Ecological processes are critical for maintaining biodiversity. Anthropogenic stress on ecological communities increases their vulnerability to extirpations and extinctions of species and with them go also the networks of relationships. These interactions may play critical roles in shaping life history strategies. In the Northern Forest, herbivory by large ungulates is a keystone ecological process with direct and indirect effects on other trophic levels. Over much of their range, browsing by moose (Alces alces) has been shown to induce changes in plant morphology, reduce tree growth and survival, and shift competitive balances. In the Northern Forest, some plants double as winter moose browse and spring/summer bird nesting substrate. By browsing only plants exposed above snowpack, moose alter nesting habitat and reshape the architecture of the understory. Here, moose and black-throated blue warbler (Setophaga caerulescens) engage in a cross-trophic relationship through their utilization of plants in the forest understory, particularly hobblebush (Viburnum alnifolium).

This research added to an existing long-term survey of bird abundance and distribution. We tested the hypothesis that the effect of moose herbivory leads to changes in the distribution and branching patterns of understory shrubs, leading to changes in habitat selection for black-throated blue warbler. Using an established sampling grid, we measured moose activity, understory composition and structure, and bird abundance. We modeled the effects of moose activity on understory plant morphology and tested for correlations in distribution of species. We also compared characteristics of hobblebush patches surrounding black- throated blue warbler nest sites with randomly selected adjacent patches.

In the Northern Forest community, moose, hobblebush and black-throated blue warbler engage in a multi-trophic relationship. This study yields new information about each species, their ecological community relationship, and the study of habitat use. Our results show that 1) moose, through the act of winter browsing on buds and twigs, have a profound physical effect on hobblebush and 2) black-throated blue warbler select nest sites based on physical attributes influenced by moose herbivory. A moose browses a patch of hobblebush and the plants develop more branches than they would otherwise. Browsed plants create more structurally complex microhabitats (also measured in terms of visual obscurity). Within their own territories, black-throated blue warblers prefer patches with more branches (moose browse effect) and greater visual obscurity for nesting. Moose returned to the Northern Forest ~30 years ago but this relationship likely preceded their extirpation. Since recolonizing, moose have become a charismatic species symbolic of the Northern Forest. Moose populations contribute to wildlife watching and ecotourism in the region. While these two species’ ranges overlap, under forces of global change, they may not always, and understanding the community dynamics and habitat preferences of species may be critical for long-term management as habitats and communities reorganize.