Plant recruitment and range size: Transition probabilities from ovule to adult in two Ipomoea

In plants, narrow geographic distributions are generally associated with low colonization and persistence abilities, therefore narrowly distributed plants are expected to have lower plant recruitment success than widespread species. Determining the association between recruitment success and range size requires the comparison of the success in multiple life-history stages among narrowly distributed and widespread congeners sharing the same habitat, an integrated approach rarely considered. We compared transition probabilities from ovule to reproductive adult between the narrowly distributed annual vine Ipomoea rubriflora O’Donnell (Convolvulaceae) and the widespread Ipomoea purpurea (L.) Roth at sites in Chaco woodland where they co-occur. I. rubriflora had marginally lower ovule fertilization success, a lower probability of seed maturation and lower seedling establishment than I. purpurea. The lowest transition probability for both species was seedling emergence. Seedling establishment in I. rubriflora was similar to seedling emergence. Plant recruitment success in I. rubriflora was an order of magnitude lower than that of I. purpurea. Indeed, I. rubriflora had lower total seed production per plant and smaller seed mass. Our results suggest that understanding processes determining regeneration (i.e., seedling emergence and establishment) may be of high importance for understanding narrow distributions in annual plants. The results also suggest that the narrowly distributed species is not only at a “numerical disadvantage” for colonizing new sites—i.e., lower total seed production per plant—but it also has a lower per-seed probability of establishing and thus to persist. However, as lower establishment success in this species seemed to be associated with the lower ability to survive adverse conditions of seedlings from smaller-seeded species, a narrower regeneration niche might also explain its narrow distribution.