Early View (EV): 1-EV Climate induced changes in matrix suitability explain gene flow in a fragmented landscape – the effect of interannual rainfall variability Niels Blaum, Monika Schwager, Matthias C. Wichmann and Eva Rossmanith N. Blaum (blaum@uni-potsdam.de) and E. Rossmanith, Univ. of Potsdam, Dept of Plant Ecology and Conservation Biology, Maulbeerallee 2, DE-14469 Potsdam, Germany. – M. Schwager, Biodiversity and Climate Research Centre (BiK-F), Senckenberganlage 25, DE-60325 Frank- furt (Main), Germany. – M. C. Wichmann, Centre for Ecology and Hydrology, CEH Wallingford, Maclean Building, Benson Lane, Crowmarsh Gifford, Wallingford, Oxfordshire, OX10 8BB, UK. In fragmented landscapes, the survival of species and the maintenance of populations with healthy genetic structures will largely depend on movement/dispersal of organisms across matrix areas. In this article, we highlight that efects of fragmentation and climate change occur simultaneously and may enhance or mitigate each other. We systematically ana- lyzed the efect of increasing interannual variation in rainfall on the genetic structure of two neighbouring small mammal subpopulations in a fragmented savanna landscape. he efect of interannual rainfall variation is analyzed for two contrast- ing scenarios that difer in mean annual rainfall and are both close to a dispersal threshold. Scenario 1 (low mean annual rainfall) lies slightly below this threshold and scenario 2 (high mean annual rainfall) slightly above, i.e. the amount of rainfall in an average rainfall year prevents dispersal in scenario 1, but promotes gene low in scenario 2. We show that the temporal dynamics of the matrix was crucial for gene low and the genetic structure of the neighbouring small mammal subpopulations. he most important result is that the increase in rainfall variability could both increase and decrease the genetic diference between the subpopulations in a complex pattern, depending on the scenario and on the amount of variation in rainfall. Finally, we discuss that the relevance of the matrix as temporarily suitable habitat may become a key aspect for biodiversity conservation. We conclude to incorporate temporal changes in matrix suitability in metapopulation theory since local extinctions, gene low and re-colonization are likely to be afected in fragmented landscapes with such dynamic matrix areas. Global changes in climate and land use are the most seri- ous threats to biodiversity (Sala et al. 2000, Vellend et al. 2006, huiller 2007). Changes in land use have led to habitat loss and fragmentation with major implications not only for the demographic structure but also for the genetic structure of plants and animals (Whittington et al. 2005, Ewers and Didham 2006, Lindenmayer and Fischer 2007). Decreasing size and increasing isolation of subpopulations lead to a higher risk of genetic drift or founder efects, and thus reducing population itness and increasing the chance of local extinctions (Floyd et al. 2005, Wiegand et al. 2005, Smith and Batzli 2006). hus, the ability of species to sur- vive and keep a healthy genetic structure in fragmented landscapes will largely depend on their ability to move and disperse across degraded areas with poor resources conditions (e.g. food) (Boudjemadi et al. 1999, Blaum and Wichmann 2007). he ability of animals to move or disperse across degraded areas may depend on the climatic conditions within the area. In semiarid grasslands and savannas, vegetation growth and thus the hospitability of the landscape is largely deter- mined by climate, i.e. rainfall. he yearly amount of rainfall dominates recruitment, growth and reproduction of animals and plants, nutrient cycling, and net ecosystem productivity (Weltzin et al. 2003, Schwinning and Sala 2004). As rainfall is highly variable in these systems, plants respond to heavy rain- fall events with explosive production of seeds and vegetative growth (Chesson et al. 2004) while productivity is suppressed in low rainfall years. his creates not only a generally low level of productivity in the system due to its aridity, but also a highly variable environment in time with oscillating abundance of resources for primary and secondary consumers. he variability in resources may be important for maintaining gene low in fragmented populations in these landscapes. In savannas worldwide, heavy grazing has cre- ated shrub dominated areas (Skarpe 1990, Gillson and Hofman 2007) that usually separate subpopulations of small mammals in pristine savanna habitats (grassland with sparse trees and shrubs) (Blaum et al. 2007). However, in exceptionally good rainfall years, the degraded shrub areas can transform into hospitable habitat for a short time period when rare events of heavy rainfall trigger mass germina- tions and growth of dormant grass-seeds (Chesson et al. 2004). his short-term habitat transformation of the matrix may then allow gene low between the otherwise separated subpopulations (Blaum and Wichmann 2007). Ecography 34: 001–011, 2011 doi: 10.1111/j.1600-0587.2011.07154.x © 2011 he Authors. Journal compilation © 2011 Nordic Society Oikos Subject Editor: horsten Wiegand. Accepted 15 August 2011