IN July, delicate blue butterflies in Western Europe know that it is time to produce the next generation. To accomplish that, though, the butterflies need more than a mate. They also need the services of blooming blue marsh gentians and hungry red ants. Why? What role do plants and ants play in the life cycle of these butterflies?

One place to observe this intriguing three-way relationship is at Dwingelderveld National Park in the northern part of the Netherlands. This park is home to a large concentration of these blue butterflies. In spring and summer, the Dwingelderveld heaths are transformed into a multicolored carpet made up of many flowering plants, including blue marsh gentians, pink bog heather, and yellow bog asphodels. The blue butterflies are especially attracted to the dainty flowers of bog heather and the fringed flowers of blue marsh gentians—but for two different reasons. The flowering bog heather is a popular food stop serving nectar, and the marsh gentian is viewed as a potential storage place.

To ensure that Dwingelderveld National Park will remain a safe haven for the blue butterfly, the park’s caretakers now try to maintain the heathland by applying the same farming methods that were used by farmers centuries ago. As in the past, shepherds with flocks of sheep roam the heaths, and cattle graze on fields covered with tougher grasses. The grazing sheep and cattle clear spots where ling, bog heather, and other plants can germinate. (Presently, some 580 species of plants grow in the park.) In response, the blue butterflies in Dwingelderveld also do their share—their numbers are growing. In fact, this largest and most important heathland park in Europe is such a hospitable home for butterflies in general that 60 percent of all butterfly species living in the Netherlands can be seen there.