Cross-scale effects of spruce budworm outbreaks on boreal warblers in eastern Canada.

Mark C Drever,Adam C Smith,Lisa A Venier,David A Maclean,Darren J.H Sleep

doi:10.1002/ece3.4244

Abstract

Insect outbreaks are major natural disturbance events that affect communities of forest birds, either directly by affecting the food supply or indirectly by changing the vegetation composition of forest canopies. An examination of correlations between measures of bird and insect abundance across different spatial scales and over varying time lag effects may provide insight into underlying mechanisms. We developed a hierarchical Bayesian model to assess correlations between counts of eight warbler species from the Breeding Bird Survey in eastern Canada, 1966 to 2009, with the presence of spruce budworm (Choristoneura fumiferana Clem.) at immediate local scales and time‐lagged regional scales, as measured by extent of defoliation on host tree species. Budworm‐associated species Cape May warbler (Setophaga tigrina), bay‐breasted warbler (Setophaga castanea), and Tennessee warbler (Oreothlypis peregrina) responded strongly and positively to both local and regional effects. In contrast, non‐budworm‐associated species, Blackburnian warbler (Setophaga fusca), magnolia warbler (Setophaga magnolia), Canada warbler (Cardellina canadensis), black‐throated blue warbler (Setophaga caerulescens), and black‐throated green warbler (Setophaga virens), only responded to regional effects in a manner that varied across eastern Canada. The complex responses by forest birds to insect outbreaks involve both increased numerical responses to food supply and to longer term responses to changes in forest structure and composition. These effects can vary across spatial scales and be captured in hierarchical population models, which can serve to disentangle common trends from data when examining drivers of population dynamics like forest management or climate change.

Highlights

Outbreaks of eastern spruce budworm (Choristoneura fumiferana Clem.) are major natural disturbance events affecting long-term cycles of growth and succession in forests of eastern North America (Baskerville, 1975)
Budworm-associated species showed weaker regional responses in the Atlantic Provinces than in Ontario or Quebec, and such strata-scale differences in responses may be related to variation in both forest management and the overall scale of the outbreak among provinces
In both New Brunswick and Nova Scotia, the defoliation had relatively lower spatial extents and very rarely exceeded 25% of the region (3 years in New Brunswick, and 2 years in Nova Scotia), while in the other regions the peak in regional defoliated area ranged from 55%–95% of the region and exceeded 25% in 12–17 years (Figures 3-7)

Summary

| INTRODUCTION

Outbreaks of eastern spruce budworm (Choristoneura fumiferana Clem.) are major natural disturbance events affecting long-term cycles of growth and succession in forests of eastern North America (Baskerville, 1975). After an outbreak begins to subside and tree mortality eventually reduces available food for the insect, abundance of budworm-associated species should increase in neighboring areas not directly affected, as the increased population redistributes to suitable habitat across the broader landscape These ecological processes suggest that population-level effects of budworm on forest birds should occur at two distinct spatial and temporal scales: Species should respond at local scales in a given year (higher territory density from preferential habitat selection) and at a broader, regional scale after a few years of sustained outbreak (increased reproductive rate followed by dispersal as local outbreaks decline). We predicted that if a warbler species can track budworm availability in the region and the hyperabundant food resource leads to increases in their population, we should expect both a positive local response to budworm in a given year on a given BBS route and a regional response after a short time lag reflecting new recruits to the population that are dispersing out of the budworm-affected areas

| METHODS

Findings

| DISCUSSION