Response of fish communities in five north temperate lakes to exotic species and climate

Exotic species invasions and climatic variation that occurred in north central Wisconsin provided the opportunity to contrast influences of climate and invasive species in shaping fish community dynamics in five Wisconsin lakes from 1981 to 2001. Year (passage of time) was positively correlated (r . 0.9) with invasive species. Year, invasive species, and climatic variables were the principal determinants of fish community dynamics according to redundancy analysis (RDA). The same invasive species negatively affected the same general group of species in different lakes; benthic invertivores declined in lakes invaded by crayfish, Orconectes spp., and pelagic zooplanktivores declined in lakes invaded by rainbow smelt, Osmerus mordax. Micropterus dolomieu, Pimephales notatus, and Notropis volucellus increased in abundance in lakes invaded by crayfish. When year was included in RDAs as a covariable (partial RDA), fish community changes in three lakes did not differ significantly from ordinations of Monte Carlo simulated data; for two lakes, growing season length and lake productivity were significant explanatory variables under the reduced model. The resulting shifts in the fish community might represent declines in those species that used resources similar to the invading crayfish and smelt or were eaten by the invading smelt. Subsequent community compensation might have occurred, with other species increasing in response to the declines in species affected directly by the invasive species. Sala et al. (2000) identified climatic variation and biotic exchange as two of the most pervasive threats to biodiversity in freshwater lakes over the next century. However, until recently, documenting the ecological effects of biotic exchange has been far more tractable than documenting ecological effects of climate change. Predation, competition, and loss of habitat are often and readily documented effects of invasive species. For fish, classic case studies can be found in Lake Victoria, Africa, the Laurentian Great Lakes, and Panama (Zaret and Paine 1973; Gitay et al. 2001; Vanderploeg et al. 2002). Invasive species introduced either by accident or through fisheries management (Rahel 2002) often radically change the functioning of aquatic ecosystems; ripple effects created by the extirpation of native fish fauna include reduced ecosystem goods and services that affect resource users economically and culturally (Gitay et al. 2001). Bridging the relationships between climatic variation and fish and fisheries has become more tenable in the last decade with the emergence of more long-term records and techniques. Long-term records of both climate and fisheries landings are the most readily available data combinations that demonstrate a response of fish to changing climate (Krovnin et al. 2001). Records for centuries of climate variation developed from lake core paleolimnology indicate that natural climate variation is a strong actor on lake productivity (Rusak et al. 2004; Smol et al. 2005) and fish life history (Finney et al. 2000). Evolving data sets from long established ecological programs in freshwater systems (e.g., Experimental Lakes Area, the English Lake District, Lake Washington, etc.) have made it possible to look across decades and consider multiple fish taxa when estimating how climate, invasive species, and anthropogenic influences affect the functions of freshwater ecosystems. It is possible that the combination of climatic variation, now widely regarded as climate change, and invasive species will interact to exacerbate changes in fish species composition in freshwater lakes, especially in temperate and northern regions. The resulting marriage of novel invaders and climate could push lake ecosystems and fish communities in unexpected directions. Habitats in northern regions will become more susceptible to invaders as seasonal temperatures that prevented establishment of less cold adapted species become more moderate (Magnuson et al. 1997; Gitay et al. 2001). Warming could benefit native fishes, provided sufficient food resources exist to satisfy the energetic needs of natives and the additional demands that invaders place on ecosystems (Gitay et al. 2001). However, unexpected interactions between lake ecosystems and climate (e.g., higher water temperatures affecting composition and quality of primary producers, which ripples up the food web [Beisner et al. 1997]) might change the nature of competitive interactions between native and invasive fish 1 To whom correspondence should be addressed. Present address: Aquatic Systems Group, University of Southern Maine, 350 Commercial Street, Portland, Maine 04101 (theowillis06 @aim.com). Acknowledgments We thank the staff at the University of Wisconsin’s Trout Lake Station of the Center for Limnology for field assistance and support over many years. This research would not have been possible without the help of many volunteers, graduate students, and summer workers who collected the data presented here, including T. Kratz, T. Meinke, and P. Montz. B. Benson and D. Balsinger provided invaluable database support to the North Temperate Lakes Long-Term Ecological Research (NTL-LTER) project. The manuscript was improved with many comments from K. Wilson, as well as E. Stanley, D. Jackson, R. Paavola, and four anonymous reviewers, and the considerable patience of N. Yan and the Limnology and Oceanography editorial staff. Support for T.V.W. and J.J.M. was provided in part by the National Science Foundation (DEB 9632853; NTL-LTER) and the University of Wisconsin-Madison. Limnol. Oceanogr., 51(6), 2006, 2808–2820 E 2006, by the American Society of Limnology and Oceanography, Inc.