Employing Stable Isotopes to Investigate the Impacts of Invasive Species on Hawaiian Stream Food Webs

ABSTRACT EMPLOYING STABLE ISOTOPES TO INVESTIGATE THE IMPACTS OF INVASIVE SPECIES ON HAWAIIAN STREAM FOOD WEBS by Megan Layhee Master of Science in Biological Sciences California State University, Chico Spring 2011 Anthropogenic disturbance around the world is restructuring ecosystems and changing interactions within ecological communities. In the Hawaiian Islands, one of the most significant forces of disturbance is the widespread presence of invasive species. In this study stable isotopes of carbon and nitrogen were used to examine trophic and energy flow changes occurring in Hawaiian stream communities due to biotic disturbances including invasive species. On the island of Kaua’i, we extensively sampled all members of the stream community, including primary producers and consumers, from four streams with varying levels of disturbance. The streams chosen in this study included Limahuli, Kapa’a, Hulē’ia and ‘Opaeka’a. δ13C, δ15N signatures, and trophic positions of stream members were calculated to determine differences in overall food web structure, changes to native consumer isotopic signatures, and differences in food web diversity and trophic redundancy. Comparison of streams with varying levels of disturbance showed that 1) overall food web structure varied dramatically among locations, 2) natives were nearly absent in heavily disturbed and highly invaded streams while at the same time a suite of non-native consumers are replacing the trophic roles left by the natives, 3) isotopic signatures and trophic position of native consumers were not significantly different across streams, and 4) heavily disturbed and highly invaded streams had more trophic diversity and generally lower trophic redundancy. Our study was able to quantitatively depict and compare differences in both the structure and trophic interactions of Hawaiian freshwater ecosystems webs due to varying levels of disturbance and species invasion.