Trophic and non-trophic seasonal interaction network for boreal forest tetrapods

Aim: Understanding the organization of the wide variety of ecological interactions is crucial to advancing our understanding and management of real ecosystems. We aimed to compile a “complete” network of tetrapod trophic and non-trophic interactions for the entire North American boreal forest biome that could be analyzed to gain insights into community organization and function. In particular, we aimed to identify functionally important units (modules) and species within the boreal network, and to compare how these changed seasonally and with the inclusion of non-trophic interactions.

Location: Boreal North America

Time period: 1950 – present day

Major taxa studied: Tetrapods

Methods: We compiled published ecological interactions for boreal tetrapods into a food web (trophic interactions) and inclusive network (trophic and non-trophic interactions). We partitioned interactions by season, creating four networks representing the two network types per season. We examined how the modular structure, composition of modules, assortativity of traits within modules, and importance of different species compared across these networks.

Results: We compiled a network of 5037 ecological interactions between 421 boreal tetrapod species. Most of these interactions (87%) occur in summer. The summer and winter boreal food webs and inclusive networks are modular (i.e., contain subsets of species interacting more with each other than with species outside of the subset). Several species attributes explain which species assort together into modules, including physical and behavioural traits, taxonomic class, and trophic niche. A small set of species come out as most functionally important (central, module hubs, or responsible for the greatest network change when non-trophic interactions are included) across all versions of the network, and other species are important within a certain season or interaction context.

Main conclusions: Potential conservation management units (modules) exist in the boreal forest network, and considering species’ function at the community level highlights new priorities for species-level management.

Methods
Using a systematic search, trophic and non-trophic interactions between tetrapod species of the North American boreal forest were extracted from the literature. All extant tetrapods that regularly live, breed, or migrate in the nearly 600 million hectares of the North American boreal biome were included. Wild, non-native species known to occur in self-sustaining populations were included, such as feral horses. Not all are woodland species, as the boreal biome encompasses wetlands, urban areas, meadows, and some agricultural matrix. Given the wide scope of possible non-trophic interactions and varying degrees of ecological importance, the non-trophic interaction search focused primarily on interactions that affected nesting or denning, feeding, and access to territory, because these non-trophic interactions include both summer and winter processes and can affect species distributions and individual survival in an obvious way. We included only recorded ecological interactions. Species other than terrestrial tetrapods were classified into groups, as it was infeasible to include every organism at the species level for this spatial scale, and species-level information was often lacking. We treated these groups as nodes that could be the "giver" or the "receiver" of an interaction, like the species-level nodes in the network.