QUANTIFYING CHANGE IN WETLAND EXTENT AND ITS EFFECT ON A WESTERN AND CLARK'S GREBE BREEDING POPULATION AT EAGLE LAKE, CA 1997-2010

Renee E Robison; Colibri Ecological Consulting; rrobison@colibri-ecology.com; Daniel W. Anderson, Kristofer M. Robison

Western and Clark's grebes (Aechmophorus occidentalis and A. clarkii; respectively) are sympatric water birds that breed at inland lakes and reservoirs. Both species are wetland-obligate nesters that build floating nests constructed-of and anchored-to emergent vegetation. Despite grebe dependence on flooded wetland habitat for nesting, the relationships among lake level, wetland availability, and reproductive success have not been well quantified. We modeled thirteen years of reproductive data collected at Eagle Lake, California. This lake is a closed-drainage system, whose annual lake level varies drastically with annual rainfall and evaporation. We used remote sensing techniques to quantify changes in wetland availability relative to inter-annual lake level changes. An information-theoretic approach was used to assess the relationship among and importance of these habitat variables in relation to grebe reproductive success. Our results indicate that lake level is a key factor affecting wetland availability and population-level reproductive success. Our findings also indicate an optimum water level where management is likely to be most effective. We strongly recommend that wetland managers consider the effects of water level on breeding habitat availability and reproductive success of grebes and other over-water nesting birds.

The Effects of Drought and Water Management on Wildlife