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.