Nutrient fluxes from insect herbivory increase during ecosystem retrogression in boreal forest

Abstract

Ecological theory, developed largely from ungulates and grassland systems, predicts that herbivory accelerates nutrient cycling more in productive than unproductive systems. This prediction may be important for understanding patterns of ecosystem change over time and space, but its applicability to other ecosystems and types of herbivore remain uncertain. We estimated fluxes of nitrogen (N) and phosphorus (P) from herbivory of a common tree species (Betula pubescens) by a common species of herbivorous insect along a -5000-yr boreal chronosequence. Contrary to established theory, fluxes of N and P via herbivory increased along the chronosequence despite a decline in plant productivity. The herbivore-mediated N and P fluxes to the soil are comparable to the main alternative pathway for these nutrients via tree leaf litterfall. We conclude that insect herbivores can make large contributions to nutrient cycling even in unproductive systems, and influence the rate and pattern of ecosystem development, particularly in systems with low external nutrient inputs.

The biogeochemical consequences of litter transformation by insect herbivory in the Subarctic: a microcosm simulation experiment

“…During these outbreaks, insect deposits (including excreta/frass, cadavers, moults, pupae, greenfall) constitute considerable nutrient fluxes from the canopy to the soil, with the largest single constituent being frass (Hunter 2001;Arnold et al 2016). Understanding how these outbreaks influence element cycling and plant growth is essential for improving predictions of C and N cycling (Throop et al 2004;Metcalfe et al 2016) and plant community composition (Jepsen et al 2013) across the Scandinavian Arctic, particularly as the extent and intensity of outbreaks appear to be increasing due to climate warming (Jepsen et al 2008;Kozlov and Zvereva 2017).…”

Below‐ground responses to insect herbivory in ecosystems with woody plant canopies: A meta‐analysis

“…The increased transfer of insect deposited organic matter from the canopy to soils has been quantified in various forest ecosystems (l‐M‐ Arnold et al, ; Lovett et al, ; Metcalfe et al, ; Metcalfe, Crutsinger, Kumordzi, & Wardle, ; Schowalter, Fonte, Geaghan, & Wang, ), but few studies examine it in concert with below‐ground responses. Observed soil responses may be direct consequences of the increased canopy‐to‐soil input of herbivore‐induced deposits, which have been shown to stimulate soil decomposer microbial growth and organic matter turnover in simple microcosms (Kristensen, Metcalfe, & Rousk, ; Lovett & Ruesink, ).…”

Hiding in the background: community-level patterns in invertebrate herbivory across the tundra biome

“…In contrast, under non-outbreak densities, invertebrates are responsible for low but chronic biomass removal, referred to as background herbivory . At these low densities the immediate effects of invertebrates appear minimal (Kotanen and Rosenthal 2000), but the longer-term nature of background herbivory may have prolonged effects on plant growth (Zvereva et al 2012), community interactions (Barrio et al 2013), and nutrient fluxes (Metcalfe et al 2016). The current understanding of the patterns of background invertebrate herbivory in tundra environments is based on only a few studies that focused on either a single host plant species (Betula glandulosa-nana complex, Barrio et al 2017) or on specific growth forms (shrubs, Kozlov et al 2015a).…”