Ecosystem retrogression leads to increased insect abundance and herbivory across an island chronosequence

1 Ecosystem retrogression, the decline-phase of ecosystem development, occurs during the long-term absence of catastrophic disturbance. It usually involves increased nutrient limitation over time, and leads to reductions in primary productivity, decomposition, and nutrient cycling. 2 As a consequence, retrogression can alter the quality and abundance of host plants as food resources, but little is known about how these changes influence herbivore densities and foliage consumption. 3 In this study, we used a 5000-year-old chronosequence of forested islands in northern Sweden on which retrogression occurs in the absence of lightning-induced wildfire. We asked whether retrogression affected the abundance and herbivory of a dominant herbivorous weevil (Deporaus betulae) and the quality and productivity of a dominant host-tree, mountain birch (Betula pubescens). 4 Betula pubescens trees on retrogressed islands were less productive and produced smaller, tougher leaves that were lower in nutrients and higher in secondary metabolites than did those trees on earlier-successional islands. 5 Despite the lower density and what ecologists might perceive as poorer quality of host plants, we observed several-fold higher weevil abundance and damage on retrogressed islands. This suggests that weevils might prefer the poorer quality leaves with higher secondary metabolites that occur on nutrient stressed host trees. 6 Our results show that ecosystem retrogression increases susceptibility of B. pubescens trees to attack by herbivorous weevils. 7 Our study provides evidence that ecosystem retrogression and associated shifts in the quantity and quality of available resources can operate as an important driver of abundance of a dominant insect herbivore.