The density of the two species depends strongly on the density of forage. Moose prefer birch and aspen trees, which used to grow plentifully on the island, but over a century of moose browsing have been largely replaced by the less nutritious balsam fir, which now makes up 59% of a moose's diet. Even this has declined dramatically: as of 2002, understory growth of balsam fir was at 5%, down 40% from 19th century observations. The plant is more plentiful on the east side of the island, which draws a higher concentration of moose. Because balsam fir does not give sufficient moisture, moose have recently been spotted eating snow, a very rare occurrence. They have also been sighted eating lichen, which researcher Rolf Peterson has compared to eating dust. When the moose population grows too high, the balsam fir population crashes, leading to a crash in the moose population, in a continuing population "see-saw". Moose mostly die from the consequences of malnutrition: they become emaciated and slowed down by arthritis, until they are easy prey for a wolf pack. Also, calves suffer from malnutrition when they are born during a winter with snow too deep for easy foraging.