THE LAND COVER CASCADE: LINKING TERRESTRIAL AND AQUATIC SUBSYSTEMS

Cascades can be thought of as a series of cause-effect couples that are relevant to the transfer of materials and energy in physical and biological systems. Here we present the land cover cascade (LCC) concept as both a heuristic and mechanistic approach to understanding how terrestrial disturbance is propagated through watershed subsystems to affect biota in the streams that drain them. We provide operational definitions for intervening components that occur at different spatial scales (elements), variables that characterize cascade elements (entities), cause-effect couples that exist among elements (links), and interactions among elements that join land cover change to biotic responses (cascades). Using a path analysis statistical approach we addressed how watershed hydrology, geomorphology, and sediment transport and deposition participated in cascades altering fish and macroinvertebrate assemblages in 10 watersheds with different extent of land cover change. Land cover was quantified from Multi-Resolution Land Characteristics Consortium (MRLC) Landsat imagery using GIS. Twenty-nine MRLC land-cover categories were reduced to agriculture, urban, forest, and other, and included with estimates of riparian cover and cover in zones related to water travel time as measures of variable land use. Path analysis identified 13 significant (Chi square goodness-of-fit test) cascades linking land cover to invertebrate (7 total) and fish (6 total) community metrics. Two cascades were evidently stronger than all other models (P > 0.70) linking urban and agricultural cover to total fish density and collector-filterer insect abundance. These cascades propagated disturbance through geomorphic (bank height, width:depth), hydrologic (rating curve slope), and sediment characteristics (e.g., D50, percent organic matter). Thus, the LCC characterized mechanistic pathways linking land and water and identified key response variables sensitive to terrestrial disturbance. While the LCC is specifically designed to assess the influence of anthropogenic activities on aquatic communities, the approach is broadly applicable to studies of terrestrial-aquatic interaction.