Disentangling the effects of population mixing and propagule amount in rare plant translocations

Genetic diversity and propagule amount have been suggested to be pivotal determinants for translocation success. Population mixing, as a proxy for higher genetic diversity, may increase mean plant fitness and resilience, while higher propagule amount buffers against environmental stochasticity. However, population mixing may also decrease mean individual fitness in translocations (e.g. via maladaptation) and higher propagule amount may increase intra-specific competition or attraction of antagonists. To disentangle the role of population mixing and propagule amount on the early fitness of translocated plants, we transplanted material from single or mixed source populations and manipulated propagule amount by introducing smaller or larger plant numbers in translocation plots of four threatened herbaceous species. By employing aster models with survival and reproduction data over two consecutive years, we assessed effects of population mixing and propagule amount on the mean individual-plant fitness. Contrary to our expectations, high propagule amount negatively affected plant fitness during the early establishment phase of the translocation of two species, while population mixing slightly negatively affected plant fitness of the two other species. Increased attraction of plants' antagonists and source populations' maladaptation and low fitness may explain this. Although the long-term effects of our treatment will emerge in subsequent generations, our findings indicate that population mixing and high propagule amount may not always be the most effective strategies for achieving successful early establishment. Instead, we suggest creating multiple small translocation plots rather than a few large ones and carefully considering source populations' vitality and suitability when designing plant translocations.