FRUITING PLANT ATTRACTIVENESS TO AVIAN SEED DISPERSERS: NATIVE VS. INVASIVE CRATAEGUS IN WESTERN OREGON

Crataegus monogyna, a hawthorn native to Europe, has successfully invaded much of North America since its introduction approximately 200 years ago. Successful dispersal by avian frugivores, relative to the native biota, may be one reason why C. monogyna is so invasive. To address this hypothesis I compared the attractiveness of С. monogyna and С. douglasii suksdorfii (the native hawthorn) to their primary dispersal agent (American robins) in western Oregon. A companion study in western Oregon (Sallabanks 1993) identified three Crataegus traits correlated with pome removal by robins: pome crop size, mean pome size, and mean pome pulp-to-pyrene ratio. With respect to these dispersal-related traits, C. monogyna was found to be superior to its native counterpart, producing larger displays of higher quality pomes. These results offer an explanation for the observed patterns of distribution and abundance of C. monogyna and C. douglasii suksdorfii at the study site specifically, and in western Oregon in general. To conclude I suggest possible recommendations for management against С. monogyna and for frugivorous birds. Some animal-dispersed plants produce fleshy fruits as a nutritional reward to frugivores in exchange for seed dissemination (Howe 1 986). The attractiveness of fruiting plants to frugivores plays an essential role in dispersal success and may have important implications for plant species invasion. Frugivorous mammals have significantly accelerated the invasion of the western United States by Carpobrotus edulis L. (Aizoaceae) (D'Antonio 1990). Birds were found to play an important role in the invasion of mediterranean-climate zone South Africa by Acacia cyclops A. Cunn. ex G. Don. (Mimosoideae) (Glyphis et al. 1981). Similarly, birds dispersed the seeds of Myrica faya Ait. (Myricaceae), enabling this species to colonize Hawaii (LaRosa et al. 1985; Walker 1990; Woodward et al. 1990). In the western United States, the native hawthorn, Crataegus douglasii Lindl, var. suksdorfii Sarg. (Rosaceae), is restricted in its range from British Columbia to northern California, and east through Idaho to Montana (Hitchcock et al. 1961; Brunsfield and Johnson 1990; T. A. Dickinson and R. M. Love personal communications). The invasive European hawthorn, С. monogyna Jacq. , arrived in the United States in the early 19th century (Douglas 1914) and has 1 Current address: Department of Zoology, North Carolina State University, Box 7617, Raleigh, NC 27695, USA. Madrono, Vol. 40, No. 2, pp. 108-116, 1993 This content downloaded from 157.55. 39.27 on Wed, 07 Sep 2016 06:27:15 UTC All use subject to http://about.jstor. org/terms 1993] SALLABANKS: DISPERSAL OF INVASIVE CRATAEGUS 109 since spread rapidly throughout much of North America, doing particularly well in the Pacific Northwest and the Great Lakes-New England regions. Crataegus is dispersed by producing polypyrenous pomes (fruits) to attract primarily avian frugivores (De Boer 1979; Courtney and Manzur 1985; Herrera 1987; Dickinson and Campbell 1991; Sallabanks 1992). In this paper I focus on a study site in the Willamette Valley of western Oregon to examine differences in the dispersal potential between the native and invasive Crataegus . The results presented here may be applicable to other geographic regions; in Australia, for example, C. monogyna is expanding its range at a rate of 1 20 m/yr (Smith 1985) due to dispersal by birds (Mulvaney 1986; Bass 1989) and marsupials (Bass 1990). Specifically, I compare the attractiveness of both C. monogyna and C. douglasii suksdorfii (henceforth "C. suksdorfii ") to their primary dispersal agent, the American robin, Tur dus migratorius L. In a companion study conducted at the same study site (Sallabanks 1993), it was determined which plant traits were correlated with the foraging choices of robins. Here I compare С. monogyna and C. suksdorfii with respect to these dispersal-related traits and discuss implications for the native and invasive species. More extensive data on fruiting and frugivory in the C. monogyna T. migratorius system per se are documented elsewhere (Sallabanks 1992).