AbstractIntroductions of mammalian herbivores to remote islands without predators provide a natural experiment to ask how temporal and spatial variation in herbivory intensity alter feedbacks between plant and soil processes. We investigated ecosystem effects resulting from introductions ofRangifer tarandus(hereafter “Rangifer”) to native mammalian predator‐ and herbivore‐free islands in the Aleutian archipelago of Alaska. We hypothesized that the maritime tundra of these islands would experience either: (1) accelerated ecosystem processes mediated by positive feedbacks between increased graminoid production and rapid nitrogen cycling; or (2) decelerated processes mediated by herbivory that stimulated shrub domination and lowered soil fertility. We measured summer plant and soil properties across three islands representing a chronosequence of elapsed time post‐Rangiferintroduction (Atka: ~100 yr; Adak: ~50; Kagalaska: ~0), with distinct stages of irruptive population dynamics ofRangifernested within each island (Atka: irruption, K‐overshoot, decline, K‐re‐equilibration; Adak: irruption, K‐overshoot; Kagalaska: initial introduction). We also measuredRangiferspatial use within islands (indexed by pellet group counts) to determine how ecosystem processes responded to spatial variation in herbivory. Vegetation community response to herbivory varied with temporal and spatial scale. When comparing temporal effects using the island chronosequence, increased time since herbivore introduction led to more graminoids and fewer dwarf‐shrubs, lichens, and mosses. Slow‐growingCladonialichens that are highly preferred winter forage were decimated on both long‐termRangifer‐occupied islands. In addition, linear relations between more concentratedRangiferspatial use and reductions in graminoid and forb biomass within islands added spatial heterogeneity to long‐term patterns identified by the chronosequence. These results support, in part, the hypothesis thatRangiferpopulation persistence on islands is facilitated by successful exploitation of graminoid biomass as winter forage after palatable lichens are decimated. However, the shift from shrubs to graminoids was expected to enhance rates of nitrogen cycling, yet rates of net N‐mineralization,pools, and soil δ15N declined markedly along the chronosequence and were weakly associated with spatial use within islands. Overall plant and soil patterns were disrupted but responded differently to intermediate (50 yr) and long‐term (100 yr) herbivory, and were correlated with distinct stages of irruptive population dynamics.