The causes of plant species rarity are of interest across a range of systems and have profound implications for conservation efforts targeting threatened and endangered species (TES). There are myriad reasons why plant species may be rare in a landscape, each with different effects on management decisions. One reason for species rarity is the possibility that their functional traits may be mismatched with their current environment, resulting in decreased fitness. In such cases it may not be possible to preserve species at a particular location, so identifying these mismatches is important for prudent allocation of limited resources. Comparing the functional traits of TES with abundant species provides an important first step in determining whether TES are well-matched for current environmental conditions. However, while our understanding of trait diversity in ubiquitous plants has grown immensely in recent years, very little is known about the functional trait diversity of TES and how they compare to common members of their communities. To address this critical gap in our understanding we measured a suite of 18 important functional traits for 19 TES and 134 common, co-occurring understory plant species from a fire adapted pine savanna and explored the relationships between their multivariate trait distributions.
Using data from a muti-year survey of TES diversity, we show that TES are significantly different (p<0.01) from common species in 6 of 18 traits in univariate analyses, but multivariate analyses of traits using permutational analysis of variance show a more complex picture. We found that the joint distribution of all 18 traits is significantly different (p=0.042) from that of common species even when controlling for environmental differences such as hydrology, light availability, and burn frequency. Further, rare species occupy a restricted subset of the trait space occupied by their more common neighbors. These results indicate that TES, as a group, are functionally constrained relative to abundant species, and imply that they are more limited in the range of microhabitats they can successfully occupy across the landscape. On the other hand, the high degree of overlap between TES and common species in trait space shows that that many, if not most, TES in this system have functional trait profiles close to those of abundant members of the larger community and are hence functionally capable of persisting and thriving at many of the same locations.