SAGUARO NATIONAL PARK, Ariz. — When Steve Bolyard checked out a report of black paint on some of the park’s majestic saguaros — cactuses whose towering bodies and upraised arms are as emblematic of the American West as red-rock buttes and skittering tumbleweeds — he did not expect to see ganglike calligraphy covering more of them than he could easily count.

“It was too much,” said Mr. Bolyard, a park ranger. The same sort of symbols one might see on a subway train were scattered along the spiny forest last month. Rangers eventually found at least 45 graffiti tags in the park, including 16 on the slow-growing and fragile saguaro, the paint obscuring part of the green skins where the plants store the chlorophyll to draw nourishment from the sun.

It was the latest example of a trend that has been unnerving park officials from Rocky Mountain National Park in Colorado to Arches in Utah and Joshua Tree in California. Just as drought and rapid development have caused a rise in encounters between humans and wild animals on the edges of many American cities, the wilder side of urban life — vandalism, graffiti and litter — has found its way into the wilderness.