Resource dynamic plays a key role in regional fluctuations of the spruce bark beetles Ips typographus

Abstract

Abstract 1 We analysed time series of spruce bark beetles ( N t ) caught in pheromone traps from 1979 to 2000 in approximately 100 localities throughout south‐east Norway. 2 The analysis was performed at two spatial scales (whole area and 12 subregions), and included additional analyses of the time‐series sequences before and after a large windfelling (1987) that appeared to induce a shift in the dynamics. 3 Regression of R t (logarithmic per‐capita growth rate) on N t − d indicated that the endogenous dynamics were dominated by lag 1 density dependence. This was particularly so in the sequence after the large windfelling ( R 2 = 0.79), probably due to intraspecific competition for breeding substrate. 4 The sequence before the windfelling (1979–1987) was declining without significant density dependence. We suggest this to be a transition period, reflecting a drop in carrying capacity due to depletion of susceptible trees during the preceding outbreak period (1970s) and a drop in beetle number to below the density required to kill trees. 5 Windfelling ( W t − 1 ) appears to be an important predictor of the dynamics; however, it was difficult to evaluate the statistical significance of this factor due to only one large windfall event. 6 The weak influence of drought stress ( D t − 1 ) could be due to the absence of severe drought periods within the time series.