An experimental study of how variation in deer density affects vegetation and songbird assemblages of recently harvested boreal forests

Etienne Cardinal,Jean-Louis Martin,Steeve D Côté,Jean-Pierre Tremblay

doi:10.1139/z2012-037

Abstract

Intense browsing by abundant large herbivores can threaten the ecological integrity of ecosystems by inducing modifications in the structure and composition of vegetation that trigger trophic cascades affecting plant and animal communities. We investigated the relationships between density of white-tailed deer ( Odocoileus virginianus (Zimmermann, 1780)), forest succession after clear-cut, and songbird communities on Anticosti Island, Quebec, Canada. We hypothesized that lower deer densities would alter the trajectory of forest succession after clear-cutting and lead to a rapid recovery of habitat attributes favorable to songbirds associated with a dense complex shrub layer. Six years after establishing a controlled browsing experiment (0, 7.5, 15, and >27 deer·km–2) in recent clearcuts, reducing deer densities ≤7.5 deer·km–2initiated the restoration of balsam fir ( Abies balsamea (L.) Mill.) forests and increased the regeneration of paper birch ( Betula papyrifera Marshall). Increasing birch ground cover from 10% to 20% increased songbird total abundance, species richness, and diversity by 17%, 39%, and 31%, respectively. Alder Flycatcher (Empidonax alnorum Brewster, 1895) was only present at ≤7.5 deer·km–2and strongly associated with birch regeneration. The regeneration of browse-resistant plants such as white spruce ( Picea glauca (Moench) Voss) in some areas at high deer density favored the maintenance of many shrub-dependent songbirds but also species usually associated with forest canopy. Active management of deer populations in Canadian harvested boreal forests will mitigate losses in vegetation and songbirds caused by over-browsing.

Highlights

Several populations of large wild herbivores in Europe and North America have reached abundance levels likely exceeding their natural range of variability (Côté et al 2004)
We investigated the relationships among deer density, habitat structure, and songbird community composition with a canonical correspondence analysis (CCA; ter Braak 1986; Palmer 1993; CANOCO software: ter Braak and Šmilauer 2002)
Vegetation compositional responses to treatments The general pattern of vegetation composition was correlated with deer density in clearcuts, but variability was high within deer density treatments (Figs. 1a, 1b)

Summary

Introduction

Several populations of large wild herbivores in Europe and North America have reached abundance levels likely exceeding their natural range of variability (Côté et al 2004). High herbivore density has modified biogeochemical cycles (Pastor et al 1998; Persson et al 2005), reduced primary productivity (Hobbs 1996; Persson et al 2005), and modulated tree regeneration and forest structure (Alverson et al 1988; Rooney 2001; Tremblay et al 2007) Such changes can lead to alternate succession pathways (Healy 1997; Augustine et al 1998) and alter the attributes of ecosystems (Augustine et al 1998; Suding et al 2004; Royo et al 2010). They trigger indirect effects on food webs (e.g., Berger et al 2001; Côté et al 2004; Martin et al 2010; Ripple and Beschta 2006; Suominen and Danell 2006) such as decreasing the abundance and diversity of songbirds (Casey and Hein 1983; Allombert et al 2005a), small mammals (McShea and Rappole 1992; Moser and Witmer 2000), and some invertebrates (Stewart 2001; Allombert et al 2005b)

Methods

Results

Conclusion