What Makes a Plant Rare?


RTE Plants

Plant species may be defined as rare, threatened, or endangered (RTE) based on limited or declining distributions, selective growth requirements, and species-specific conflicts, to name a few. Some species are "naturally rare," meaning they prefer a particular habitat or relationship to another organism (mycorrhizal fungus, for example). These species may live in an isolated population where seed dispersal is difficult. Rarity may also occur due to changes in an environment, including loss of habitat, pollinators, or competitive pathogens and pests (USDA, 2017).

Before we delve further into why species are rare, we will first clarify the terminology. How do we define or classify the rarity of these species? In each state? Across the nation? Globally?


Ranks and statuses are used to indicate the rarity and vulnerability of a species on a state, federal, and global level. Ranks are typically non-legal, whereas statuses represent the legal standing of a species under the federal Endangered Species Act and other state legislation. 

For example, the small whorled Pogonia (pictured below) is ranked a G2, S1, LT, E. G2 describes a species that is globally very rare and imperiled, with six to twenty known occurrences. In Tennessee, the species is ranked S1, or extremely rare and critically imperiled, with five or fewer occurrences.

 This species is known from well-drained second-growth hardwood or pine forests. It is possibly the rarest orchid east of the Mississippi River, according to the Tennessee Flora Committee (2015).

This species is known from well-drained second-growth hardwood or pine forests. It is possibly the rarest orchid east of the Mississippi River, according to the Tennessee Flora Committee (2015).

In legal terms, the species is federally listed threatened (LT) and state listed endangered (E). LT means that the Pogonia is likely to become endangered within the foreseeable future. In Tennessee, E means that the prospect of survival or recruitment is in jeopardy. For clarification, the full ranks and status listings can be found here.


With the lists of rarer species increasing annually, how do we keep track of everything? NatureServe is a large non-profit organization involved with biodiversity, conservation, and data collection. They oversee globally rarer species, buy and manage critical habitat for species conservation, and employ thousands of biologists who excel in data collection and analysis. NatureServe also provides sets of habitat data, mapping applications, and research databases.

In Tennessee, the Natural Heritage Program (NHP) uses a methodology based on that of NatureServe, its parent organization, to collect and track the specific locations of rare plant and animal species in a database based on standardized data collection protocols. Through the NHP, the Department of Environment and Conservation (TDEC) publishes a rare plant list for the state. This ability to legally list plant species as threatened, endangered, and of special concern is made possible by the Rare Plant Protection and Conservation Act of 1985.

 State Nature Preserves are managed by the Tennessee Department of Environment and Conservation. Many rare species of plants and animals can be observed at these sites. Falling Water Falls (right) and Piney Falls (left) are two of 85 managed areas.

State Nature Preserves are managed by the Tennessee Department of Environment and Conservation. Many rare species of plants and animals can be observed at these sites. Falling Water Falls (right) and Piney Falls (left) are two of 85 managed areas.

The NHP tracks rare species to collect baseline inventory data about our lands so that conservationists, biologists, and land managers can make informed decisions. For example, developers are often encouraged - or mandated, depending on the project - to submit an Environmental Request as part of the development process. NHP representatives query their databases to locate any rare species within a mile of a project area. Using this data, developers work with environmental consultants to understand the distribution of rare species in their project area. Often, the developer is asked to move the project, but in some cases, transplanting the rare species prior to development is an option.


So what exactly makes one species rarer than another? According to Rabinowitz (1981), there are many different reasons why a species may become rare, including changes in geographic range, habitat specificity, and local population size. Some species may occur across a narrow range in locally high abundance, while others are widely dispersed and locally rare. Generally, species with both narrow ranges and habitat specificity are often endangered or threatened. Rarer species were also thought to be inferior competitors as compared to their more common counterparts, though this has been debunked in a number of experimental treatments. In fact, rare species tend to outcompete neighboring common species.

However, despite various explanations, why some species are rare and others are not is still unknown (Jain et al., 2013; Wamelink et al., 2014). To investigate the phenomenon, a group of researchers in Tennessee and the surrounding region are currently tackling this question. Jennifer Boyd, an associate professor at the University of Tennessee at Chattanooga, is leading this team in a four-year project sponsored by the National Science Foundation. In her research, she considers the dynamic nature of the natural world and the species that must respond successfully to environmental change to persist. Two possible ways to do this include adaptation and acclimation.

Adaptation involves a genetic change in a population over generations, which is influenced by the size of its gene pool. This may be limited for rare species, which often are found in small numbers. Acclimation involves the adjustment of an individual over a lifetime that is possible due to its existing genetic makeup. Acclimation may be especially important in the face of rapid environmental change because adaptation takes time. Boyd’s research therefore focuses on the potential for rare and common plant species to adapt and acclimate to environmental change.

In the past, Boyd has researched the responses of federally threatened Scutellaria montana (large-flower skullcap) (right) and Platanthera integrilabia (white fringeless orchid) (left) to environmental change to better direct the conservation efforts for these species. She hopes that her new research project will accomplish a similar goal as well as address the incredibly fundamental scientific question of why some species are rare while others are common.


One of the most notable conservation efforts in the state for rare plant species is the Plant Conservation Alliance (PCA). Modeled after similar efforts made in the state of Georgia, Tennessee's PCA is comprised of networks of private and public entities that agree to work together to leverage expertise and resources for the common goal of conserving over 500 of Tennessee's imperiled native plants. Let us connect you to the group.

The Reflection Riding Arboretum and Nature Center hosts a number of conservation-oriented events, allowing participants to meet with community leaders. Chatt About Science hosts monthly meetings at Stone Cup Cafe, where speakers address a topic for about an hour followed by discussion. It is a great way to meet other people within the conservation network.

Photograph your Finds

As we've said before, if you are ever unsure about species identification, we invite you to snap a photo and Contact Us. If we cannot answer your question, the professionals within our network can. Taking photos of plant and animal species with your phone is a great way to document a species in question. The more, the better, and multiple shots of flowers, leaves (both front and back), and stems are helpful. We would like to thank everyone who has sent us plant identification photos (a few are pictured below). While they are not rare, a few are less common and more difficult to find. We look forward to fall! 


Jain, M., D. F. B. Flynn, C. M. Prager, G. M. Hart, C. M. Devan, F. S. Ahrestani, M. I. Palmer, D. E. Bunker, J. M. H. Knops, C. F. Jouseau, S. Naeem. 2013.The importance of rare species: a trait-based assessment of rare species contributions to functional diversity and possible ecosystem function in tall-grass prairies. Ecology and Evolution 4(1): 104-112. Accessed September 2017. Available online at: http://onlinelibrary.wiley.com/doi/10.1002/ece3.915/full.

Rabinowitz, D. "Seven forms of rarity." The Biological Aspects of Rare Plant Conservation. Hugh Synge, Editor. Ann Arbor: John Wiley & Sons, Ltd. 1981. Accessed September 2017. Available online at: http://www.esf.edu/efb/parry/Invert_Cons_14_Readings/Rabinowitz_1981.pdf.

Tennessee Flora Committee. Guide to the Vascular Plants of Tennessee. University of Tennessee Press: Knoxville. 2015. Print.

USDA Forest Service. 2017. Why are some plants rare? Rangeland Management & Vegetation Ecology - Botany Program. Washington, DC. Accessed September 2017. Available online at: https://www.fs.fed.us/wildflowers/Rare_Plants/whyare.shtml.

Wamelink G.W.W., P.W. Goedhart, and J. Y. Frissel. 2014. Why Some Plant Species Are Rare. PLoS ONE 9(7). Accessed September 2017. Available online at: http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0102674.