Molecular Ecology (2007) 16, 1303–1314 doi: 10.1111/j.1365-294X.2006.03204.x © 2007 The Authors Journal compilation © 2007 Blackwell Publishing Ltd Blackwell Publishing Ltd Extensive pollen dispersal in a bird-pollinated shrub, Calothamnus quadrifidus, in a fragmented landscape M. BYRNE, C. P. ELLIOTT, C. YATES and D. J. COATES Science Division, Department of Conservation and Land Management, Locked Bag 104, Bentley Delivery Centre, WA 6983, Australia Abstract Pollen dispersal was investigated in six populations of Calothamnus quadrifidus, a bird- pollinated shrub in the fragmented agricultural region of southern Western Australia. Paternity analysis using six microsatellite loci identified a pollen source within popula- tions for 67% of seedlings, and the remainder were assumed to have arisen from pollen sources outside the populations. Outcrossing was variable, ranging from 5% to 82%, and long-distance pollen dispersal was observed in all populations with up to 43% of pollen sourced from outside the populations over distances of up to 5 km. This extensive pollen immigration was positively associated with population size but not isolation. Comparison of two populations of similar size but different density showed greater internal pollination and less selfing in the denser population, suggesting an influence of density on pollinator behaviour. The study revealed extensive long-distance pollen dispersal for C. quadrifidus within this fragmented agricultural landscape and highlighted the interaction between reserve populations and isolated road verge remnants in maintaining genetic connectivity at the landscape scale. Keywords: Calothamnus, dispersal function, fragmentation, paternity analysis, pollen dispersal, twogener Received 25 June 2006; revision received 1 September 2006; accepted 14 October 2006 Introduction Landscape fragmentation impacts on many biological pro- cesses including the movement and dispersal of organisms, rates of gene flow and invasion by exotic competitors, and is now regarded as one of the most important factors contributing to the loss of plant biodiversity (Heywood & Iriondo 2003). Investigations on the genetic effects of fragmentation on plant populations have most commonly focused on reduction in population size and the effects of small population processes, particularly loss of genetic diversity and increase in inbreeding (Young & Clarke 2000), yet the amount of pollen dispersal and the distance over which it occurs will have significant impacts on genetic and reproductive processes in fragmented populations. Interpretation of ecological and genetic parameters, such as outcrossing, seed set and seed viability, may be influenced by the pattern of pollen dispersal across the landscape. For example, a reduction in seed set could occur through inbreeding due to reduction in pollen dispersal or change in pollination pattern, or it could occur through outbreeding depression due to extensive pollen dispersal from differenti- ated populations (Ellstrand 1992). Therefore, knowledge of the amount and pattern of pollen dispersal, and the factors that influence it, is important for management of plant populations in disturbed landscapes. Anthropogenic disturbance has dramatically increased the physical isolation of populations and it has been assumed that such isolation will lead to reduced gene flow and consequently reduced genetic diversity in populations. However, many trees in which pollen dispersal has been investigated in fragmented landscapes have shown that gene flow is extensive and may well have increased due to physical isolation of populations (Nason & Hamrick 1997; Aldrich & Hamrick 1998; White et al . 2002; Dick et al . 2003; Bacles et al . 2005; Byrne et al . 2006). The relative level of pollen dispersal into a population is expected to increase with decrease in population size since there will be fewer plants within the population to contribute to the pollen pool (Ellstrand 1992). Plant density is known to impact on out- crossing rates as it influences the extent of near-neighbour foraging activity of pollinators (Ellstrand et al . 1978; Correspondence: Margaret Byrne, Fax: +618 93340327; E-mail: margaret.byrne@dec.wa.gov.au