ORIGINAL PAPER Exotic grass invasion impacts fitness of an endangered prairie butterfly, Icaricia icarioides fenderi Paul M. Severns Received: 31 March 2007 / Accepted: 26 June 2007 / Published online: 25 July 2007 Ó Springer Science+Business Media B.V. 2007 Abstract Fender’s blue butterfly is an endangered spe- cies restricted to fragmented, grassland remnants that are becoming increasingly dominated by tall, invasive grasses in western Oregon, USA. I performed a removal experi- ment to assess the impacts of structural degradation accompanying the invasion of Arrhenatherum elatius, tall oat grass, on butterfly fitness and fitness related behaviors. Clipping of A. elatius to native grass sward height resulted in 2.5–5 times as many eggs laid per leaf of host plant. Both male and female butterflies basked more frequently in areas removed of A. elatius inflorescences and upon encountering the treatment edge butterflies had a high rate of return into a large area removed of the grass inflores- cences. Although butterfly behavior appeared to be affected by the change in sward height on the treatment edge, there was no evidence for the edge causing a disproportionate egg load. Invasion and dominance by A. elatius appeared to diminish host plant apparency which may result in over- loading of eggs on conspicuous host plants, increased incidence of emigration, and a decrease in the likelihood of colonization because female butterflies appeared indiffer- ent to larval resources beneath A. elatius inflorescences. Dominance of natural shortgrass prairies by tall stature grasses like A. elatius may be an insidious form of habitat degradation for grassland Lepidoptera worldwide, but it may go largely unnoticed because larval and adult re- sources can persist under the unnaturally tall grass canopy. Keywords Invasive species Á Grassland Á Oviposition preference Á Habitat quality Á Thermoregulation Á Butterfly behavior Introduction Exotic plant invasion may change the behavior of animals by modifying fundamental aspects of the invaded native habitat. For example, invasion and dominance by exotic plants that change the structure of open spaces may be detrimental to fauna requiring open habitats. Butterflies are sensitive to changes in habitat structure such as the edges of grasslands (Reis and Debinski 2001; Schultz and Crone 2001), hilltops (Shields 1967; Lederhouse 1982), forest edges (DeVries et al. 1999; Haddad 1999), perches taller than the surrounding habitat (Rutowski 2000), roads (Munguira and Thomas 1992), and microtopographic changes that afford protection from wind and access to basking sites (e.g., Thomas et al. 1986). Butterfly sensi- tivity and preference for specific habitat features suggests that exotic plants that change habitat structure may directly impact butterfly behavior. For example, changes in vege- tation structure may degrade basking site quality and sub- sequently interfere with thermoregulation. This may further impact reproductive related behaviors like mate searching, territorial defense, predator avoidance, oviposition, and nectaring (Clench 1966; Heinrich 1986; Shreeve 1986; Stutt and Willmer 1998; Ide 2002; Berwaerts and Van Dyck 2004). Host plant apparency may diminish if the presence of taller plants physically obscures the host plant (Wiklund 1984; Karban 1997; Floater and Zalucki 2000) or oviposition likelihood may decline if the taller plants alter preferred egg laying environments (Williams 1981; Tho- mas et al. 1986). P. M. Severns (&) Department of Botany and Plant Pathology, Oregon State University, 2082 Cordley Hall, Corvallis, OR 97331, USA e-mail: severnsp@onid.orst.edu 123 J Insect Conserv (2008) 12:651–661 DOI 10.1007/s10841-007-9101-x