Range disequilibrium in an ant-dispersed forest herb: An experimental test of abiotic, biotic, and dispersal limits on the distribution of Jeffersonia diphylla (Berberidaceae)

Background/Question/Methods

Species distributions are the result of an ecological filtering process that includes limits imposed by dispersal, abiotic conditions, and biotic interactions.  However, most species distribution models focus exclusively on abiotic factors, like climate and soils, even though recent studies suggest that the geographic ranges of some species may be dispersal-limited over long periods of time and that biotic factors, like soil pathogens, often have significant impacts on plant performance.  In this study, two parallel experiments have examined the roles of seed dispersal, and abiotic and biotic factors in controlling the northeastern range edge of the ant-dispersed forest herb Jeffersonia diphylla.  In 2006, seeds were sown in 3 natural populations in New York and in 3 unoccupied, but apparently suitable, habitat patches ~200 km beyond the species range edge in western Massachusetts.  We have tracked survival and performance for 6 years as these long-lived perennials have matured.  In a parallel greenhouse experiment launched in Fall 2011, we manipulated soil biota and physical properties to test the influence of key biotic and abiotic factors on seed germination.  Seeds were sown in native field-collected soil that was sterilized or amended with perlite, and non-native soil inoculated with soil biota from the native range.  

Results/Conclusions

In the field-based experiment, J. diphylla seeds sown beyond the range edge outperformed controls in natural populations in terms of germination, growth, and maturation over 6 years.  Starting in 2011, experimental plants beyond the range edge began to produce seed, resulting in new seedlings in 2012, indicating the potential for positive population growth outside the native range.  Further, the increased performance documented beyond the range edge suggested the possibility that negative biotic factors, such as soil pathogens, might reduce performance in native populations.  However, in the greenhouse study, we detected no evidence of significant effects of soil biota on germination and growth in Spring 2012 for either sterilized native soil or non-native soil inoculated with native biota.  Likewise, modifications of soil bulk density had no significant effect on germination rates.  Overall, our results suggest that highly suitable habitat for J. diphylla exists beyond its current range edge in the Northeast and that its absence from these areas may result from severe dispersal limitation.  This range ‘disequilibrium’ may trace to limited northward range expansion following the Pleistocene.  These findings underscore the potential for dispersal to substantially limit some species abilities to respond to rapid climate change in the future.