The lateral extent of the subsidy from an upland stream to riparian lycosid spiders


  • Robert A. Briers,

  • Helen M. Cariss,

  • Rory Geoghegan,

  • John H. R. Gee

R. A. Briers (, H. M. Cariss, R. Geoghegan and J. H. R. Gee, Inst. of Biological Sciences, Univ. of Wales Aberystwyth, Edward Llwyd Bldg, Aberystwyth, Ceredigion, UK SY23 3DA (present address of R. A. B.: School of Life Sciences, Napier Univ., Merchiston Campus, Edinburgh, UK EH10 5DT. Present address of H. M. C.: Dept of Environmental and Geographical Sciences, Manchester Metropolitan Univ., Chester St., Manchester, UK M1 5GD).


Adult aquatic insects emerging from streams can subsidize riparian food webs, but little is known of the spatial extent of these subsidies. Stable isotope (15N) enrichment of aquatic insects, principally a species of stonefly (Plecoptera: Leuctridae), emerging from an upland stream was used to trace the subsidy from the stream ecosystem to riparian spiders (Lycosidae). The downstream profile of spider δ15N correlated closely with that of adult stoneflies, indicating that they were deriving nutrition from aquatic sources. The contribution of adult aquatic insects to spider diets was determined using a two-source mixing model. Adult aquatic insects made up over 40% of spider diets adjacent to the stream, but <1% at 20 m from the stream. Enrichment of riparian spiders declined exponentially with distance from the stream channel. Aquatic-terrestrial subsidies were spatially restricted, but locally important, to riparian lycosid spiders at the study site.