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Hydroecology of Intermittent and Ephemeral Streams: Will Landscape Connectivity Sustain Aquatic Organisms in a Changing Climate?
Dr. Julian Olden | University of Washington
This project aims to understand how Southwest intermittent and ephemeral (dryland) streams provide critical habitat and population connectivity for obligatory aquatic species. The project examined how hydrology, hydrologic connectivity, and other riverine characteristics influence the community structure and population genetics (e.g., gene flow, structure, diversity) of amphibian and aquatic insect species across a gradient of flow permanence within Fort Huachuca and the surrounding Sky Island mountain ranges. Specifically, the project addressed three main objectives that aim to provide both the science and management tools needed to ensure the conservation of aquatic species on Department of Defense (DoD) military lands in a rapidly changing environment. First, stream flow was measured to quantify flow permanence and hydrologic connectivity at multiple spatial scales. Second, the distribution and abundance of aquatic insects were characterized and modeled in relation to hydrology, riparian vegetation, and geomorphology. Third, population structure (gene flow) of insect and amphibian species was evaluated with contrasting life histories along a gradient of flow permanence and hydrologic connectivity.
Quantifying the extreme spatiotemporal variability in streamflow of dryland rivers remains an ongoing challenge. This project addressed this knowledge gap by deploying electrical resistance sensors (novel approach to quantify streamflow occurrence at fine temporal intervals) at 40+ locations across Fort Huachuca to quantify network‐scale longitudinal hydrological connectivity. Next, patterns and drivers of aquatic invertebrate communities were accessed using empirical field collections and statistical modeling. Invertebrate diversity was examined from multiple streams in Fort Huachuca that span a flow permanence continuum from highly intermittent to perennial, and the relative roles of flow permanence, habitat size, season, and microhabitat in determining taxonomic and functional (trait) structure were quantified. Then, information was combined on local and regional habitat characteristics to explain spatial patterns of invertebrate diversity and test whether these patterns were predictable based on species’ dispersal abilities. Finally, population structure (gene flow) and landscape genetics of amphibian and aquatic invertebrate species with contrasting life histories were investigated. How species’ ecological strategies affect the regional balance of gene flow was examined within three amphibians [the canyon treefrog (Hyla arenicolor), red-spotted toad (Anaxyrus punctatus), and Mexican spadefoot (Spea multiplicata)] and three aquatic insects [Abedus herberti (Hemiptera: Belostomatidae), Mesocapnia arizonensis (Plecoptera: Capniidae), and Boreonectes aequinoctialis (Coleoptera: Dytiscidae)]. These species utilize a range of ecological strategies, driven primarily by different water dependencies and dispersal abilities, enabling species survival in arid and semiarid environments. Finally, the project examined a suite of hypothesized relationships between genetic connectivity and landscape connectivity across all species.
Substantial within and across canyon hydrologic variability was evident during the study period, which led to differences in patterns of longitudinal connectivity. Increased flow permanence of streams in Fort Huachuca was associated with increased functional richness, functional evenness, and taxonomic richness of invertebrate communities. Conversely, drying events reduced functional diversity across all measured indices. A saturating relationship was identified between functional richness and taxonomic richness, indicating functional redundancy in species-rich communities, which may promote resilience of ecosystem function to environmental variation. The results also suggest that both local and regional factors influenced the structure of invertebrate communities, and the importance of each factor depended on the dispersal capacities of the organisms. Local and weak dispersers were more affected by site-specific factors, intermediate dispersers by landscape-level factors, and strong dispersers showed no discernable pattern. Unlike most other studies of dendritic networks, the results suggest that overland pathways, using perennial refugia as stepping-stones, might be the main dispersal routes in fragmented stream networks.
A positive relationship existed between population differentiation and water dependency for amphibians, e.g., longer larval development periods and site fidelity for reliable water sources. Global genetic differentiation was highest for canyon treefrogs, intermediate for red-spotted toads, and lowest for Mexican spadefoots. Strong hierarchical clustering was present for canyon treefrogs with spatial clustering by mountain range. Red-spotted toads had moderate hierarchical structure with complex spatial patterns of genetic connectivity. Mexican spadefoots had little hierarchical structure with diffuse spatial clustering. Aquatic connectivity exhibited importance for all amphibian species, particularly when considered with topography (slope). The effect of spatial scale differed by species, with canyon treefrogs and Mexican spadefoots characterized by relatively consistent results at different scales in contrast to the stark differences in results for red-spotted toads at different scales. Direction and strength of the relationships between genetic distances and geographic distances between sampling localities matched predictions of population genetic structure according to invertebrate species’ dispersal abilities. Moderate-disperser
Mesocapnia arizonensis has a strong isolation-by-distance pattern, suggesting migration-drift equilibrium; whereas, population structure in the flightless Abedus herberti is influenced by genetic drift, and gene flow is the dominant force in the strong-flying Boreonectes aequinoctialis. Analyses also identified a strong spatial scale-dependence, in which landscape genetic methods performed well only for species that were intermediate dispersers.
Rapid environmental change and limited management resources necessitate efficient and effective conservation planning that promotes the persistence of aquatic species in dryland environments. This research highlights the key role of hydrology in determining aquatic invertebrate diversity in dryland streams of Fort Huachuca and surrounding Sky Island mountain ranges. These findings emphasize the need to manage river systems for organisms that span a wide variety of dispersal abilities and local ecological requirements, and they highlight the need to preserve perennial refugia in fragmented networks, as they may ensure the viability of aquatic communities by facilitating recolonization after disturbance.
Knowledge of population attributes such as structure, connectivity, and genetic integrity are a fundamental part of successful conservation. In this project, landscape genetic approaches were applied to integrate population genetics with emerging spatial statistics to examine how hydrology and the terrestrial matrix affects population genetic structure, diversity, and differentiation of obligatory aquatic species. Research findings highlight the utility and potential of species’ ecological traits, in this case water dependency for amphibians and dispersal for aquatic invertebrates, in characterizing relationships between genetic and structural (landscape) connectivity. Genetic diversity is often a missing component in conservation planning and resource allocation despite its recognized role in species persistence. With increasing human demand for aquatic resources in arid environments, environmental change and habitat alteration will likely outpace the resources and time necessary for single-species population genetics studies for many species of conservation concern. Using ecological information to predict relationships between genetic and landscape connectivity is a promising approach for multi-taxa inference and may help inform conservation efforts in which single-species genetic studies are not possible.