Transient top‐down and bottom‐up effects of resources pulsed to multiple trophic levels

Abstract

Pulsed fluxes of organisms across ecosystem boundaries can exert top‐down and bottom‐up effects in recipient food webs, through both direct effects on the subsidized trophic levels and indirect effects on other components of the system. While previous theoretical and empirical studies demonstrate the influence of allochthonous subsidies on bottom‐up and top‐down processes, understanding how these forces act in conjunction is still limited, particularly when an allochthonous resource can simultaneously subsidize multiple trophic levels. Using the Lake Mývatn region in Iceland as an example system of allochthony and its potential effects on multiple trophic levels, we analyzed a mathematical model to evaluate how pulsed subsidies of aquatic insects affect the dynamics of a soil–plant–arthropod food web. We found that the relative balance of top‐down and bottom‐up effects on a given food web compartment was determined by trophic position, subsidy magnitude, and top predators’ ability to exploit the subsidy. For intermediate trophic levels (e.g., detritivores and herbivores), we found that the subsidy could either alleviate or intensify top‐down pressure from the predator. For some parameter combinations, alleviation and intensification occurred sequentially during and after the resource pulse. The total effect of the subsidy on detritivores and herbivores, including top‐down and bottom‐up processes, was determined by the rate at which predator consumption saturated with increasing size of the allochthonous subsidy, with greater saturation leading to increased bottom‐up effects. Our findings illustrate how resource pulses to multiple trophic levels can influence food web dynamics by changing the relative strength of bottom‐up and top‐down effects, with bottom‐up predominating top‐down effects in most scenarios in this subarctic system.

A methodological roadmap to quantify animal‐vectored spatial ecosystem subsidies

“…Animals of all sizes can have important impacts on ecosystems through the deposition of subsidies. For example, midges have been shown to translocate resources from aquatic to terrestrial systems (Gratton et al, 2017), altering soil-plant-arthropod food webs (McCary et al, 2021). Increasingly, large-bodied animals are recognized for playing an especially important role as landscapescale vectors of ecosystem subsidies (Bauer & Hoye, 2014;McInturf et al, 2019).…”

Quantifying community responses to environmental variation from replicate time series

“…This study fits into a larger effort to understand the role of allochthonous inputs on recipient communities (Polis et al 1997, McCary et al 2020). One of the challenges in identifying the effects of allochthonous pulses from long-term observational data is that pulses are likely to be conflated with other factors that vary through space and time (Yang et al 2008).…”

Body mass and latitude predict the presence of multiple stressors in global vertebrate populations

“…Given that body mass is often positively related with trophic level, particularly in aquatic environments (Riede et al 2011), our analysis suggest that predators are likely to be disproportionately threatened by multiple stressors, which may in part account for these observed losses. These impacts on predators are known to have cascading effects on the stability of food webs by changing the strength of direct and indirect top-down effects (McCary et al 2020). By altering the stability of predator populations, changes in biodiversity (Levin & Lubchenco 2008), biomass (Soliveres et al 2016), disease and carbon sequestration are all possible (Estes et al 2011), with extensive cascading effects seen throughout ecosystems worldwide (Beschta & Ripple 2020).…”