Mechanistic insights into the role of large carnivores for ecosystem structure and functioning

Selwyn Hoeks,Michael B. J. Harfoot,Mark A. J. Huijbregts,Michela Busana,Luca Santini,Jens‐Christian Svenning

doi:10.1111/ecog.05191

Abstract

Large carnivores can exert top–down effects in ecosystems, but the size of these effects are largely unknown. Empirical investigation on the importance of large carnivores for ecosystem structure and functioning presents a number of challenges due to the large spatio‐temporal scale and the complexity of such dynamics. Here, we applied a mechanistic global ecosystem model to investigate the influence of large‐carnivore removal from undisturbed ecosystems. First, we simulated large‐carnivore removal on the global scale to inspect the geographic pattern of top–down control and to disentangle the functional role of large carnivores in top–down control in different environmental contexts. Second, we conducted four small‐scale ecosystem simulation experiments to understand direct and indirect changes in food‐web structure under different environmental conditions. We found that the removal of top–down control exerted by large carnivores (> 21 kg) can trigger large trophic cascades, leading to an overall decrease in autotroph biomass globally. Furthermore, the loss of large carnivores resulted in an increase of mesopredators. The magnitude of these changes was positively related to primary productivity (NPP), in line with the ‘exploitation ecosystem hypothesis’. In addition, we found that seasonality in NPP dampened the magnitude of change following the removal of large carnivores. Our results reinforce the idea that large carnivores play a fundamental role in shaping ecosystems, and further declines and extinctions can trigger substantial ecosystem responses. Our findings also support previous studies suggesting that natural ecosystem dynamics have been severely modified and are still changing as a result of the widespread decline and extinction of large carnivores.

Highlights

Large terrestrial carnivores have experienced severe population declines and geographic range contractions since the Late Pleistocene (Beschta and Ripple 2009, Prugh et al 2009, Dirzo et al 2014, Ripple et al 2014, Galetti et al 2018)
We analyzed the influence of top–down control exerted by large carnivores on ecosystem structure by performing mechanistic ecosystem simulations using the Madingley model
Our results support the idea that large carnivores play key roles in ecosystem structure and functioning (Ripple et al 2014, Wolf and Ripple 2018)

Summary

Introduction

Large terrestrial carnivores have experienced severe population declines and geographic range contractions since the Late Pleistocene (Beschta and Ripple 2009, Prugh et al 2009, Dirzo et al 2014, Ripple et al 2014, Galetti et al 2018). The strength of a possible response after the extinction of large carnivores can depend on the trophic structure and environmental conditions present in the pre-disturbed ecosystem (Pace et al 1999, Terborgh 2015, Ford et al 2017). These properties arise from the combination of the community composition, primary productivity, seasonality and other environmental factors (Hopcraft et al 2010, Ripple and Van Valkenburgh 2010). The loss of regulating feedbacks exerted by megafauna species can, in many cases, be directly linked to drastic changes in ecosystem structure and functioning in combination with a loss in biodiversity (Miller et al 2001, Prugh et al 2009, Ripple et al 2015)

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Results

Conclusion