Vol.:(0123456789) 1 3 Oecologia (2017) 185:221–231 DOI 10.1007/s00442-017-3941-1 POPULATION ECOLOGY – ORIGINAL RESEARCH Habitat fragmentation leads to reduced pollinator visitation, fruit production and recruitment in urban mangrove forests Tyge D. Hermansen 1  · Todd E. Minchinton 1  · David J. Ayre 1   Received: 7 July 2016 / Accepted: 23 August 2017 / Published online: 8 September 2017 © Springer-Verlag GmbH Germany 2017 the importance of conserving the largest possible stands. However, additional work is needed to determine (1) the pro- portion of recruits within small stands that originate within large stands, (2) how seedling performance varies with fruit size and genotype, and (3) how seedling size and perfor- mance vary with the abundance and diversity of pollen. Keywords Apis mellifera · Australia · Invasive species · Mating system · Population ecology Introduction Although the impacts of habitat fragmentation on repro- ductive success and regeneration of terrestrial plant species have been well documented (e.g. reviewed by Aizen et al. 2002; Ghazoul 2005; Aguilar et al. 2006), the demographic effects of fragmentation in mangrove forests have been largely ignored. This is surprising because, although man- groves have naturally fragmented distributions (Tomlinson 1986; Duke 2006; West et al. 1985), urbanisation and coastal development have increased the fragmentation of mangrove forests (reviewed by Rogers 2004), producing stands rang- ing from thousands of trees to isolated individuals (West et al. 1985). Urban mangrove populations may therefore experience demographic impacts of reduced stand size similar to those of terrestrial plant species (Collinge 2009; Nayak and Davidar 2010; Newman et al. 2013), afecting their capacity for regeneration and persistence. Nevertheless, in some regions, mangroves have expanded their distribu- tions, sometimes due to anthropogenic development that has increased sedimentation into estuaries (e.g. New Zealand: Stokes 2010; North America: Cavanaugh et al. 2013: Aus- tralia: Adam 2002; Duke 2006). Newly created stands, in combination with remaining fragments, provide a setting Abstract Mangrove forests worldwide undergo anthropo- genic fragmentation that may threatentheir existence, and yet there have been few tests of the efects of fragmentation on demographic processes critical for mangrove regeneration. Predicting the efects of habitat fragmentation on mangroves is problematic as pollinators may move more freely across water than terrestrial habitat, and propagules can be widely dispersed by water. Here, within each of two estuaries, we compared pollinator diversity and activity, reproductive efort and output, and rates of recruitment for sets of three large (>1500 trees), medium (300–500) and small (<50) stands. As predicted, most measures of reproductive activity and success were inversely related to stand size with large stands typically producing signifcantly more and larger fruit, and signifcantly more seedlings. Most strikingly, we found the efect of fragmentation on the abundance of pol- linators (honeybees), the production and quality of fruit and the survival rate of seedlings to be similar, showing signifcant reduction of recruitment in small stands. This study provides the frst rigorous evidence that recruitment of mangroves, like for many terrestrial plants, is negatively impacted by habitat fragmentation. From a management perspective, we argue that in the short term our data imply Communicated by Anne Worley. Electronic supplementary material The online version of this article (doi:10.1007/s00442-017-3941-1) contains supplementary material, which is available to authorized users. * Tyge D. Hermansen tdhermansen@speedpost.net 1 Centre for Sustainable Ecosystem Solutions and School of Biological Sciences, University of Wollongong, Wollongong, NSW 2522, Australia