Importance of Weak Trophic Interactions in the Structure of the Food Web in La Paz Bay, Southern Gulf of California: A Topological Approach

Andrés F. Navia, SQUALUS
Sergio R. Maciel-Zapata, Instituto Politécnico Nacional
Adrián F. González-Acosta, Instituto Politécnico Nacional
Robert T. Leaf, University of Southern Mississippi
Victor H. Cruz-Escalona, Instituto Politécnico Nacional


In this work we describe the trophic structure of the La Paz Bay, Baja California Sur, Mexico and, using a suite of topological indices, evaluate the role of weak trophic interactions in the ecosystem. We used information derived from published and unpublished stomach contents analysis studies from the region to construct an unweighted and directional trophic network (comprising 250 nodes and 1,528 associated interactions), to assess the impact of predators in three scenarios. Each scenario simulated the removal of weak interactions corresponding to removal of those nodes with trophic contributions lower than 5%, 10%, and 15%. In each removal scenario, the nodes phytoplankton and zooplankton exhibited the greatest betweenness and closeness centrality, suggesting that they are critical groups in the structure of the web, especially in the transference of indirect effects to the largest number of possible nodes in the network. The multiphyletic nature of the phytoplankton and zooplankton and their ubiquity in diets served to increase their connectivity to all taxonomic groups. We note the presence of numerous weak interactions (67% of taxa have weak trophic interactions) and speculate that this characteristic confers ecological resilience in the Bay of La Paz coastal marine ecosystem.