While tourists trod the beaches in southern Spain from July to August, seashell abundance declined by 70 percent, according to a study published Wednesday in PLOS ONE. Even in the off-season, shell numbers on Llarga Beach declined by 60 percent. These seasonal declines formed part of a 30-year drop in shell abundance.
"It's too early to tell whether this depletion is substantial enough to trigger major environmental changes," said lead author Michal Kowalewski, of the Florida Museum. "However, our results suggest that we should not ignore this issue."
BLOG: Time to Bring Back New York's Oysters
Kowalewski's team conducted monthly surveys of seashell abundance on Llarga Beach from 1978 to 1981 and again from 2008 to 2010. During the 30 years between surveys, hotel records show a tripling of tourist visits. During the same period, shell numbers declined by 60 percent.
The researchers noted that Llarga Beach hadn't witnessed any other major changes in fishing or industry, besides tourism, that could have seriously affected the shells or the mollusks that create them.
"Humans may play a significant role in altering habitats through activities that many would perceive as mostly harmless, such as beachcombing and seashell collecting," Kowalewski said. "It is important that we continue to investigate the more subtle aspects of tourism-related activities and their impact on shoreline habitats."
Simple seashells play a crucial, yet subtle, role in the life of the shore. The cast-off coverings of mollusks, such as oysters and conch snails, take on new life as homes for other animals, including hermit crabs. Tiny lifeforms cling to the shells and form diverse communities.
BLOG: Ocean Indigestion? Take Oyster Antacids
Shells also help anchor a beach and reduce erosion. A solid beach or protective row of sandbars can shield a city from storm surges, like the one that inundated the U.S. East Coast after Sandy. Less shells may mean weaker beaches.
The shells also act like oceanic antacids because calcium carbonate makes up much of their structure. Just as antacids neutralize stomach acid with pH-basic calcium carbonate, seashells slowly disolve in the sea and help neutralize carbonic acid dissolving into the ocean from the air.
Increasing levels of carbon dioxide in the air cause more acid to be absorbed by the ocean. Seashells act as a buffer against ocean acidification, but not if they are carted away in a tourist's bucket.
Photo: A hermit crab emerges from its shell. Credit: U.S. Fish and Wildlife Service, Wikimedia Commons