Angus R. McIntosh

Interactions between fish, grazing invertebrates and algae in a New Zealand stream: a trophic cascade mediated by fish-induced changes to grazer behaviour?

Experiments in laboratory stream channels compared the behaviour of Deleatidium mayfly nymphs in the absence of fish with that in the presence of either native common river galaxias (Galaxias vulgaris Stokell) or introduced brown trout (Salmo trutta L.). Galaxias present similar predation risks to prey during day and night but are more active at night. Whereas, trout present a higher predation risk during the day. Deleatidium maintained a fixed nocturnal drift periodicity that is characteristic of streams containing visually feeding fish regardless of the nature of the predation regime presented in the laboratory. However, the number on the substratum surface, and therefore able to graze algae, was lower when fish were present than when they were absent. The number was lower during the day in the presence of trout, when they present the highest predation risk, and lower during the night compared to the day in trials with galaxias when galaxias activity disturbs Deleatidium from the substratum. Increases in the probability of Deleatidium leaving a patch, reductions in the proportion of mayflies on high quality patches and reductions in the distance travelled from refuge also reflected variations in the predation regime. Similar differences in positioning were observed under the same predation regimes in in situ channels in the Shag River and these were associated with differences in algal biomass. Algal ash-free dry mass (AFDM) and chlorophyll a (chl a) were higher on the tops of cobbles when fish were present. Fish also affected the biomass and the distribution of algae on cobbles as AFDM and chl a were higher on the sides of cobbles from channels with trout compared to those with galaxias. Changes in grazing behaviour, caused by predator avoidance, are likely to have been responsible for differences in algal biomass because no significant differences were detected between treatments in the biomass of Deleatidium or of total invertebrates.

The impact of introduced brown and rainbow trout on native fish: the case of Australasia

Introduction page 218 Trout introductions to Australasia 218 Brown trout Rainbow trout Native fish communities 219 Australia New Zealand Evidence for effects of introduced trout 220 Non-overlapping distributions Shifts in relative abundance Mechanisms underlying effects of trout 224 Competition for food Competition for space Predation Discussion 228 Towards an invasion ecology Introduced species, invasive species and colonizers Features of the environment that increase susceptibility to invasion Biological parameters of introduced species that promote the likelihood of invasion Factors associated with increased risk to the native fauna The impact of trout in Australasia Potential reasons for the successful introduction of trout to Australasia The impact of trout on Australasian species a predictable pattern? Summary and conclusions 233 Brown trout appear to be more damaging than rainbow trout Stream fish faunas appear to be less affected than lake faunas Trout are more often invasive in New Zealand than in Australia Both trout introductions and land-use changes have affected native fishes Acknowledgements 234 References 235

Fitness and community consequences of avoiding multiple predators

Abstract We investigated the fitness and community consequences of behavioural interactions with multiple predators in a four-trophic-level system. We conducted an experiment in oval flow-through artificial-stream tanks to examine the single and interactive sublethal effects of brook trout and stoneflies on the size at emergence of Baetis bicaudatus (Ephemeroptera: Baetidae), and the cascading trophic effects on algal biomass, the food resource of the mayflies. No predation was allowed in the experiment, so that all effects were mediated through predator modifications of prey behaviour. We reared trout stream Baetis larvae from just before egg development until emergence in tanks with four treatments: (1) water from a holding tank with two brook trout (trout odour), (2) no trout odour + eight stoneflies with glued mouthparts, (3) trout odour + stoneflies and (4) no trout odour or stoneflies. We ended the experiment after 3 weeks when ten male and ten female subimagos had emerged from each tank, measured the size of ten male and ten female mature nymphs (with black wing pads), and collected algal samples from rocks at six locations in each tank. To determine the mechanism responsible for sublethal and cascading effects on lower trophic levels we made day and night observations of mayfly behaviour for the first 6 days by counting mayflies drifting in the water column and visible on natural substrata in the artificial streams. Trout odour and stoneflies similarly reduced the size of male and female Baetis emerging from artificial streams, with non-additive effects of both predators. While smaller females are less fecund, a fitness cost of small male size has not been determined. The mechanism causing sublethal effects on Baetis differed between predators. While trout stream Baetis retained their nocturnal periodicity in all treatments, stoneflies increased drift dispersal of mayflies at night, and trout suppressed night-time feeding and drift of mayflies. Stoneflies had less effect on Baetis behaviour when fish odour was present. Thus, we attribute the non-additivity of effects of fish and stoneflies on mayfly growth to an interaction modification whereby trout odour reduced the impact of stoneflies on Baetis behaviour. Since stonefly activity was also reduced in the presence of fish odour, this modification may be attributed to the effect of fish odour on stonefly behaviour. Only stoneflies delayed Baetis emergence, suggesting that stoneflies had a greater sublethal effect on Baetis fitness than did trout. Delayed emergence may reduce Baetis fitness by increasing risks of predation and parasitism on larvae, and increasing competition for mates or oviposition sites among adults. Finally, algal biomass was higher in tanks with both predators than in the other three treatments. These data implicate a behavioural trophic cascade because predators were not allowed to consume prey. Therefore, differences in algal biomass were attributed to predator-induced changes in mayfly behaviour. Our study demonstrates the importance of considering multiple predators when measuring direct sublethal effects of predators on prey fitness and indirect effects on lower trophic levels. Identification of an interaction modification illustrates the value of obtaining detailed information on behavioural mechanisms as an aid to understanding the complex interactions occurring among components of ecological communities.

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.

Interpopulation Variation in Mayfly Antipredator Tactics: Differential Effects of Contrasting Predatory Fish

Introduced brown trout (Salmo trutta) have replaced native common river galaxia (Galaxias vulgaris) as the principal predators in many streams in the Taieri River system, New Zealand. Brown trout and common river galaxias present prey with contrasting predation risks as galaxias forage mainly at night using mechanical cues and trout present a higher predation risk during the day by foraging using visual cues. To determine if this change has affected the behavior of a siphlonurid mayfly, Nesameletus ornatus, we compared the behavior of N. ornatus nymphs from three neighboring streams that have different fish predation regimes (trout, galaxias, fishless) when confronted with all possible predation regimes in the laboratory (trout, galaxias, no fish). N. ornatus from a trout stream were consistently nocturanally active whether trout, galaxias, or no fish were present. In contrast, when mayflies from streams without trout were confronted with trout, no consistent diel periodicities in their behavior were observed. However, when mayflies from stream without trout were tested with and without galaxias they altered their behavior according to the presence/absence of the native predator. The possibility that N. ornatus accumulates evolutionary experience of trout through adults flying between streams may be ruled out: mayflies with no possible experience of trout did not respond differently to those from streams without trout but with the possibility of migrating from trout streams close by. The presence of different predatory fish may have resulted in differential trade—offs between foraging and predator avoidance. The probability of mayflies leaving food patches was higher during the day in the presence of trout. In contrast, the probability was higher during the night in the presence of galaxias. Trout and galaxias were also associated with differential reductions in the number of mayflies on the substrate surface and the proportion of mayflies on food patches, with consequent effects on time spent foraging. These responses indicate that both the history of the prey population and the nature of the predation risk have considerable influence on antipredator behavior and indicate a possible important influence on stream communities of predatory fish through sublethal effects on prey behavior.

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 manipu- lation 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 fon- tinalis) 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 chem- icals 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.

Extrapolating from Individual Behavior to Populations and Communities in Streams

Biological processes, such as predator-prey or competitive interactions, occur at multiple spatial and temporal scales, but their impacts on the distribution, abundance, and fitness of organisms may only be detectable at some scales. This is because 1) small scale (local) processes may be constrained by large-scale (regional) processes, 2) the influence of organism movement changes with spatial scale, and 3) multiple small-scale processes may interact and produce variation that obscures large-scale patterns. Most ecologists would like to know the relevance of small-scale observations and experiments for large-scale patterns and processes acting over long time periods. In this paper we consider whether patterns of individual behavior translate in a straightforward way to patterns in population dynamics, community structure, or individual fitness at larger spatial and longer temporal scales. We illustrate our discussion with data from communities living in high altitude streams in western Colorado, coastal streams in southern California, and streams on the South Island of New Zealand. We describe patterns of individual behavior including selective predation, predator-avoidance behavior, and competitive interactions. Our examples indicate that large scale patterns of distribution and abundance of organisms sometimes deviate from those expected from patterns of individual behavior. We hypothesize that such discrepancies are often due to multiple confounding factors, including abiotic disturbances, that have effects on populations and communities at larger scales. Similarly, our data illustrate that present behaviors may not have measurable fitness consequences over longer time scales. We emphasize that only by studying all life history stages of organisms with complex life cycles can we interpret the long-term fitness consequences of individual behaviors. We conclude that our ability to extrapolate from individual behaviors to populations and communities in streams will improve when we consider the repercussions of individual behaviors and environmental factors over broader spatial and temporal scales.

Dual influences of ecosystem size and disturbance on food chain length in streams

The number of trophic transfers occurring between basal resources and top predators, food chain length (FCL), varies widely in the worlds ecosystems for reasons that are poorly understood, particularly for stream ecosystems. Available evidence indicates that FCL is set by energetic constraints, environmental stochasticity, or ecosystem size effects, although no single explanation has yet accounted for FCL patterns in a broad sense. Further, whether environmental disturbance can influence FCL has been debated on both theoretical and empirical grounds for quite some time. Using data from sixteen South Island, New Zealand streams, we determined whether the so-called ecosystem size, disturbance, or resource availability hypotheses could account for FCL variation in high country fluvial environments. Stable isotope-based estimates of maximum trophic position ranged from 2.6 to 4.2 and averaged 3.5, a value on par with the global FCL average for streams. Model-selection results indicated that stream size and disturbance regime best explained across-site patterns in FCL, although resource availability was negatively correlated with our measure of disturbance; FCL approached its maximum in large, stable springs and was <3.5 trophic levels in small, fishless and/or disturbed streams. Community data indicate that size influenced FCL, primarily through its influence on local fish species richness (i.e., via trophic level additions and/or insertions), whereas disturbance did so via an effect on the relative availability of intermediate predators (i.e., predatory invertebrates) as prey for fishes. Overall, our results demonstrate that disturbance can have an important food web-structuring role in stream ecosystems, and further imply that pluralistic explanations are needed to fully understand the range of structural variation observed for real food webs.

Top-down and bottom-up processes in grassland and forested streams

The influence of predatory fish on the structure of stream food webs may be altered by the presence of forest canopy cover, and consequent differences in allochthonous inputs and primary production. Eight sites containing introduced brown trout (Salmo trutta) and eight sites that did not were sampled in the Cass region, South Island, New Zealand. For each predator category, half the sites were located in southern beech (Nothofagus) forest patches (range of canopy cover, 65–90%) and the other half were in tussock grassland. Food resources used by two dominant herbivores-detritivores were assessed using stable isotopes. 13C/12C ratios were obtained for coarse particulate organic matter (CPOM), fine particulate organic matter (FPOM), algal dominated biofilm from rocks, and larvae of Deleatidium (Ephemeroptera) and Olinga (Trichoptera). Total abundance and biomass of macroinvertebrates did not differ between streams with and without trout, but were significantly higher at grassland sites than forested sites. However, taxon richness and species composition differed substantially between trout and no-trout sites, irrespective of whether streams were located in forest or not. Trout streams typically contained more taxa, had low biomass of predatory invertebrates and large shredders, but a high proportion of consumers with cases or shells. The standing stock of CPOM was higher at forested sites, but there was less FPOM and more algae at sites with trout, regardless of the presence or absence of forest cover. The stable carbon isotope range for biofilm on rocks was broad and encompassed the narrow CPOM and FPOM ranges. At trout sites, carbon isotope ratios of Deleatidium, the most abundant invertebrate primary consumer, were closely related to biofilm values, but no relationship was found at no-trout sites where algal biomass was much lower. These results support a role for both bottom-up and top-down processes in controlling the structure of the stream communities studied, but indicate that predatory fish and forest cover had largely independent effects.

Habitat loss drives threshold response of benthic invertebrate communities to deposited sediment in agricultural streams

Agricultural land uses can impact stream ecosystems by reducing suitable habitat, altering flows, and increasing inputs of diffuse pollutants including fine inorganic sediment (< 2 mm). These changes have been linked to altered community composition and declines in biodiversity. Determining the mechanisms driving stream biotic responses, particularly threshold impacts, has, however, proved elusive. To investigate a sediment threshold response by benthic invertebrates, an intensive survey of 30 agricultural streams was conducted along gradients of deposited sediment and dissolved nutrients. Partial redundancy analysis showed that invertebrate community composition changed significantly along the gradient of deposited fine sediment, whereas the effect of dissolved nitrate was weak. Pollution-sensitive invertebrates (%EPT, Ephemeroptera, Plecoptera, Trichoptera) demonstrated a strong nonlinear response to sediment, and change-point analysis indicated marked declines beyond a threshold of -20% fine sediment covering the streambed. Structural equation modeling indicated that decreased habitat availability (i.e., coarse substrate and associated interstices) was the key driver affecting pollution-sensitive invertebrates, with degraded riparian condition controlling resources through direct (e.g., inputs) and indirect (e.g., flow-mediated) effects on deposited sediment. The identification of specific effects thresholds and the underlying mechanisms (e.g., loss of habitat) driving these changes will assist managers in setting sediment criteria and standards to better guide stream monitoring and rehabilitation.