Cross‐ecosystem effects of a large terrestrial herbivore on stream ecosystem functioning

Abstract

Large terrestrial consumers have direct and indirect effects on forest ecosystem function, but few studies have investigated the impacts of terrestrial consumers on freshwater ecosystems. In the Cape Breton Highlands of Nova Scotia, browsing by hyper‐abundant moose following a spruce budworm outbreak has transformed boreal forest into grasslands. We conducted a field study to investigate the potential for cross‐ecosystem effects of hyper‐abundant moose following budworm outbreak on small boreal stream ecosystem structure and function. With our field study, we tested the prediction that watersheds with higher levels of moose‐mediated grasslands in their sub‐basin would have higher stream temperatures, total nitrogen, electrical conductivity, periphyton biomass and macroinvertebrate abundances. While our data supported several of our predictions pertaining to moose impacts on the abiotic variables (i.e. temperature range, total nitrogen, electrical conductivity) we found evidence of variable moose impacts on the benthic community. Specifically, we observed lower relative abundance of predatory invertebrates in streams with high moose impacts compared to streams with low moose impacts in their watersheds but no evidence of moose impacts on the relative abundance of shredders, filterers, gatherers, and grazers. This empirical study fills a key gap in our understanding of spatial ecosystem ecology by providing insight into the effects of large terrestrial consumers across ecosystem boundaries with potential implications for landscape‐scale management of hyper‐abundant ungulates. Given the broad availability and improvement in remote sensing technology, the novel integration of remote sensing and field studies employed here may provide a roadmap for future studies of meta‐ecosystem dynamics.

Decadal Shift in Nitrogen Inputs and Fluxes Across the Contiguous United States: 2002–2012

“…However, this top‐down approach to estimating fixation may fail to capture areas of high fixation rates due to the abundant presence of N‐fixing tree species. For example, Cleveland et al () predict low rates of fixation in the Pacific Northwest, but this region is dominated by high‐fixing species like red alder where N inputs from fixation are known to be high (MacSween et al, ).…”

Herbivore Impacts on Carbon Cycling in Boreal Forests

“…Recent research demonstrated that invertebrate herbivores [e.g., mountain pine beetle (Dendroctonus ponderosae) [4]] and invasive earthworms [31] can shift large regions of boreal forest from being a C sink to a C source during outbreak years. We surmise that large herbivore impacts will be greatest when mediated by natural and human disturbances [15,32] and that such impacts can lead to failed natural forest regeneration and alternative ecosystem states (Figure 2 and Box 1; [13,17,19]). Large herbivores have the capacity to respond quickly (e.g., increase in twinning rates of female moose) to favorable forage conditions created by large canopy openings, which ultimately increases their abundance.…”

Animal-vectored nutrient flows across resource gradients influence the nature of local and meta-ecosystem functioning

“…While the influences of abiotic vectors on ecosystem dynamics have been extensively studied (Gravel, Guichard, et al 2010; Gravel, Mouquet, et al 2010; Gounand, Mouquet, et al 2014; Loreau, Daufresne, et al 2013), less is known about how flows mediated by biotic vectors affect ecosystem processes and functions (Gounand, Harvey, et al 2018; but see Subalusky, Dutton, Njoroge, et al 2018; Schmitz, Wilmers, et al 2018). Evidence from both the fossil record and present-day events shows that biotic drivers of ecosystem flows affect ecosystem functions at extents ranging from local to continental (Bauer and Hoye 2014; Schmitz, Raymond, et al 2014; Doughty 2017; MacSween, Leroux, and Oakes 2019). However, organism-driven exchanges have diminished over time as humankind began modifying the biosphere (from the late Quaternary onwards; Doughty et al 2016).…”