Persistence, changes and robustness of nest webs along a latitudinal gradient in the
\nCanadian boreal forest

Introduction: In eastern Canada, the boreal forest is associated with an \nimportant latitudinal shift in forest composition and structure, which occurs in \nthe transition between the mixed southern boreal forest and the coniferous \nnorthern boreal forest. Along this transition, upland mixedwood stands with large \ndeciduous trees (important for cavity-dependent vertebrate species) are \ngradually replaced by forests with smaller conifer trees, primarily black spruce \n(Picea mariana). Concomitantly, the availability of lowland forests flooded by the \nAmerican beaver (Castor canadensis), which can provide adequate conditions for \ntree-cavity users, is also decreasing. \nMethods: We hypothesized that this latitudinal gradient would bring important \nchanges in the functional diversity and network structure of vertebrate cavityusing \ncommunities. Along this latitudinal gradient we used a nest web approach \nto analyze the structure and robustness of networks of cavity users in upland \nforests and in lowland forests flooded by beavers. \nResults: Despite their low availability in the northern forest region, we found that \nmixedwood stands persisted throughout the boreal forest in being the main \ndrivers of nest webs network structure of upland forests whereas old black \nspruce stands contribution was low. In lowland forests, beaver ponds harbored \nnest webs with a rich and complex structure in both forest regions. Species \nremoval simulations revealed that across our latitudinal gradient upland and \nlowland forest nest webs responded differently. In upland forests, the removal of \ntrembling aspen and the Pileated Woodpecker (Dryocopus pileatus) caused the \nhighest proportions of secondary extinctions, showing low robustness of nest \nwebs given that these two species were highly connected to the other species. \nContrastingly, nest webs in beaver ponds were more robust mainly because \nexcavator species used a higher diversity of tree species despite the removal of \nthe Northern Flicker (Colaptes auratus) which induced numerous secondary \nextinctions. The Pileated Woodpecker remained the pivotal species across the \ntwo forest regions in upland forests whereas the Northern Flicker became the \nmain large cavity provider in beaver ponds across the latitudinal gradient. \nDiscussion: We discuss how mixedwood forests and beaver ponds, which are key \nhabitat types for the cavity-using vertebrate community across our latitudinal \ngradient, should be maintained and protected in landscapes under industrial \ntimber harvesting.

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