Population dynamics of tree-killing bark beetles

During outbreak periods, the European spruce bark beetle and the North American mountain pine beetle are able to kill millions of coniferous trees. Throughout the 20th century, six outbreaks have occurred in Sweden and four in British Columbia, with about 20-year intervals in both regions. The outbreaks of the mountain pine beetles seem to grow much larger and last longer compared to the outbreaks of the spruce bark beetles. Over the years, the mountain pine beetle has killed about 60 million ha forest or 550 million m3 trees in British Columbia, which is at least one hundred times more than for the Spruce bark beetle in Sweden. Damages of both species have increased markedly in the last forty years. About 750 spruce bark beetles per m2 are necessary to kill a healthy spruce, whereas seven times fewer, i.e., about 110 mountain pine beetles per m2, are needed to kill a healthy pine. Furthermore, twice as many offspring per m2 bark are produced by the spruce bark beetle compared to the mountain pine beetle. An explanation for the large differences in population dynamics between these two beetle species may spring from differences in (1) the availability of host trees, (2) number of specimens required to kill a tree, and (3) reproductive success. The latter is in turn affected by the intraspecific competition, nutrient content, and occurrence of fungi