Engaging Birds in Vegetation Restoration after Elwha Dam Removal

Abstract

Elwha River restoration is humanity's largest dam removal project to date. Reservoir revegetation will be most effective if it is rapid and spatially extensive to stabilize residual sediment, impede invasive plants, and restore other riparian functions. I evaluated how birds could enhance Elwha restoration by dispersing native seeds to reservoir sediment deposits. I evaluated three factors affecting avian seed dispersal: (1) number of native woody plants with bird-dispersed seeds; (2) identity and abundance of seed-dispersing birds; and (3) distribution of avian seed deposition. (1) I sorted Elwha early seral woody plants into species dispersed by birds vs. other mechanisms. (2) I recorded avian flights from seed sources, and I estimated disperser abundances from Elwha avian inventory data. (3) I assessed patterns of avian seed dispersal by evaluating eight causal hypotheses, using fresh scat as a seed surrogate. I recorded scat density and habitat characteristics along random transects in Lake Mills reservoir delta and Geyser Valley floodplain. I evaluated models for each hypothesis using Akaike's Information Criterion. Birds disperse seeds of most (59%) native woody plants likely to establish in the reservoirs. American robins ( Turdus migratorius ) accounted for most flights between seed sources and sediment deposits, with mean abundance 1 bird/ha. Most avian scats occurred on logs, and scat density increased with log volume. These results suggest birds can disperse native seeds throughout Elwha restoration sites. Revegetation programs associated with dam removal can leverage birds as restoration agents by retaining or placing large woody debris to recruit fruit-bearing plants.