This article is from Hakai Magazine, an online publication about science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.
The clock is ticking for many low-lying coastal areas. Sea levels are rising faster than at any time in recorded history, promising to radically redraw the map. On a broad level we know this to be true. But knowing exactly which properties will flood and which will remain dry is a much more daunting task. This effort may have an ally that almost no one would have guessed: one of the smallest and most inconspicuous life forms – lichens.
More than 18,000 lichen species have been described worldwide. Each is a community made up of one or more fungal species and an alga or cyanobacteria. This combination has allowed lichens to survive in diverse and often hostile conditions, from tropical heat to bitter Antarctic cold.
In order to carve out its niche, each species has evolved to tolerate different factors of temperature, light, air quality, and other factors. Because of this sensitivity, lichens are already being used by scientists to assess environmental disturbances such as the impact of logging or nitrogen pollution. Lichens also differ in their salt tolerance. It’s this property, says botanist Roger Rosentreter of Boise State University in Idaho, that makes them so useful for understanding sea-level rise.
“Lichens are a good indicator of site history,” says Rosentreter, who has been studying lichens and related species for over 40 years. In particular, the lichen species growing in a coastal site can be a powerful indicator of low levels of saltwater intrusion and sea spray, which can be caused by infrequent flooding or storm events. As sea levels continue to rise, any site that has experienced occasional saltwater exposure in the past is likely to experience more frequent flooding and storms in the future.
Recently, Rosentreter and his wife, Ann DeBolt, a Boise State botanist, surveyed the lichen communities of two state parks near West Palm Beach, Florida. One park on a barrier island is subject to frequent salt spray and storm surges, while the other is just 500 meters inland. The scientists found two surprisingly different lichen communities at each site. By comparing the two, they began compiling a list of lichen species that can be useful indicators of long-term or historical saltwater presence.
It takes more than just salt sensitivity to make a lichen a good indicator of whether a site has experienced the first effects of sea level rise. The lichen’s own life story also comes into play.
Species like powdery medallion lichen (left photo) can die if exposed to too much salt water from a storm or flood. But the rapid reproduction of this lichen means it will quickly recolonize after the sea recedes. Larger species with slower growth and reproduction and low salt tolerance, such as the ruffled blue jellyskin (right photo), are better able to tell a site’s saltwater history. These salt-intolerant lichens would not have been able to survive and grow if a saltwater event such as storm spray or flooding occurred at any point in their lives. Because some lichen species can live for decades or longer, the records they provide can be both spatially hyperlocal and temporally extensive.
Of the 48 different lichen species that Rosentreter and DeBolt found at their two Florida study sites, 11 are reliable indicators of the presence of salt water. Seven of the species only like to grow in places with very little saltwater exposure, while four are salt-tolerant. So if they are growing, it suggests the site has a moderate salinity history and is at greater risk of being affected by rising seas.
In general, they found that the species that best indicate whether a site is relatively safe from sea-level rise and saltwater inundation are lichens, which are larger, more leafy, and often light green or blue in color. But lichens can be difficult to identify, and some promising indicator species look quite similar to less useful ones. “You have to be at least an advanced plant expert to find out,” says Rosentreter.
“The good thing is that these aren’t unique to Florida. They’re all over the southeast coastal plain,” he says. Reports on iNaturalist, for example, are bringing the ruffled blue jellyskin all over the US East Coast and beyond.
Borja G. Reguero, an expert in maintaining natural defenses against sea-level rise at the University of California, Santa Cruz, who was not involved in the research, sees parallels between the way coastal communities and lichens deal with environmental change. “It makes a lot of sense to find these indicators [species] where the frequency of spray or flooding events is above a threshold beyond which some species can no longer survive,” he says. “The same could be said about people and coastal infrastructure. You reach a tipping point where certain parts of the city are flooded so regularly that they don’t get insurance.”
Modern science offers a range of tools to study sea level rise, from satellite data to groundwater and soil samples. Lichens could be another way to see, on smaller, site-specific scales, where the sea is next and, just as important, where it isn’t.
This article first appeared in Hakai Magazine and is republished here with permission.