DISEASE
The spread of disease among animal populations is another density-dependent factor. In the past, the wolf population on Isle Royale dropped considerably due to the outbreak of a deadly disease called canine parvovirus. You can see this drop in wolf population starting just before 1980 in the graph on the left.

With the resulting lower wolf population, fewer predators threatened the moose. Their numbers slowly increased each year beyond the carrying capacity of the island (the number of moose the island's food supply can support).

When too many moose overgrazed the island's resources, hundreds of moose abruptly died due to lack of food. This was combined with a frigid winter that stressed the health of the moose population.