Habitat fragmentation provides a competitive advantage to an invasive tree squirrel, Sciurus carolinensis
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Changes in the composition of biological communities can be elicited by competitive exclusion, wherein a species is excluded from viable habitat by a superior competitor. Yet less is known about the role of environmental change in facilitating or mitigating exclusion in the context of invasive species. In these situations, decline in a native species can be due to the effects of habitat change, or due to direct effects from invasive species themselves. This is summarized by the “driver-passenger” concept of native species loss. We present a multi-year study of tree squirrels that tested the hypothesis that tree canopy fragmentation, often a result of human development, influenced the replacement of native western gray tree squirrels (Sciurus griseus) by non-native eastern gray tree squirrels (Sciurus carolinensis). We tested this hypothesis along a continuum of invasion across three study sites in central California. We found that within the developed areas of the University of California at Santa Cruz campus and city of Santa Cruz, S. carolinensis excluded S. griseus from viable habitat. The competitive advantage of S. carolinensis may be due to morphological and/or behavioral adaptation to terrestrial life in fragmented hardwood forests. We classify S. carolinensis as a “driver” of the decline of native S. griseus in areas with high tree canopy fragmentation. Future habitat fragmentation in western North America may result in similar invasion dynamics between these species. Our study warrants consideration of existing and predicted interactions between potentially invasive species that co-occur with native species where land use change is proposed.
KeywordsHabitat fragmentation Invasive species Competitive exclusion Squirrel Sciurus carolinensis Sciurus griseus
We thank the James Stuart Chanley Memorial Foundation for providing funds for this study. We also thank Gage Dayton and Alex Jones of the UC Natural Reserve System and Chris Lay, curator of the UCSC Museum of Natural History, for their generous input throughout the study. Gratitude is also due to the numerous graduate and undergraduate students, including the UCSC Small Mammal Research Team, who contributed to the field work. Finally, we thank Rianne Diepstraten and two anonymous reviewers for their thoughtful comments and edits.
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