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1 January 2010 Invasive Hollies (Ilex, Aquifoliaceae) and Their Dispersers in the Pacific Northwest
Peter F. Zika
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Naturalized Ilex aquifolium L. (English holly) was first collected in the Pacific Northwest in 1953, based on herbarium records. Field surveys showed it is now commonly naturalized from northwestern California to coastal British Columbia. Ilex crenata Thunb. and I. opaca Aiton were also found growing outside of cultivation, but rarely. A key and seed illustrations are provided to distinguish these three Ilex species. Between 2003 and 2006 twice-weekly visits to naturalized and cultivated hollies in Seattle revealed seven species of birds disseminating seeds by eating the fruits. American robins, Turdus migratorius, accounted for 96% of 2796 frugivory observations on I. aquifolium, followed by European starlings, Sturnus vulgaris (3.2%). Ilex aquifolium fruits ripened in October and persisted for six months, yet 99% of all fruit was consumed between November and February. A study of I. aquifolium seed fate found pre-dispersal diurnal seed predation was rarely observed. Bird-regurgitated seed was more frequently attacked by nocturnal rodents in a sheltered forested setting in Clark Co., Washington (39% losses), compared to an exposed urban setting in Seattle (2% losses). The percentage of viable seed surviving rodent attack was higher in the urban sample (66%) than in the forest sample (24%). Commercial and ornamental use of I. aquifolium is extensive in the coastal region and less-invasive alternatives should be considered, to provide food and cover for urban avians without degrading natural areas.

Peter F. Zika "Invasive Hollies (Ilex, Aquifoliaceae) and Their Dispersers in the Pacific Northwest," Madroño 57(1), 1-10, (1 January 2010).
Published: 1 January 2010

American Robin
English holly
Ilex aquifolium
invasive plants
seed dispersal
seed predators
Turdus migratorius
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