were induced within the natural variability inherent in streams. These results demonstrate the strength of this predator–prey interaction and indicate that brook trout are an important agent of natural selection on mayfly life history traits.

VARIATION IN MAYFLY SIZE AT METAMORPHOSIS AS A DEVELOPMENTAL RESPONSE TO RISK OF PREDATION

Animals with complex life cycles often show large variation in the size and timing of metamorphosis in response to environmental variability. If fecundity increases with body size and large individuals are more vulnerable to predation, then organisms may not be able to optimize simultaneously size and timing of metamorphosis. The goals of this study were to measure and explain large-scale spatial and temporal patterns of phe- notypic variation in size at metamorphosis of the mayfly, Baetis bicaudatus (Baetidae), from habitats with variable levels of predation risk. Within a single high-elevation watershed in western Colorado, USA, from 1994 to 1996 we measured dry masses of mature larvae of the overwintering and summer generations of Baetis at 28 site-years in streams with and without predatory fish (trout). We also estimated larval growth rates and development times at 16 site-years. Patterns of spatial variation in mayfly size could not be explained by resource (algae) standing stock, competitor densities, or physical-chemical variables. However, size at metamorphosis of males and females of summer generation Baetis was smaller in fish streams than in fishless streams and decreased as densities of predatory stoneflies increased. Furthermore, overwintering individuals matured at larger sizes than summer generation Baetis, and the size of emerging Baetis declined over the summer, but predominantly in trout streams. Theoretical consideration of the effect of predation risk on size and timing of metamorphosis accurately predicted the observed temporal variation in size and timing of mayflies at emergence in fish and fishless streams. Baetis populations had similar growth rates but followed different developmental trajectories in high and low risk environments. In risky environments larval development was accelerated, resulting in metamorphosis of younger and smaller individuals, minimizing exposure of larvae to risk of mortality from trout predation, but at the cost of future reproduction. In safe environ- ments, larvae extended their development, resulting in larger, more fecund adults. Thus, we propose that large-scale patterns of variation in size and timing of metamorphosis represent adaptive phenotypic plasticity, whereby mayflies respond to variation in risk of predation, thereby maximizing their fitness in variable environments.

Variation in Mayfly Size at Metamorphosis as a Developmental Response to Risk of Predation

Animals with complex life cycles often show large variation in the size and timing of metamorphosis in response to environmental variability. If fecundity increases with body size and large individuals are more vulnerable to predation, then organisms may not be able to optimize simultaneously size and timing of metamorphosis. The goals of this study were to measure and explain large-scale spatial and temporal patterns of phenotypic variation in size at metamorphosis of the mayfly, Baetis bicaudatus (Baetidae), from habitats with variable levels of predation risk. Within a single high-elevation watershed in western Colorado, USA, from 1994 to 1996 we measured dry masses of mature larvae of the overwintering and summer generations of Baetis at 28 site-years in streams with and without predatory fish (trout). We also estimated larval growth rates and development times at 16 site-years. Patterns of spatial variation in mayfly size could not be explained by resource (algae) standing stock, competitor densities, or physical–chemical variables. However, size at metamorphosis of males and females of summer generation Baetis was smaller in fish streams than in fishless streams and decreased as densities of predatory stoneflies increased. Furthermore, overwintering individuals matured at larger sizes than summer generation Baetis, and the size of emerging Baetis declined over the summer, but predominantly in trout streams. Theoretical consideration of the effect of predation risk on size and timing of metamorphosis accurately predicted the observed temporal variation in size and timing of mayflies at emergence in fish and fishless streams. Baetis populations had similar growth rates but followed different developmental trajectories in high and low risk environments. In risky environments larval development was accelerated, resulting in metamorphosis of younger and smaller individuals, minimizing exposure of larvae to risk of mortality from trout predation, but at the cost of future reproduction. In safe environments, larvae extended their development, resulting in larger, more fecund adults. Thus, we propose that large-scale patterns of variation in size and timing of metamorphosis represent adaptive phenotypic plasticity, whereby mayflies respond to variation in risk of predation, thereby maximizing their fitness in variable environments.

Nocturnal drift of mayfly nymphs as a post‐contact antipredator mechanism

1. The predominantly nocturnal constrained drift of stream invertebrates is commonly regarded as a behaviour that avoids encounters with visually foraging fish in the water column. The alternative explanation, that drift peaks are caused by bottom‐feeding, nocturnal predators, has rarely been tested. 2. We examined these hypotheses by collecting invertebrate drift in five streams in northern Finland: one with brown trout (Salmotrutta, a drift‐feeding fish), one with alpine bullhead (Cottuspoecilopus, a benthic fish), one with both species, and two fishless streams. 3. Drift by Baetis mayflies was aperiodic or slightly diurnal in both fishless streams on all sampling occasions. In contrast, drift was nocturnal in streams with trout and, to a lesser extent, in the stream with bullhead. Non‐dipteran prey drifted mainly nocturnally in all streams with fish, whereas Diptera larvae were less responsive to the presence of fish. 4. In laboratory experiments, bullheads were night‐active, causing a much higher frequency of drift by touching Baetis at night than during the day. Thus, increased nocturnal drift may serve to avoid both visual predators (a pre‐contact response) and benthic fish (a post‐contact response). In streams with bottom‐feeding fish, nocturnal drift should be caused by increased drift by night rather than by reduced drift by day.

Effects of fish predation in erosional and depositional habitats in a temperate stream

Combined effects of predation by benthic and drift-foraging fish (prickly sculpin (Cottus asper) and coho salmon (Onchorhynchus kisutch) parr) on benthic invertebrate community and trophic structure were evaluated in Mayfly Creek, a previously fishless stream in the Coast Range Mountains of British Columbia. The role of microhabitat (substrate) in mediating predation effects was assessed by comparing invertebrate community structure on unglazed ceramic tiles and gravel baskets nested within enclosures. The role of macrohabitat was evaluated by placing enclosures in pool and riffle habitats. Effects of fish predation were most pronounced on tile substrate and in riffle habitat and least pronounced on gravel substrate in pool habitat. The presence of fish caused a decrease in abundance of larger-bodied herbivores (primarily the mayflies Ameletus and Baetis) and had positive indirect effects on algae and smaller invertebrates (primarily Orthocladiinae chironomids and nemourid stoneflies), probably through competitive release. In contrast with herbivores, detritivorous invertebrates were less influenced by fish predation and more highly correlated with the abundance of organic detritus. The distribution and abundance of detritivores in Mayfly Creek appear to be primarily influenced by bottom-up forces (implying resource limitation), while grazers in algal-based food chains are more strongly influenced by top-down effects (fish predation).

Contrasting effects of fish predation in a fishless and fish-bearing stream

The effects of fish predation on invertebrate community structure and algal biomass were contrasted in a fish-bearing and previously fishless stream in the coast mountains of British Columbia. Predator impact was assessed by stocking coho salmon (Onchorhynchus kisutch) juveniles and prickly sculpin (Cottus asper) in replicate enclosures. Abundance of larger invertebrates (primarily herbivorous mayflies) decreased in the presence of fish, while algal biomass and density of smaller epibenthic herbivores, primarily orthoclad chironomids, correspondingly increased. Effect sizes of fish predation were generally larger in the previously fishless stream than the fish-bearing stream, where invertebrates were presumably better adapted to fish predation. Density of larger invertebrates was lower in the fish-bearing stream, and density of smaller invertebrates and algal biomass were higher, indicating that the effects of fish predation on algal biomass and invertebrate community structure at a larger scale between streams with and without fish are similar to the effects of fish predation at a smaller scale within a single stream.

Effects of size at metamorphosis on stonefly fecundity, longevity, and reproductive success.

Many organisms with complex life cycles show considerable variation in size and timing at metamorphosis. Adult males of Megarcyssignata (Plecoptera: Perlodidae) are significantly smaller than females and emerge before females (protandry) from two western Colorado streams. During summer 1992 stoneflies from a trout stream emerged earlier in the season and at larger sizes than those from a colder fishless stream, and size at metamorphosis did not change over the emergence period in either stream. We performed two experiments to determine whether variation in size at metamorphosis affected the fecundity, reproductive success and longevity of individuals of this stonefly species and if total lifetime fecundity was affected by the number of matings. In the first experiment, total lifetime fecundity (eggs oviposited) was determined for adult females held in small plastic cages in the field. Males were removed after one copulation, or pairs were left together for life and allowed to multiply mate. Most copulations occurred in the first few days of the experiment. Females in treatments allowing multiple matings had significantly lower total lifetime fecundity and shorter adult longevity than females that only mated once. Multiple matings also reduced longevity of males. Fecundity increased significantly with female body mass at emergence, but only for females that mated once. While multiple matings eliminated the fecundity advantage of large female body size, number of matings did not affect the significant positive relationship between body mass at metamorphosis and longevity of males or females. In a second experiment designed to determine if body mass at emergence affected male mating success, we placed one large and one small male Megarcys in an observation arena containing one female and recorded which male obtained the first mating. The large and the small male had equal probabilities of copulating with the female. Copulations usually lasted all night, and the unmated male made frequent, but unsuccessful attempts to take over the copulating female. Our data suggest that selection pressures determining body size at metamorphosis may operate independently on males and females, resulting in evolution of sexual size dimorphism, protandry, and mating early in the adult stage. We emphasize the importance of interpreting the fitness consequences of larval growth and development on the timing of and size at metamorphosis in the context of the complete life cycle.

The influence of upstream predation on the expression of fish effects in downstream patches

1. Enclosures were installed in a fishless stream and divided transversely into upstream and downstream sections. Downstream sections were further divided longitudinally, and one of the downstream sections in each enclosure was stocked with juvenile coho salmon (Oncorhynchus kisutch), and the other side was left as a fishless control. Densities of juvenile coho were then manipulated in upper sections of enclosures to determine the effect of upstream predation intensity on fish predation effects in downstream sections.2. Significantly fewer mayflies (primarily Ameletus sp.) were observed grazing on unglazed ceramic tiles in lower enclosure sections with fish present. There was no detectable effect of fish density in upstream enclosure sections on the number of mayflies observed grazing on ceramic tiles in lower sections of the enclosures.3. There was a significant positive effect of both fish presence and upstream density on chlorophyll a concentrations on ceramic tiles in lower enclosure sections, but not on chlorophyll a on natural gravel substratum.4. Behavioural experiments with mayflies and coho in streamside troughs suggest that Ameletus sp. responds primarily to mechanical rather than chemical cues from coho parr.