Evidence for the Dominance of Indirect Effects in 50 Trophically-Based Ecosystem Networks

TL;DR

This study demonstrates that indirect effects overwhelmingly dominate direct effects in trophic ecosystem networks, with robustness confirmed through uncertainty analysis and highlighting the microbial food web's mediating role.

Contribution

It provides empirical evidence that indirect flows are the primary drivers in ecosystem networks, emphasizing the significance of the microbial food web and network cycling.

Findings

Indirect flows dominate in 74% of models.

Dominance increases to 88.5% in models with cycling and microbial web.

Results are robust to data uncertainty.

Abstract

Indirect effects are powerful influences in ecosystems that may maintain species diversity and alter apparent relationships between species in surprising ways. Here, we applied Network Environ Analysis to 50 empirically-based trophic ecosystem models to test the hypothesis that indirect flows dominate direct flows in ecosystem networks. Further, we used Monte Carlo based perturbations to investigate the robustness of these results to potential error in the underlying data. To explain our findings, we further investigated the importance of the microbial food web in recycling energy-matter using components of the Finn Cycling Index and analysis of Environ Centrality. We found that indirect flows dominate direct flows in 37/50 (74.0%) models. This increases to 31/35 (88.5%) models when we consider only models that have cycling structure and a representation of the microbial food web. The uncertainty analysis reveals that there is less error in the I/D values than the $\pm$ 5% error introduced into the models, suggesting the results are robust to uncertainty. Our results show that the microbial food web mediates a substantial percentage of cycling in some systems (median = 30.2%), but its role is highly variable in these models, in agreement with the literature. Our results, combined with previous work, strongly suggest that indirect effects are dominant components of activity in ecosystems.