Influence of Bio-Pollution on Ecosystem Processes: The Impact of Introduced Lake Trout on Streams, Predators, and Forests in Yellowstone National Park

Abstract

Recently an unauthorized introduction of lake trout (Salvelinus namaycush) to Yellowstone Lake was documented. Recent investigation at the University of Wyoming indicated that in-lake predation by lake trout on juvenile and sub-adult native Yellowstone cutthroat trout (Oncorhynchus clarki bouvieri) could negatively influence recruitment of cutthroat trout (Stapp and Hayward 2002). This may lead to significant reductions in numbers of spawning adult cutthroat if current management actions are ineffective or if they are not continuously pursued (Stapp and Hayward 2002). While lake trout invasion in Yellowstone Lake will likely have detrimental effects on in-lake communities and processes, a reduction in the native cutthroat trout population could potentially impact other aquatic and terrestrial ecosystems outside of Yellowstone Lake. Cutthroat trout in Yellowstone Lake annually migrate into tributary streams and rivers to spawn (Varley and Gresswell 1988), with runs up to 60,000 trout per season into small streams such as Clear Creek (Gresswell and Varley 1988). This spawning migration may significantly affect in­stream communities (cf. Power 1990) and alter nutrient cycling within tributary streams (Peterson et al. 1993) and in the adjacent riparian forests (Ben­David et al. 1998; Hilderbrand et al. 1999). Therefore, spawning cutthroat trout not only have trophic effects on their ecosystem but also act as "ecosystem engineers" (i.e., species that influence structure and function of ecosystems through non­trophic processes) because of their role in transporting large amounts of nutrients between ecosystems (Jones et al. 1994). Reductions in spawning adult cutthroat trout will likely alter in­stream processes. In addition, for piscivorous (fish­ eating) predators, a significant decline in the number of adult spawning cutthroat trout may reduce recruitment and survival, and it could threaten viability of predator populations. In this project we are investigating the importance of cutthroat trout to a representative fish­predator - the river otter (Lontra canadensis), and possible effects on terrestrial plants through nutrient transport by otters to latrine sites (Ben-David et al. 1998; Hilderbrand et al. 1999). We hypothesized that the spawning migration of cutthroat trout will result in transport of nutrients from lake to streams, and from streams to terrestrial forests, through the activity of river otters. Documentation of such transport will enable us to predict how trout predators and the terrestrial landscape will be affected following cutthroat trout declines. Specifically, we predicted that: 1. Spawning cutthroat trout will be seasonally a major food resource to river otters. 2. Spawning migrations of cutthroat trout and the resulting predation by river otters will create a flux of nitrogen (N) to riparian forests. 3. Tree-ring widths and stable nitrogen isotope values of tree-rings from river otter latrine sites will reflect temporal changes in cutthroat trout and otter abundance. 4. Future reductions in spawning cutthroat trout will lead to declines in number of otters.