by This article is from Hakai Magazine, an online publication on science and society in coastal ecosystems. Read more stories like this at hakaimagazine.com.
The watch 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 scale, we know this to be true. But knowing precisely which plots will flood and which will remain dry is a much more daunting task. This effort may have had an ally that almost no one would have guessed: one of the smallest and least visible forms of life – lichens.
More than 18,000 lichen species have been described worldwide. Each is a community consisting of one or more species of fungus 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.
To distinguish its niche, each species has evolved to tolerate different levels of temperature, light, air quality and other factors. Because of this sensitivity, lichens are already used by scientists to measure environmental disturbance, such as the effect of logging or nitrogen pollution. Lichens also differ in their salt tolerance. It’s this property, says botanist Roger Rosentreter at Boise State University in Idaho, that makes them so useful for understanding sea-level rise.
“Lichens are a good indicator of the history of the site,” says Rosentreter, who has studied lichens and related species for more than 40 years. In particular, lichen species growing in a coastal site can be an effective indicator of low levels of seawater intrusion and spray, which can be caused by infrequent floods or storms. As sea levels continue to rise, any location that had occasional salt water in the past is likely to see more frequent flooding and storms in the future.
Recently, Rosentreter and his wife, fellow Boise State botanist Ann DeBolt, studied 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 flooding from storms, while the other is inland just 500 meters away. The scientists found two surprisingly different lichen communities at each site. By comparing the two, they began to compile a list of lichen species that might be useful indicators of the long-term or historical presence of saltwater.
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 life history of the lichen itself also comes into play.
Species such as the powdery medallion lichen (left photo) can be killed if exposed to very salty water from a storm or flood. But the rapid reproduction of this lichen lets it repopulate quickly after the sea recedes. Larger species with slower growth and reproduction, as well as low salt tolerance, such as blue jelly (right photo), can better tell a location’s saltwater story. These salt-intolerant lichens could not have survived and grown if a seawater event such as storm spray or flooding had occurred at any point in their lifetime. Since some lichen species can live for decades or more, the record they provide can be supralocal in space and extensive in time.
Of the 48 different lichen species Rosentreter and DeBolt found at the two Florida research sites, 11 are reliable indicators of the presence of saltwater. Seven of the species like to grow only in places with very low salt water impact, while four are salt tolerant, so finding them growing suggests the site has a moderate salt history and a higher risk of being affected by sea level rise.
In general, they found that the species that best indicate whether a site will be relatively safe from sea-level rise and seawater inundation are lichens that are larger and leafier and often pale 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 a middle man to figure it out,” says Rosentreter.
“The good thing is that these are not just in Florida. They are found throughout the southeastern coastal plain,” he says. Reports on iNaturalist, for example, put the troubled blue jelly all over the US East Coast and beyond.
Borja G. Reguero, an expert on maintaining natural defenses against sea-level rise at the University of California, Santa Cruz, who was not involved in the research, sees parallels between how 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 flood events is above a threshold where some species can no longer live,” he says. “You could say the same thing about people and coastal infrastructure. You get to a tipping point where certain neighborhoods flood so regularly that they don’t have insurance.”
Modern science offers a range of tools for studying sea-level rise, from satellite data to groundwater and soil sampling. Lichens could be another way to see, at smaller site-specific scales, where the sea is coming, and just as importantly, where it isn’t.
This article first appeared in Hakai Magazine and is republished here with permission.
