Dispersal limitation and seed predation drive rarity of a plant species at its range edge

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1. 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 suit- ability of abiotic conditions, dispersal limitation and biotic interactions. Theory emphasizes the importance of the availability of abiotically suitable habitat in de- termining occurrence frequency, especially for species at their range edge, where the amount of suitable habitat is predicted to decline. However, the relative influ- ence of these filters in driving species rarity is mostly unknown. 2. 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 limi- tation by planting seeds in unoccupied sites that varied in their predicted habitat suitability and measured seedling emergence and seedling survival over 2 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. 3. The habitat suitability estimated by the SDMs did not predict seedling emergence or short-term seedling survival. We found that dispersal limitation coupled with seed pre- dation was a significant predictor of seedling emergence, while microclimate, specifi- cally microsite temperature, was a significant predictor of short-term seedling survival. 4. Synthesis. 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 limita- tion 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 appropri- ate microsites and the management of biotic interactions to establish new popu- lations and ensure long-term persistence.

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