Heterostylous plants in an era of global change: a review on the consequences of habitat loss and fragmentation.

The widespread loss and fragmentation of habitats have caused significant declines in biodiversity. Among plants, animal-pollinated species are particularly threatened because of the negative effects of these factors on pollinators. Heterostyly is a unique reproductive system defined by two or three floral morphs having a distinct position of anthers and style. The spatial separation of reproductive organs, accompanied by a self-incompatibility system, restricts self-pollination and favours pollinator-mediated pollen transfer between different morphs. In this review, we synthesize knowledge about the effects of loss in the area and connectivity of habitats, and related reduction in population size on heterostylous plants. We conducted a literature search to obtain an overview of studies investigating the short- and long-term consequences of the decreased area and connectivity of habitats as well as plant population size for heterostylous species. To quantify the relationship between plant population size and morph ratio bias, we applied a meta-analytical approach. First, the meta-analysis showed that reductions in population size can significantly disrupt the optimal morph ratio, leading to fewer compatible mates and lower reproductive output. Second, the literature review highlights the negative consequences of biased morph ratios for population viability and genetic diversity of heterostylous plants. Finally, heterostylous species may adapt to the loss of pollination by shifting their mating system to homostyly and selfing. This review demonstrates that habitat loss and fragmentation have various consequences for heterostylous plants, e.g. reduced population size, morph ratio bias, and disruption of pollination. With ongoing environmental changes, there are still important knowledge gaps that need to be addressed more systematically. These include the long-term impact of skewed morph frequencies on population viability, the selective forces driving variation in anther-stigma separation and intra-morph compatibility, the role of habitat loss and connectivity, and related reduction in pollinator abundance and diversity in the selection for homostyly.