Community Ecology of Stream Fishes: Concepts, Approaches, and Techniques

<em>Abstract</em>.—Migratory fishes are common in freshwaters throughout the world and can fundamentally alter recipient ecosystems. We describe different types of fish migrations and consider their importance from the perspective of ecosystem subsidies—that is, landscape-scale flows of energy, materials, and organisms that are important in driving local food web and ecosystem dynamics. We distinguish between two general categories of subsidies, which we term here material subsidies and process subsidies. Material subsidies are the transfer of energy, nutrients, and other resources resulting in direct changes in resource pools within ecosystems. We posit that material subsidies occur under only a subset of life history strategies and ecological settings, and the potential for migratory fish to represent major material subsidies is greatest when (1) the biomass of migrants is high relative to recipient ecosystem size, (2) the availability of nutrients and energy is low in the recipient ecosystem (i.e., oligotrophic), and (3) there are effective mechanisms for both liberating nutrients and energy from migratory fishes and retaining those materials within the food web of the recipient ecosystem. Thus, anadromous semelparous Pacific salmon <em>Oncorhynchus </em>spp. with en masse programmed senescence in oligotrophic Pacific Northwest streams can be large material subsidies. In contrast, process subsidies arise from feeding or other activities of migratory species that directly affect process rates within recipient ecosystems. For example, the physical and chemical effects of grazing and sediment-feeding fishes such as prochilodontids, as well as seed dispersal by large-bodied frugivorous characins, represent potentially key process subsidies by migratory fishes in some of the great rivers of South America. We speculate that process subsidies are more widespread than material subsidies from migratory stream fishes because they are independent of the type of migration patterns, life history, and distance traveled. Nevertheless, the magnitude of process subsidies is likely to be greatest under a specific subset of ecological conditions, which can differ from those where material subsidies might be most important. In addition to migrant biomass, the potential for migratory fish to represent strong process subsidies is regulated by migrant interaction strength and the degree to which a migratory species is functionally unique in a particular ecological setting. Unlike material subsidies, which require high migrant biomass as conveyor belts of materials, migratory fishes can be crucial process subsidies, even when migrant biomass is low, if they are functionally unique and strong interactors. We provide specific examples of these different types of subsidies and outline key directions of research for furthering our understanding of the functional significance of migratory stream fishes. Our aim is to highlight the diversity of subsidies provided by migratory fishes in order to foster a more comprehensive perspective on fishes as essential components of riverine ecosystems.

Community Ecology of Stream Fishes: Concepts, Approaches, and Techniques

<em>Abstract</em>.—Timing of reproduction differs among fish species in nearly all rivers, and reproductive phenology is predicted to strongly influence freshwater fish community structure in some systems. Despite its potential importance, few long-term studies of reproductive phenology in river fishes have been conducted in a community context. Here, we evaluated timing and sequence of reproduction of fishes in the Rio Grande, New Mexico over 9 years. Dates and rank order of first appearance of larvae varied among species and years, but three consistent spawning guilds were evident: early season, late season, and species that were intermediate in rank order of spawning. We hypothesized that appropriate reproductive timing enhanced recruitment to the extent that spawning cues predicted future availability of critical resources for larvae. Analysis of historical discharge records indicated that present and future discharge exhibited positive autocorrelation for up to 90 d. Likewise, larval fish densities were highest at moderate flows and coincident with high food resource abundance. However, stable isotope data for larval and adult fishes indicated considerable overlap in food resource use among larvae and adult fishes. There may be pressure for spawning time to converge among species to match the appearance of seasonal resources, but to diverge to lessen competition among young-of-year fishes in a classical trade-off scenario. More long-term studies are needed, and we propose that an integrated research program that combines detailed analysis of reproductive phenology, food web dynamics, and comparative genomic analyses could forge connections between environmental variation in spawning cues, recruitment success, and community assembly in river fishes. Such an integrated program could lead to better predictions about fish community responses to global warming, especially in vulnerable arid-land systems like the Rio Grande.

Community Ecology of Stream Fishes: Concepts, Approaches, and Techniques

<em>Abstract</em>.—Stream fish are expected to be influenced by climate change as they are ectothermic animals living in lotic systems. Using fish presence–absence records in 1,110 stream sites across France, our study aimed at (1) modeling current and future distributions of 35 stream fish species, (2) using an ensemble forecasting approach (i.e., several general circulation models [GCM] × greenhouse gas emission scenarios [GES] × statistical species distribution models [SDM] combinations) to quantify the variability in the future fish species distribution due to each component, and (3) assessing the potential impacts of climate change on fish species distribution and assemblage structure by using a consensus method that accounted for the variability in future projections.

Community Ecology of Stream Fishes: Concepts, Approaches, and Techniques

<em>Abstract</em>.—Stream fishes carry out their life histories across broad spatial and temporal scales, leading to spatially structured populations. Therefore, incorporating metapopulation dynamics into models of stream fish populations may improve our ability to understand mechanisms regulating them. First, we reviewed empirical research on metapopulation dynamics in the stream fish ecology literature and found 31 papers that used the metapopulation framework. The majority of papers applied no specific metapopulation model, or included space only implicitly. Although parameterization of spatially realistic models is challenging, we suggest that stream fish ecologists should incorporate space into models and recognize that metapopulation types may change across scales. Second, we considered metacommunity theory, which addresses how trade-offs among dispersal, environmental heterogeneity, and biotic interactions structure communities across spatial scales. There are no explicit tests of metacommunity theory using stream fishes to date, so we used data from our research in a Great Plains stream to test the utility of these paradigms. We found that this plains fish metacommunity was structured mainly by spatial factors related to dispersal opportunity and, to a lesser extent, by environmental heterogeneity. Currently, metacommunity models are more heuristic than predictive. Therefore, we propose that future stream fish metacommunity research should focus on developing testable hypotheses that incorporate stream fish life history attributes, and seasonal environmental variability, across spatial scales. This emerging body of research is likely to be valuable not only for basic stream fish ecological research, but also multispecies conservation and management.

Community Ecology of Stream Fishes: Concepts, Approaches, and Techniques

<em>Abstract</em>.—The flow regime is considered the primary driver of physical processes in riverine ecosystems; thus we expect that the trait composition of fish assemblages might respond similarly to hydrologic variability, even at broad spatial scales. Here, we test the hypothesis that freshwater fish life history strategies on two continents (southern United States and eastern Australia) converge along gradients of hydrologic variability and primary productivity at the drainage scale. Our results show that the fishes of the United States and Australia conform to the three-dimensional adaptive space arising from the trade-offs among three basic demographic parameters of survival, fecundity, and onset and duration of reproductive life. Species from both continents represent the endpoints in adaptive space defining the periodic (19% versus 33% for the United States and Australia, respectively), opportunistic (69% versus 52%), and equilibrium life history strategies (12% versus 15%). We found evidence that fish life history composition of drainage basins in the two continents have converged across similar gradients of hydrologic variability and productivity despite phylogenetic and historical differences. Moreover, these relationships were largely consistent with predictions from life history theory. Increasing hydrologic variability has promoted the greater prevalence of opportunistic strategists (a strategy that should maximize fitness in environmental settings dominated by unpredictable environmental change) while concurrently minimizing the persistence of periodic-type species (a strategy typically inhabits seasonal, periodically suitable environments). Our study provides a conceptual framework of management options for species in regulated rivers because life history strategies are the underlying determinants for population responses to environmental change and therefore can be used to classify typical population responses to flow alteration or mitigation via environmental flow prescriptions.

Community Ecology of Stream Fishes: Concepts, Approaches, and Techniques

<em>Abstract</em>.—Community ecology increasingly seeks to integrate the influences of regional and historical processes with species interactions within local habitats. This broadened perspective is largely based on comparative approaches that employ “natural experiments” to identify factors shaping community structure. Because coastal rivers are separated from one another by insurmountable barriers (oceans or land), freshwater fishes are particularly well suited for comparative analyses of factors that influence fish community organization. In this chapter, we review how this comparative approach shed light on large-scale biodiversity gradients, community saturation, community convergence, density compensation, and the role of intrinsic and extrinsic factors in community dynamics. The main factors (e.g., river mouth discharge and history) empirically related to species richness of a river are well identified, and metacommunity ecology provides a fruitful conceptual framework for understanding how regional (river) species richness translates into local species richness. Much work remains to identify factors explaining differences among whole river basin assemblages with regard to ecological traits (e.g., trophic status and life history) composition and to assess whether trait-related environmental and biotic local filters act similarly over large spatial scales. One important conclusion that can be drawn from the studies reviewed here is that history cannot be neglected whatever the scale of investigation (global, river, or site). A second conclusion is that historical effects are not strong enough to blur the occurrence of qualitatively repeatable patterns of community structure over large spatial scale, which is encouraging because it suggests development of general predictive models of community structure is an attainable goal.

Community Ecology of Stream Fishes: Concepts, Approaches, and Techniques

<em>Abstract</em>.—Ecological stoichiometry refers to the relative availability of elements in ecosystems as both an influence upon and result of ecological interactions. Nutrient ratios have long been analyzed in primary producers, but their application to animals is more recent. Here, we summarize the ecological stoichiometry framework and highlight three key contexts in stream fish ecology: body stoichiometry, dietary stoichiometry, and roles in ecosystem nutrient cycling. Elemental demands for growth depend directly upon the stoichiometry of carbon and nutrients in body tissues. Body stoichiometry varies widely among the dozens of stream fish species for which data are available and exhibits some phylogenetic and size-based patterns. Due to the variety of foods consumed by stream fishes, the stoichiometry of their diets also varies widely. Consuming foods with high carbon:nutrient ratios can produce phosphorus-limited growth in algivores and potentially in insectivores as well. These expectations contrast with the prevailing belief that energy intake is the key nutritional control on growth of most fishes. Ingested nutrients that are not incorporated into body tissues must be defecated or excreted. These waste products can be a critical component of ecosystem nutrient cycles and offer the opportunity for species identity to affect ecosystem functioning. We argue that ecological stoichiometry provides an integrative framework for merging perspectives across individual, population, community, and ecosystem levels. Broader application of this approach to stream fishes will offer particular insight into consumer–resource interactions and ecosystem dynamics.