Abstract
Demographic and genetic connectivity of fragmented plant populations will depend on effective propagule flow across the landscape. We analyze functional connectivity in a holm oak (Quercus ilex) fragmented landscape by considering three important stages driving recruitment: effective pollination, acorn production and acorn dispersal. We used a network approach to (1) determine if pollen-mediated gene exchange across the landscape was spatially structured; (2) estimate the effects of limited acorn dispersal on functional connectivity; (3) identify which landscape traits could drive source–sink dynamics of gene flow.Although long distance dispersal was relatively frequent, most effective pollen flow occurred over short distances (<100m). This resulted in a significantly modular structure of the mating network, yielding higher gene flow among nearby fragments. Limited mouse acorn hoarding activity had a strong impact on landscape connectivity, decreasing male gametic immigration rates into forest patches by one order of magnitude Besides, our results show that big forest fragments (>10ha) are the main pollen sources, while small ones (<1ha) are important pollen sinks. Thus, big fragments are critical to maintain functional connectivity, while small forest fragments may provide acorn crops better representing regional genetic diversity. In addition to area effects, less isolated and more central fragments showed higher migration rates and exchanged effective pollen with more fragments. Hence, we expected that landscapes with uniform or clumped distribution of big forest fragments would show optimal connectivity traits. However, despite that simulated gene flow was more evenly distributed across the landscape, connectance and migration rates decreased. Our results call for caution before translating patch-level management guidelines to the landscape scale. They also show that the level of functional connectivity may change throughout the recruitment process, suggesting that large-scale conservation strategies may fail if local effective seed establishment is disregarded.
Highlights
Land use changes have led to intense fragmentation of many temperate woodland ecosystems, replacing large continuous forests by a mosaic of isolated fragments, embedded in agricultural or urban matrices (Riitters et al 2000)
We explored the structure of the two mating networks by constructing 100 replicates for each and performing 349 modularity analyses
We evaluated which factors determined the role of forest fragments as sources or sinks of effective pollen across the landscape
Summary
Land use changes have led to intense fragmentation of many temperate woodland ecosystems, replacing large continuous forests by a mosaic of isolated fragments, embedded in agricultural or urban matrices (Riitters et al 2000). The demographic and genetic connectivity of fragmented plant populations depend on the effective dispersal of pollen and seeds across the landscape (Sork et al 1999; Sork and Smouse 2006). The intensity and scale of both seed and pollen dispersal across forest fragments are central to the decade-long debate about the extent to which tree populations are resilient to fragmentation (Kramer et al 2008). Seed dispersal determines plant recruitment patterns, the successful establishment of both male and female migrant gametes and the final distribution of genotypes (Sork and Smouse 2006, Sork et al 2015). Pollen movement sets the template of male gametic gene flow, which depends on and is subsequently shaped by effective seed dispersal (Fig. 1). Integrating the 100 joint effects of pollen and seed dispersal is necessary to gain realism
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