Shifts in abundance and distribution reveal non-random patterns of threat to species diversity among plant functional groups

Background/Question/Methods

Identification of species experiencing decline is essential for conservation planning. This research assessed abundance and distribution shifts of 207 plant species in north-central North America and evaluated the importance of a suite of functional characteristics in predicting their persistence over 114 years (1895-2009).   Functional characteristics included native versus introduced status, pollination syndrome, symbiosis and habitat requirements, and phenological responsiveness to temperature change.

Plant specimens from the Ohio State University Herbarium were used to assess patterns of abundance and distribution shifts.  The partial Solow equation and the sighting rate model were used to calculate the average probability that a species has declined over the 114 yr study interval.  Rarefaction analysis was used to calculate the percent change in county occurrences from historic (1895-1970) to modern (1971-2009) time periods as a measure of distribution shift. Phenological responsiveness (flowering shifts with temperature increase, days/^oC) was determined by regressing the day of year of flowering for each specimen with a temperature specific to its collection date and location taken from the U.S. Historical Climatology Network.  Differences in abundance and distribution shifts within functional groups and with differing phenological responsiveness were evaluated with generalized linear models.

Results/Conclusions

Fifty-five of the 207 species significantly decreased in abundance from 1895 to 2009 while 136 species showed distribution contraction.  Species that require a symbiont for growth or reproduction were twice as likely to decrease in abundance compared to those without a symbiont requirement.  Native species were seven times more likely to decline in abundance relative to introduced species and on average showed a two-fold greater distribution contraction than introduced species. Introduced species that strongly advanced flowering with temperature increase also showed significantly greater distribution expansion than those with weak phenological responsiveness.  Upland species and facultative wetland species were less likely to experience range contraction relative to habitat generalists.

This analysis of abundance and distribution shifts indicates non-random patterns of threat to plant species diversity among functional groups. Further, this research adds to the limited number of studies world-wide that have evaluated the relationship between phenological responsiveness and performance of non-native species.  The results suggest that with climate warming, highly responsive introduced species may become more invasive across large areas. Thus, the already substantial impacts of land-use change, loss of symbionts, and non-native species invasion on species persistence may be exacerbated by climate warming.