You might think twice next time you snag a seashell from the beach and drop it into your pocket: You might be altering the seaside environment.

In a study more than 30 years in the making, researchers have found that the removal of shells from beaches could damage ecosystems and endanger organisms that rely on shells to survive.

The study 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.

At the time they weren’t thinking of ecology: They were doing research to understand what happens to shells after the organisms that inhabit them die.

Led by Michal Kowalewski, a curator at the Florida Museum of Natural History, the researchers returned to the same beach three decades later. They found that the abundance of seashells had decreased by 60 percent while tourism had increased in the area by 300 percent. Even though other factors might play a role in the shells’ decline, it is hard not to think that human behavior is to blame for the decline.

But could beach-goers who pocketed seashells – or bought them at beach shops – really account for what Kowalewski found? Probably not.

“Shell collecting by beachcombers is an intuitively obvious explanation,” he said, “but many other processes with tourism can lead to removal or destruction of shells.” Grooming the sand with heavy machinery to make a more comfortable beach-going experience, for example, can destroy shells. Such grooming is common at tourist beaches around the world. Another way in which tourism contributes to shell decline, the study suggested, is through the use of recreational vehicles on the sand.

The decline in shell abundance in the summers of 2008, 2009 and 2010 – but not in the summers of 1978 to 1981 – provides a clue that heavy machinery may indeed bear a large proportion of the blame. “This is a peak of the tourist season,” Kowalewski said, “but also the only time of the year when tractors with rakes are used regularly to clean the beach.” Those tractors weren’t in wide use in the late 1970s.

So while tourism may be at fault, it’s not individual tourists themselves who deserve the blame.