Why body size matters: how larger fish ontogeny shapes ecological network topology

Abstract

Ontogenetic development can strongly shape species interactions. Yet, rarely is stage‐structure considered when analyzing species interaction networks, particularly networks that can account for more than feeding relationships. Here, we assess 1) if body size or trophic level regulate the importance of species' ontogeny on their interactions and 2) how including relevant stage‐structure affects the topology of species interaction networks. We use a count‐based inferential method to create networks from adult and juvenile fish count data and test stage‐structure importance by comparing a model that includes stage‐structure for all species against models that include stage‐structure only for larger fishes and only for piscivorous fishes during network construction. While the inferential method we use cannot differentiate between different types of interactions, it can account for different interaction types within a network as a pairwise interaction is inferred when one species influences the abundance of another. Next, we use graphlet‐based techniques to test if including stage‐structure alters overall network topology and a linear model to measure if adult‐juvenile size differences drive interaction differences at a species‐level. We find that the model that includes stage‐structure only for larger fishes outperforms other stage‐structured models including the model with only piscivore stage‐structure, and that larger differences in body size among juveniles and adults lead to greater interaction dissimilarities. Moreover, we find topological differences between inferred networks that only include adults and those that account for the stage‐structure of larger species. Overall, our study demonstrates how stage‐structured topological changes can be measured using inferred interaction networks and illustrates how larger species' juveniles fundamentally shape the structure of stream fish communities.