Thelomma carolinianum is distinctive crustose lichen that has been lost from 72% of the total known sites (inferred from a comparison of historical vs. modern (pre-1990 vs. post-1990) occurrence data supported by voucher specimens), together with a 68% loss of Area of Occupancy and 95% loss of Extent of Occurrence. The causes of this reduction are considered to be the large scale degradation and loss of habitat, as well as loss of suitable substrates, throughout the range of the species. These losses have occurred in the past, are ongoing at present at smaller scales, and will likely continue in the future. Therefore, it is listed as Endangered under criteria A2bce+4bce; B2ab(i,ii,iii,v).
Thelomma carolinianum is narrowly endemic to the Coastal Plain of south-eastern North America, where it was historically known from scattered locations in Florida, Louisiana, and South Carolina. Online databases (CNALH) report several vouchers from North Carolina, however, these are erroneous and the vouchers are actually derived from localities in Florida and South Carolina. A single voucher (Rolfs 215, NY) is labelled as having been collected in Clemson, South Carolina in 1900. That site is well outside of all other known occurrences and is treated as a probable error.
This species has been considered rare since its initial discovery, as is evidenced by the note “rare” on labels of some specimens collected by William Wirt Calkins in Florida in the late 1800’s. Of the 25 total documented sites, 18 were located prior to 1990 and the majority of those were collected prior to 1950 in areas that are now heavily fragmented and developed (e.g. the metro-regions of Jacksonville and Orlando). Extensive post-1990 fieldwork has been conducted within the range of the species by multiple experts. Only seven populations are known to have been located post-1990, and two of these are in protected management units (Archbold Biological Station and Big Cypress National Preserve, Florida).
Population Trend: decreasing
The species occurs on both naturally occurring, and introduced old, weathered conifer wood, often in low-lying coastal swamps, including that of juniper (Juniperus) and pine (Pinus). Many collections are from old wooden fence rails, a substrate that has become less common in the last century due to the introduction of plastic and metal building materials.
Thelomma carolinianum has been significantly impacted by large scale loss and degradation of suitable habitat throughout its range historically (Drummond and Loveland 2010, Napton et al. 2010). The major threat to the species is the continuation of these trends into the future as the majority of extant sites are not within existing protected management units. The lack of protection by state, federal, and international legislation is a further threat to the species. In addition to intense pressure from development and other forces (i.e. industry, urbanisation), much of the remaining habitat in the region where this species occurs is imperilled by sea-level rise. Additional threats include pollution, road expansion and maintenance, logging, and other threats that would further degrade the remaining habitats where this species is known, as well as where it is predicted to occur.