Land use effects on ecological linkages between small streams and their surrounding terrestrial habitats

Abstract: Adult aquatic insects are important vectors for aquatic transfers to terrestrial consumers and an integral component of riparian and terrestrial food webs. Incorporation of aquatic subsidies into terrestrial food webs depends heavily on the dispersal and life history traits of aquatic insects. Agricultural land use often results in the degradation of in-stream and riparian habitats which may affect the efficiency of cross-habitat exchanges. In this thesis I studied; (i) how land use (forested and agriculture) affects stream invertebrate assemblages, with special focus on aquatic insects, (ii) how riparian habitats differ with land use and how these differences affect dispersal of emerged adult aquatic insects, and (iii) if the distribution of riparian arthropod consumers is related to food resources (aquatic and terrestrial dipterans) and/or riparian habitat (e.g., substratum and microclimate). Subsidy production (abundances of adult aquatic insects) was over five times greater in agricultural streams, however, most emerging insects dispersed no further than 10m from the stream edge. In contrast, dispersal of adult aquatic insects declined little with distance in the forested sites. Furthermore, the abundance of arthropod consumers was lower at agricultural sites, compared to forested sites. Taken together, these results indicate that agricultural practices result in reduced subsidy resource exchange efficiency, where energy flow to riparian habitats is weakened relative to the productivity of the stream. Specifically, cross-habitat transfers are constrained by factors associated with land use, microhabitat and phenology. Land use has been largely ignored in studies of aquatic to terrestrial cross-habitat resource exchanges. This thesis illustrates the interdependence of spatial resources and how anthropogenic alterations can disrupt cross-scale linkages. This thesis brings attention to the importance of aquatic-terrestrial linkages in forested and agricultural landscapes, demonstrating that anthropogenic impacts may have deeper consequences for aquatic and terrestrial ecosystems than have been previously identified.

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