Urban Plants as Genetic Reservoirs or Threats to the Integrity of Bushland Plant Populations DAVID G. ROBERTS ∗ , DAVID J. AYRE, AND ROBERT J. WHELAN Institute for Conservation Biology, School of Biological Sciences, University of Wollongong, N.S.W. 2552, Australia Abstract: Remnant plants in urban fringes and native plants in gardens have the potential to contribute to the conservation of threatened plants by increasing genetic diversity, effective size of populations, and levels of genetic connectedness. But they also pose a threat through the disruption of locally adapted gene pools. At Hyams Beach, New South Wales, Australia, four bushland stands of the rare shrub, Grevillea macleayana McGillivray, surround an urban area containing remnant and cultivated specimens of this species. Numbers of inflorescences per plant, fruits per plant, and visits by pollinators were similar for plants in urban gardens and bushland. Urban plants represented a substantial but complex genetic resource, displaying more genetic diversity than bushland plants judged by H e , numbers of alleles per locus, and number of private alleles. Of 27 private alleles in urban plants, 17 occurred in a set of 19 exotic plants. Excluding the exotic plants, all five stands displayed a moderate differentiation (F ST = 0.14 ± 0.02), although the urban remnants clustered with two of the bushland stands. These patterns may be explained by high levels of selfing and inbreeding in this species and by long-distance dispersal (several seeds in the urban stand were fathered by plants in other stands). Genetic leakage (gene flow) from exotic plants to 321 seeds on surrounding remnant or bushland plants has not occurred. Our results demonstrate the conservation value of this group of urban plants, which are viable, productive, genetically diverse, and interconnected with bushland plants. Gene flow has apparently not yet led to genetic contamination of bushland populations, but high levels of inbreeding would make this a rare event and difficult to detect. Remnant plants in urban gardens could successfully contribute to recovery plans for endangered and vulnerable species. Keywords: bushland plant populations, gene flow, genetic diversity, Grevillea, habitat fragmentation, pollen dispersal, urban gardens Plantas Urbanas como Reservorios Gen´ eticos o Amenazas a la Integridad de Poblaciones de Plantas de Matorrales Xer´ ofitos Resumen: Las plantas remanentes en los bordes urbanos y las plantas nativas en los jardines tienen el potencial para contribuir a la conservaci´ on de plantas amenazadas al incrementar el tama˜ no poblacional efectivo y los niveles de conectividad gen´ etica. Pero tambi´ en constituyen una amenaza por la disrupci´ on de pozas g´ enicas adaptadas localmente. En Hyams Beach, Nuevo Gales del Sur, Australia, cuatro poblaciones del arbusto raro, Grevillea macleayana McGillivray, rodean un ´ area urbana que contiene especimenes remanentes y cultivados de esta especie. El n´ umero de inflorescencias por planta, frutos por planta y visitas de polinizadores fue similar para plantas en jardines urbanos y en el matorral xer´ ofito. Las plantas urbanas representaron un recurso gen´ etico sustancial pero complejo, ya que presentaron mayor diversidad gen´ etica que las plantas silvestres, considerando H e , el n´ umero de alelos por locus y el n´ umero de alelos privados. De 27 alelos privados en plantas urbanas, 17 ocurrieron en un conjunto de 19 plantas ex´ oticas. Excluyendo las plantas ex´ oticas, las cinco poblaciones mostraron una diferenciaci´ on moderada (F ST = 0.14 ± 0.02), aunque los remanentes urbanos se agruparon con dos de las poblaciones silvestres. Estos patrones pueden ser explicados por los altos niveles de autogamia y endogamia en esta especie y por la dispersi´ on de distancia larga (varias semillas en la poblaci´ on urbana fueron producidas por plantas en otras poblaciones). El derrame gen´ etico ( flujo g´ enico) de ∗ email dgr042@uow.edu.au Paper submitted August 9, 2006; revised manuscript accepted November 15, 2007. 842 Conservation Biology Volume 21, No. 3, 842–852 C 2007 Society for Conservation Biology DOI: 10.1111/j.1523-1739.2007.00691.x