Effects of Topology, Number and Location of Nodes, Population Density, and Stocking Duration on Hybrids’ Dispersal Across a Network

Hybridization between native cutthroat trout (Oncorhynchus clarki sp.) and introduced rainbow trout (O. mykiss) has been a topic of fisheries research for decades in the northern Rocky Mountains, USA. Several studies suggest that the likelihood of introgression at any location in a stream network is influenced by the distance between that location and the source of non-native genes, e.g., stocking locations or areas dominated by non-native or introgressed fish. The relationship between “distance to non-native source” and hybridization rates, however, is rarely quantified. Studies that attempt to quantify the relationship generally ignore the potential influence of stream network topology on gene movement. We have developed and applied an agent-based model that tracks the lineage and breeding location of individual fish over time, this simulating the movement of non-native genes among spawning locations within a stream network. The model considered both distances between spawning sites and network topology in determining non-natal spawning site selection by stocked and straying fish. Model results suggest that stream network topology has a strong influence on the relationship between “stream distance from genetic source” and “degree of hybridization” at spawning locations. However, the importance of topology varies depending on underlying model assumptions about, stocking duration, number and location of spawning grounds, population density, and spawning site fidelity, i.e., “straying rates,” within the river system

Seasonal Variation in Terrestrial Invertebrate Subsidies to Tropical Streams and Implications for the Feeding Ecology of Hart’s Rivulus (Anablepsoides hartii)

Terrestrial invertebrates are important subsidies to fish diets, though their seasonal dynamics and importance to tropical stream consumers are particularly understudied. In this year-round study of terrestrial invertebrate input to two Trinidadian headwater streams with different forest canopy densities, we sought to (a) measure the mass and composition of terrestrial inputs with fall-in traps to evaluate the influences of seasonality, canopy cover, and rainfall intensity, and; (b) compare terrestrial and benthic prey importance to Anablepsoides hartii(Hart’s Rivulus), the dominant invertivorous fish in these streams, by concurrently measuring benthic and drifting invertebrate standing stocks and the volume and composition of invertebrates in Rivulus guts throughout the year. The biomass of terrestrial invertebrate fall-in was 53% higher in the wet versus dry season; in particular, ant input was 320% higher. Ant biomass fall-in also increased with the density of canopy cover among sampling locations within both streams. Greater precipitation correlated with increased ant inputs to the more open-canopied stream and increased inputs of winged insects in the more closed canopy stream. Concurrently, the biomass of benthic invertebrates was reduced by more than half in the wet season in both streams. We detected no differences in the total volume of terrestrial prey in Rivulus diets between seasons, though ants were a greater proportion of their diet in the wet season. In contrast, benthic prey were nearly absent from Rivulus diets in the wet season in both streams. We conclude that terrestrial invertebrates are a substantial year-round prey subsidy for invertivores in tropical stream ecosystems like those we studied, which may contrast to most temperate streams where such terrestrial inputs are significantly reduced in the cold season. Interestingly, the strongest seasonal pattern in these tropical streams was observed in benthic invertebrate biomass which was greatly reduced and almost absent from Rivulus diets during the wet season. This pattern is essentially the inverse of the pattern observed in many temperate streams and highlights the need for additional studies in tropical ecosystems to better understand how spatial and temporal variation in terrestrial subsidies and benthic prey populations combine to influence consumer diets and the structure of tropical stream food webs

Global Patterns and Drivers of Ecosystem Functioning in Rivers and Riparian Zones

River ecosystems receive and process vast quantities of terrestrial organic carbon, the fate of which depends strongly on microbial activity. Variation in and controls of processing rates, however, are poorly characterized at the global scale. In response, we used a peer-sourced research network and a highly standardized carbon processing assay to conduct a global-scale field experiment in greater than 1000 river and riparian sites. We found that Earth’s biomes have distinct carbon processing signatures. Slow processing is evident across latitudes, whereas rapid rates are restricted to lower latitudes. Both the mean rate and variability decline with latitude, suggesting temperature constraints toward the poles and greater roles for other environmental drivers (e.g., nutrient loading) toward the equator. These results and data set the stage for unprecedented “next-generation biomonitoring” by establishing baselines to help quantify environmental impacts to the functioning of ecosystems at a global scale

Global Patterns and Controls of Nutrient Immobilization On Decomposing Cellulose In Riverine Ecosystems

Microbes play a critical role in plant litter decomposition and influence the fate of carbon in rivers and riparian zones. When decomposing low-nutrient plant litter, microbes acquire nitrogen (N) and phosphorus (P) from the environment (i.e., nutrient immobilization), and this process is potentially sensitive to nutrient loading and changing climate. Nonetheless, environmental controls on immobilization are poorly understood because rates are also influenced by plant litter chemistry, which is coupled to the same environmental factors. Here we used a standardized, low-nutrient organic matter substrate (cotton strips) to quantify nutrient immobilization at 100 paired stream and riparian sites representing 11 biomes worldwide. Immobilization rates varied by three orders of magnitude, were greater in rivers than riparian zones, and were strongly correlated to decomposition rates. In rivers, P immobilization rates were controlled by surface water phosphate concentrations, but N immobilization rates were not related to inorganic N. The N:P of immobilized nutrients was tightly constrained to a molar ratio of 10:1 despite wide variation in surface water N:P. Immobilization rates were temperature-dependent in riparian zones but not related to temperature in rivers. However, in rivers nutrient supply ultimately controlled whether microbes could achieve the maximum expected decomposition rate at a given temperature

Investigating the Influence of Habitat Heterogeneity on Diversity of Insectivorous Birds in a River-Floodplain Mosaic

Rivers are fundamentally linked to their adjacent terrestrial ecosystems. In particular, within river-floodplains, the emergence of adult aquatic insects constitutes a resource flux for local bird communities, and the structure of this emergence varies across the mosaic of aquatic habitats within these ecosystems. In our study, we are assessing the importance of habitat heterogeneity, through its influence on patterns of insect emergence, on the overall density, species richness, and biodiversity of insectivorous birds across seven unique habitats within a Snake River-floodplain. During summer 2017, we conducted three bird point-counts monthly at each habitat, separated by 50-meter line transects. Overall, we observed higher bird abundance and richness at flowing-water habitats than standing-water habitats. In total, 16 species of insectivorous birds have been observed; several species, most notably bank swallows, song sparrows, and yellow warblers, have been ubiquitous, whereas others have been exclusively observed in specific habitats. A thought-experiment, whereby we sequentially and randomly aggregated species richness from seven habitats, revealed richness increasing curvilinearly with increasing habitat heterogeneity. We hypothesize that this pattern is linked to the asynchrony of insect emergence among habitats, such that the complexity of the mosaic sustains elevated bird diversity by providing more consistent and diverse prey resources

Seasonal Variation in Terrestrial Invertebrate Subsidies to Tropical Streams and Implications for the Feeding Ecology of Hart\u27s Rivulus (Anablepsoides hartii)

Terrestrial invertebrates are important subsidies to fish diets, though their seasonal dynamics and importance to tropical stream consumers are particularly understudied. In this year-round study of terrestrial invertebrate input to two Trinidadian headwater streams with different forest canopy densities, we sought to (a) measure the mass and composition of terrestrial inputs with fall-in traps to evaluate the influences of seasonality, canopy cover, and rainfall intensity, and; (b) compare terrestrial and benthic prey importance to Anablepsoides hartii (Hart\u27s Rivulus), the dominant invertivorous fish in these streams, by concurrently measuring benthic and drifting invertebrate standing stocks and the volume and composition of invertebrates in Rivulus guts throughout the year. The biomass of terrestrial invertebrate fall-in was 53% higher in the wet versus dry season; in particular, ant input was 320% higher. Ant biomass fall-in also increased with the density of canopy cover among sampling locations within both streams. Greater precipitation correlated with increased ant inputs to the more open-canopied stream and increased inputs of winged insects in the more closed canopy stream. Concurrently, the biomass of benthic invertebrates was reduced by more than half in the wet season in both streams. We detected no differences in the total volume of terrestrial prey in Rivulus diets between seasons, though ants were a greater proportion of their diet in the wet season. In contrast, benthic prey were nearly absent from Rivulus diets in the wet season in both streams. We conclude that terrestrial invertebrates are a substantial year-round prey subsidy for invertivores in tropical stream ecosystems like those we studied, which may contrast to most temperate streams where such terrestrial inputs are significantly reduced in the cold season. Interestingly, the strongest seasonal pattern in these tropical streams was observed in benthic invertebrate biomass which was greatly Frontiers in Ecology and Evolution | www.frontiersin.org 1 February 2022 | Volume 10 | Article 788625 Owens et al. Terrestrial Subsidies to Tropical Streams reduced and almost absent from Rivulus diets during the wet season. This pattern is essentially the inverse of the pattern observed in many temperate streams and highlights the need for additional studies in tropical ecosystems to better understand how spatial and temporal variation in terrestrial subsidies and benthic prey populations combine to influence consumer diets and the structure of tropical stream food webs