Cyclic population fluctuations of herbivores as an effect of cyclic seed cropping of plants: the mast depression hypothesis

Three sets of time series on plant seed production and herbivore number were analysed in order to test the proposed ''mast depression hypothesis'', which states that cyclic population fluctuations of herbivores is a result of cyclic seed cropping of plants, because high seed crops, termed masts, are produced at the expense of chemical defence against herbivores. From 1880 to 1916, the annual export of small game, mainly willow grouse Lagopus lagopus, from Oslo was positively correlated with the quantity of berries, mainly bilberry Vaccinium myrtillus and cowberry V. vitis-idaea, exported in the previous year. In Vegarshei, South Norway, the intervals between years of high access of bilberry and cowberry varied from 3 to 5 yr during 1952-95. All mast years were followed by a year of high microtine population. In South Norway, mast years of Norway spruce Picea abies, which usually shows high flowering intensity in the same years as mountain birch Betula pubescens, occurred with a mean interval of 9.8 yr (range 7-13 yr) during 1930-85. Almost all attacks of autumnal moth Epirrita autumnata on mountain birch were reported after a mast year of forest trees. The synchronised flowering cycles of plants indicate that high seed crops are induced by some climatic conditions. Allocating a large proportion of available resources, including chemical defence compounds, to seed production in some years of advantageous conditions may improve a plant's total life-time reproduction, despite the subsequent risk of serious damage by herbivores.