Abstract

The group model is a useful tool to understand broad-scale patterns of interaction in a network, but it has previously been limited in use to food webs, which contain only predator-prey interactions. Natural populations interact with each other in a variety of ways and, although most published ecological networks only include information about a single interaction type (e.g., feeding, pollination), ecologists are beginning to consider networks which combine multiple interaction types. Here we extend the group model to signed directed networks such as ecological interaction webs. As a specific application of this method, we examine the effects of including or excluding specific interaction types on our understanding of species roles in ecological networks. We consider all three currently available interaction webs, two of which are extended plant-mutualist networks with herbivores and parasitoids added, and one of which is an extended intertidal food web with interactions of all possible sign structures (+/+, -/0, etc.). Species in the extended food web grouped similarly with all interactions, only trophic links, and only nontrophic links. However, removing mutualism or herbivory had a much larger effect in the extended plant-pollinator webs. Species removal even affected groups that were not directly connected to those that were removed, as we found by excluding a small number of parasitoids. These results suggest that including additional species in the network provides far more information than additional interactions for this aspect of network structure. Our methods provide a useful framework for simplifying networks to their essential structure, allowing us to identify generalities in network structure and better understand the roles species play in their communities.

Highlights

  • Networks are a useful tool to understand patterns of interactions in an ecological community

  • Species are grouped together if they eat and are eaten by the same other groups. This recursive definition implies that species which are far from each other in the network may still impact each other’s grouping. This reflects the ecological reality of the complex ways in which species in a network influence each others’ dynamics, for example, via trophic cascades or apparent competition [5, 6]

  • The group model is a useful way to gain a coarse-grained view of ecological dynamics and the niches that are filled in the community

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Summary

Introduction

Networks are a useful tool to understand patterns of interactions in an ecological community. Species with identical niches were considered “trophic species”, and ecological networks were often simplified by combining them [2]. This approach is highly sensitive to small changes or errors in the network structure, since a single missing or false interaction can change which species may be combined. This recursive definition implies that species which are far from each other in the network may still impact each other’s grouping This reflects the ecological reality of the complex ways in which species in a network influence each others’ dynamics, for example, via trophic cascades or apparent competition [5, 6]. The group model is a useful way to gain a coarse-grained view of ecological dynamics and the niches that are filled in the community

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