The reasons why some plant species are rare, while some others are not, is a long-standing concern for biologists. A better understanding of drivers of plant rarity is central to broaden our knowledge of the distribution of species and is crucial for conservation practitioners to protect rare and threatened species. However, despite a large literature body dedicated to this question, we are lacking understanding and evidences on the effects of factors suggested to contribute to plant species rarity and factors suggested to increase success of conservation projects of threatened plants. In this thesis, I focused on investigating whether generalist herbivory and genetic processes could contribute to further plant species rarity and how genetic diversity, propagule pressure and plant origin alter reintroduction success. To achieve this, I have conducted multi-species experiments taking into account the local and regional rarity, habitat characteristics and several traits linked to life-history and resource-allocation strategies of the plant species.
In the first chapter of this thesis, I investigated how the performance and preference of one below-ground and three aboveground invertebrate generalist herbivore species vary with the rarity, habitat characteristics and traits linked to resource-allocation strategies of 62 different plant species. I found that regionally and locally rare and common plant species did not generally differ in their defense against generalist herbivores. The results indicate that the hypothesis that rare plant species are less defended against herbivores does not hold for herbaceous plant species. Instead, they suggest that the ability of plants to allocate resources away from defense against herbivores to achieve a stronger competitive ability might have allowed plants to become locally and regionally common.
In the second chapter, I tested whether selfing and between-population outcrossing affected five fitness-related traits for 16 species of different local and regional rarity, habitat characteristics and resource-allocation strategies. The results demonstrate that regional rarity, habitat characteristics and traits linked to resource-allocation strategies shape the outcome of selfing and between-population outcrossing in plants. This study highlights that inbreeding depression may not represent a major threat to species that are already rare. Moreover, as between-population outcrossing appears beneficial for plant fitness of many species, including rare ones, we suggest considering it for plant-conservation activities.
In the last chapter, I investigated how genetic diversity, propagule pressure and population of origin alter fitness of reintroduced rare and threatened plant species. The results show that genetic diversity and population of origin interacted and were important determinants of early plant fitness. The effect of genetic diversity was positive or negative depending on how ecologically similar the populations of origin to the translocation site are. In addition, propagule pressure may affect negatively fitness of reintroduced plants, possibly due to intra-specific competition or plant antagonists. The study underlines the importance to match reintroduction and origin sites very carefully by conducting vegetation record to assess ecological similarity.
Altogether, the results of this thesis challenge important hypotheses about rare plant species and suggest that plant species rarity may be driven by complex interactions between extrinsic and intrinsic factors. Finally, it highlights the importance of considering context dependency in multi-species experiments in order to disentangle patterns related to plant rarity from those related to plant habitat and resource allocation strategies.