Rapid development of indirect effects in ecological networks

Abstract

Indirect effects are important components of ecological and evolutionary interactions that may maintain biodiversity, enable or inhibit invasive species, and challenge ecosystem assessment and management. A central hypothesis of Network Environ Analysis (NEA), one type of ecological network analysis, is that indirect flows tend to dominate direct flows in ecosystem networks of conservative substance exchanges. However, current NEA methods assume that these ecosystems are stationary (i.e. time invariant exchange rates), which is unlikely to be true for many ecosystems for interesting time and space scales. For the work reported here, we investigated the sensitivity of the dominance of indirect effects hypothesis to the stationary modeling assumption by determining the development rate of indirect effects and flow intensity, as expressed as the number of transfer steps, in thirty-one ecosystem models. We hypothesized that indirect effects develop rapidly in ecological networks, but that they would develop faster in biogeochemically based models than in trophically based models. In contrast, our results show that indirect effects develop rapidly in all thirty-one models examined. In 94% of the models, indirect flows exceeded direct flows by a pathway length of 3. This indicates that ecological systems do not need to maintain a particular configuration for long for indirect effects to dominate. Thus, the dominance of indirect effects hypothesis remains plausible. We also found that biogeochemical models tended to require more of the extended path network than the trophic models to account for 50% and 95% of the total system activity, but that both types of models required more of the power series than is typically considered in engineered systems. These results succinctly illustrate the complexity of ecological systems and help explain why they are challenging to assess and manage.