Abstract

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 of Rangifer 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‐Rangifer introduction (Atka: ~100 yr; Adak: ~50; Kagalaska: ~0), with distinct stages of irruptive population dynamics of Rangifer nested within each island (Atka: irruption, K‐overshoot, decline, K‐re‐equilibration; Adak: irruption, K‐overshoot; Kagalaska: initial introduction). We also measured Rangifer spatial 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‐growing Cladonia lichens that are highly preferred winter forage were decimated on both long‐term Rangifer‐occupied islands. In addition, linear relations between more concentrated Rangifer spatial 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 that Rangifer population 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.

Highlights

  • Herbivores strongly impact ecosystems with weak top-­down control (Hairston et al 1960, Oksanen and Oksanen 2000)

  • A combination of highly labile plant litter and large inputs of readily decomposable plant fiber in herbivore waste enhances soil nutrient cycling that further stimulates aboveground productivity of plants with high tissue quality (Hobbs 1996, Bardgett and Wardle 2010)

  • Small human communities (

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Summary

Introduction

Herbivores strongly impact ecosystems with weak top-­down control (Hairston et al 1960, Oksanen and Oksanen 2000) It follows that nonnative herbivory in ecosystems lacking predators represents a perturbation that can disrupt feedbacks that maintain a particular ecosystem state (Scheffer et al 2001, Suding et al 2004, Standish et al 2014) by altering plant community structure that shift patterns of nutrient cycling (Jefferies et al 1994, Hobbs 1996, Pastor and Cohen 1997, Ritchie et al 1998, Singer and Schoenecker 2003, Bardgett and Wardle 2010). Processes can shift from accelerating to decelerating within foraging patches when herbivory intensity surpasses some threshold (e.g., herbivore optimization) (McNaughton 1979, 1983, Hobbs 1996)

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