Pollen flow in fragmented populations of myrtus communis and pistacia lentiscusThe importance of mating, pollination systems and the landscape context

Abstract

Over the last century, habitat loss and fragmentation have been the main components of current anthropogenic global change and the greatest threats to the global biodiversity. The scientific community has shown a growing interest in understanding the consequences of the anthropogenic disturbance of the biosphere and habitat fragmentation studies have been one of the scientific fields that most flourishing in the last decades. The detrimental effects of habitat fragmentation on plant reproductive success and mating system patterns have been widely documented and there is a general agreement that these effects can ultimately compromise plant persistence. However, species responses to habitat loss and fragmentation are often variable, emphasizing the need to general knowledge on the complex and variable mechanisms of species responses to changes in habitat configuration and size. The main purpose of this Doctoral Thesis is to evaluate how habitat fragmentation (fragment size and connectivity) affects and determines both historical and contemporary gene flow and mating systems patterns, as well as the reproductive success of populations of the two common Mediterranean shrubs, Myrtus communis and Pistacia lentiscus. By comparing populations of these two species with contrasting mating and pollination systems, co-occurring in a diverse regional mosaic of Mediterranean forest patches in the Guadalquivir River Valley, this thesis highlights on the complexities surrounding fragmented landscapes and the influence of fragmentation on ecological interactions. Chapter 1 infers patterns of historical gene flow of Myrtus and Pistacia populations under different landscape contexts. The influence of mating and pollination systems characteristics and population-specific response in both study species is perceived in shaping species’ genetic patterns. Both species revealed weak detrimental effects of habitat fragmentation, such as low values of effective population size and evidence for recent genetic bottlenecks, although results were more pronounced in Myrtus than in Pistacia populations. Both Chapter 2 and Chapter 3 are comparative studies that assessed the role of habitat fragmentation in conditioning plant’s reproductive success and contemporary patterns of pollen flow of Myrtus and Pistacia populations. In these chapters we evaluated the variation in plant reproductive success and mating system estimates and also the main ecological factors responsible for the variation at both the individual and population levels. Rather than a direct effect of habitat fragmentation, ecological components are more important on species’ reproductive success and mating system patterns but, interestingly, their influence vary along the fragmentation gradient. Finally, Chapter 4 analyzes how correlated paternity shapes Myrtus and Pistacia early progeny performance both in a greenhouse environment and under natural (field) conditions. This study represents one of the few existing empirical examples that evaluate the role of the correlated paternity in shaping the early performance of maternal progenies. This Doctoral Thesis also demonstrates that the impacts of habitat loss and fragmentation, a part of being most of the times context and species dependent, they are often also more nuanced than simple and easy observed losses of genetic diversity and increase differentiation among populations. Some key consequences of forest fragmentation have now been identified as increased inbreeding depression in progeny sired in a fragmented landscape and decreased progeny fitness due to low numbers of effective pollen donors. It also confirms that variation in mating and pollination systems is an important factor in determining the type and magnitude of species response to habitat fragmentation.