Next time you're scouring the beaches for seashells to add to your collection, think again. A study 30 years in the making has found that the removal of seashells from beaches could damage ecosystems and endanger organisms that rely on shells for their survival.

The study, published in the journal PLOS ONE, focused on a stretch of coastline on Spain's northeastern Mediterranean shore called Llarga Beach, where the researchers conducted monthly surveys of seashell abundance between 1978 and 1981.

Researchers were actually doing paleontology research, but came back to that same beach three decades later after realizing what else their data could tell them.

"Only later did our research group realize that this quantitative data set offered us a unique opportunity to evaluate changes in shell abundance on a beach that was increasingly frequented by tourists," study leader Michal Kowalewski told The Washington Post.

When they returned to Llarga Beach, a popular tourist attraction, they found that the abundance of seashells - specifically the empty shells of three types of clams - had decreased by 60 percent while tourism had increased in the area by 300 percent.

Shell abundance was especially low in the tourist-heavy summer months of 2008 to 2010 compared with the rest of the year. While it may seem easy to blame humans, researchers ruled out fishing, urban development and major ecological or environmental changes as reasons for the decline.

"Shell collecting by beachcombers is an intuitively obvious explanation," Kowalewski said.

He added that sand combing, which uses heavy machinery to sift out shells to create smooth sand, also adds to the problem.

Seashells are important for coastal ecosystems, researchers advocate. They provide materials for birds' nests, a home or attachment surface for algae, sea grass, sponges and many other microorganisms. Fish use them to hide from predators, and hermit crabs use them as temporary shelters. The removal of large shells and shell fragments also has the potential to alter the rate of shoreline erosion.

"I'd rather assume that there are consequences and be proven wrong than the other way around," said University of Arizona geosciences professor Karl W. Flessa, who was not involved in the study.

"I'll keep my hands in my pockets the next time I go to the beach," he added.