Informing rare plant conservation with experimental translocations and common garden trials

Abstract

Understanding the causes of species rarity is a central goal in ecology. The three filters thought to predict if a species is present or not in a community are the suitability of abiotic conditions, dispersal limitation, and biotic interactions. Theory emphasizes the importance of the availability of abiotically suitable habitat in determining occurrence frequency, especially for species at their range edge, where the amount of suitable habitat is predicted to decline. However, the relative influence of these filters in driving species rarity is mostly unknown. We used species distribution models (SDMs) to estimate habitat suitability based on broad-scale abiotic predictors for a rare plant species (Stylophorum diphyllum) at the northern edge of its global distribution. We tested the role of dispersal limitation by planting seeds in unoccupied sites that varied in their predicted habitat suitability, and measured seedling emergence and seedling survival over two years. To manipulate the biotic interactions, we excluded seed predators by caging half of the seeds. We also measured the microclimate at each microsite, including soil moisture, temperature, and canopy cover. The habitat suitability estimated by the SDMs did not predict seedling emergence or short-term seedling survival. We found dispersal limitation coupled with seed predation was a significant predictor of seedling emergence, while microclimate, specifically microsite temperature, was a significant predictor of short-term seedling survival. Contrary to the assumption that species occur at a low frequency near their range edges due to a lack of suitable habitat, we found that dispersal-limitation coupled with biotic interactions can drive rarity. If this is the case for many rare species at risk of extinction at their range edges, effective conservation strategies must incorporate assisted dispersal (i.e., translocations) into appropriate microsites and the management of biotic interactions to establish new populations and ensure long-term persistence.