Terrestrial-aquatic linkages in human-altered landscapes

Abstract

Streams and adjacent riparian zones are intimately linked by the flow of resource subsidies between terrestrial and aquatic habitats. Landscape-level changes in land use can have profound impacts on riparian structure and stream health, and may alter the flow of resource subsidies across the stream-riparian boundary. Yet, terrestrial-aquatic linkages have not been well-studied in human-impacted landscapes. Here, I examine energy flows across the stream-riparian boundary in agricultural and suburban landscapes in Maryland. I study the effects of terrestrial resource subsidies (grass and herbaceous vegetation, periodical cicada detritus) on stream ecosystem processes and consumers and the effects of one aquatic subsidy (emerging aquatic insects) on agriculturally important consumers, wolf spiders (Lycosidae).

I present strong evidence for terrestrial-aquatic linkages where large quantities of high quality, allochthonous resources subsidize stream ecosystems. Herbaceous vegetation and grasses growing along the edges of agricultural headwater streams provide significant quantities of organic matter that are rapidly decomposed and support a diverse macroinvertebrate community. Further, the dense vegetation appears to limit light to algae growing on the stream bottom. Detritus from 17-year periodical cicadas (<em>Magicicada</em> spp.) that falls into forested suburban streams provides an intense pulse of terrestrial resources that is unusual for the summer, but is locally utilized and causes dramatic increases in whole-stream community respiration.

I provide weak evidence for a terrestrial-aquatic linkage between emerging aquatic insects and lycosid wolf spiders inhabiting agro-ecosystems in central Maryland. Results from field studies indicate that wolf spiders are generally more abundant in the riparian buffers adjacent to corn fields, and exhibit neither alternating abundance nor net movement between the field and buffer habitats throughout the year. While wolf spiders consume adult aquatic insects in the lab, I could not resolve the specific contribution aquatic insects make to the diets of field-collected wolf spiders.

Terrestrial-aquatic linkages are important in human-altered ecosystems, and have significant implications for the conservation and restoration of impacted habitats and ecosystem services. The relative strength of these linkages, however, depends on the species involved, the direction of the subsidy flux, the nature of the land-water boundary, and the temporal context in which they occur.