Interference competition between the net-building caddisfly Ceratopsyche oslari and the amphipod Gammarus lacustris

We investigated the interspecific interactions between the amphipod, Gammar us lacustris (Sars) and the cosmopolitan caddisfly, Ceratopsyche oslari (Ba nks). We believe this is an example of interference competition and may be one of several factors that limits the distribution of C. oslari from the C olorado River below Glen Canyon Dam. Field observations indicated that C. o slari was associated with fewer food capture nets in the Colorado River tha n in the Paria River tributary. Laboratory experiments indicated the amphip od Gammarus lacustris, which is abundant in the Colorado River but absent f rom the Paria River, had a detrimental effect on net-building success by C. oslari. Net numbers in laboratory chambers declined significantly after 6 d due to destruction by the swimming and feeding activities of G. lacustris . Competition with the introduced, non-native G. lacustris for interstitial spaces along with several abiotic factors, may limit the distribution of C . oslari in the Colorado River.