They examined differences between red squirrels and gray squirrels in the American state of Indiana.The scientists wanted to know how these differences could affect the growth of black walnut trees.The black walnut is the nut of choice for both kinds of squirrels.The black walnut tree is also a central part of some hardwood forests.

　　Rob Swihart of Purdue University did the study with Jake Goheen, a former Purdue student now at the University of New Mexico.The two researchers estimate that several times as many walnuts grow when gathered by gray squirrels as compared to red squirrels.Gray squirrels and red squirrels do not store nuts and seeds in the same way.Gray squirrels bury nuts one at a time in a number of places.But they seldom remember where they buried every nut.So some nuts remain in the ground.Conditions are right for them to develop and grow in the following spring.Red squirrels, however, store large groups of nuts above ground.Professor Swihart calls “death_traps_for_seeds”．

　　Gray squirrels are native to Indiana.But Professor Swihart says their numbers began to decrease as more forests were cut for agriculture.Red squirrels began to spread through the state during the past century.

　　The researchers say red squirrels are native to forests that stay green all year, unlike walnut trees.They say the cleaning of forest land for agriculture has helped red squirrels invade Indiana.Jake Goheen calls them a sign of an environmental problem more than a cause.

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