Two species of squirrel lived along a hillside in California. One species had a range that extended from sea level to about 5,000 meters in elevation; the other species had a principal range from 5,000 meters to 10,000 meters. However, when a viral epidemic greatly reduced the population of the low-elevation species, the high-elevation species increased its range down to 3,000 meters. This suggests that ________ was occurring and that coexistence was due to ________.