What Can Interaction Webs Tell Us About Species Roles?

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.

Fig 1. Example 10-species network partitioned using the group model.

Each row and column represents a species, and each dot in the heatmap represents an interaction between two species (red for negative impact of column on row, blue for positive impact of column on row, white for no interaction). Colors on the outer edge correspond to group membership. In (A), only trophic links are included, and the network is partitioned into 3 groups. In (B), both trophic and nontrophic interactions are included. The mutualism between the light purple and light green groups has caused the green and purple groups from part (A) to split into two subgroups. In this example, nontrophic interactions serve to refine trophic groups into subgroups, but additional interactions could potentially reinforce or directly conflict with groupings based on a single interaction type.

Fig 2. Mutual Information Venn Diagram for 5-species partitions A and B.

Left circle represents H(A), right circle represents H(B), and the intersection represents MI _AB. All areas are proportional to the values they represent.

Fig 3. Similarities between Tatoosh Mussel Bed partitions.

Venn Diagrams showing the similarity between pairs of partitions in the Tatoosh Mussel Bed: (A) the complete and trophic networks, (B) complete and nontrophic networks, (C) trophic and nontrophic, and complete and taxonomic groupings (D and E). Venn Diagrams are structured as in Fig 2, where the size of the left circle is proportional in area to the entropy of the first partition listed (H(A)), the right circle’s area represents the entropy of the second partition listed (H(B)), and the overlap between the circles is proportional to the Mutual Information values (MI). Stars next to MI values denote significance level (* <.05, ** <.01, *** <.001). Note that this figure includes only the partition comparisons that are discussed in the main text. For all partition comparisons, see S3 Fig.

Fig 4. Similarity between Tatoosh network groupings.

Alluvial diagrams comparing the species groupings for (A) complete and trophic webs, (B) complete and nontrophic webs, and (C) trophic and nontrophic webs. Complete network coloring matches colors in Fig 5. Note that the light red group in the complete grouping does not necessarily correspond to the light red group in the trophic group, and so on. Flows between groupings show species in common between two groups; line thickness is proportional to number of species in common. The complete Tatoosh network is organized into groups that are almost perfectly nested in the trophic grouping. The complete grouping also matches closely with the nontrophic groupings, but the trophic and nontrophic groupings are comparatively dissimilar.

Fig 5. Matrix structure of complete Tatoosh network, organized by groups.

The best complete Tatoosh network grouping, displayed in matrix form. Dot colors in the top row and leftmost column represent group identity (19 groups total). Red and blue dots in the matrix are defined as in Fig 1. Many of the groups in the partition correspond closely to a priori ecological knowledge about the system, for example in the foraging birds (dusty purple), limpets (light blue), and predatory snails (dark aqua). This highlights the success of this method in identifying relevant groups, even in the absence of specific ecological information. Full list of species and their group identities given in S1 Table.

Fig 6. Similarities between Doñana Biological Reserve plant partitions.

Venn Diagrams for similarity between pairs of plant partitions for the Doñana web: (A) complete and mutualist-removal webs, (B) complete and herbivore-removal webs, (C) mutualist-removal and herbivore-removal webs, and (D) complete network and taxonomic order. Figure structured as in Fig 3. This Figure includes only comparisons relevant to the main text; for all comparisons, see S4 Fig.

Fig 7. Similarity between Doñana plant groupings.

Alluvial diagrams comparing the plant groupings for (A) complete and herbivore-removal webs, (B) complete and mutualist-removal webs, and (C) herbivore-removal and mutualist-removal webs. All three comparisons show major areas of similarity, but the groupings in (C) have many more conflicts than (A) and (B).

Fig 8. Similarities between Norwood Farm plant partitions.

Venn Diagrams for similarity between pairs of plan partitions for the Norwood Farm webs: (A) complete mutualist-removal webs, (B) complete and herbivore-removal webs, (C) complete and parasitoid-removal webs, (D) complete and mutualist-and-parasitoid-removal webs, (E) complete web and taxonomic order, (F) mutualist-removal and herbivore-removal webs, (G) mutualist-removal and mutualist-and-parasitoid-removal webs, (H) herbivore-removal and parasitoid-removal webs, (I) herbivore-removal and mutualist-and-parasitoid-removal webs, and (J) parasitoid-removal and mutualist-and-parasitoid-removal webs. Figure structured as in Fig 3. Note that comparisons H-J are equivalent to the comparisons in Doñana, in that they show the effect of removing mutualists and herbivores in the absence of parasitoids. As in Figs 3 and 6, only partition comparisons relevant to the main text are included; for all comparisons, see S5 Fig.

Fig 9. Similarity between Norwood plant groupings.

Alluvial diagrams comparing the plant groupings for (A) complete and herbivore-removal webs, (B) complete and mutualist-removal webs, (C) herbivore-removal and mutualist-removal webs, and (D) complete and mutualist-and-parasitoid-removal webs. In general, these grouping are more dissimilar than seen in the Tatoosh and Doñana systems, and only (A) and (D) show more similarity than expected by chance.

Fig 10. Comparison between complete and taxonomic groupings.

Alluvial diagrams comparing complete web groupings with taxonomic groupings for (A) Tatoosh and kingdom, (B) Tatoosh and phylum, (C) Doñana and plant order, and (D) Norwood and plant order. All groupings are more similar than expected by chance. Kingdom matches very closely with the complete Tatoosh grouping, but has so few categories that it still provides very limited information. The other taxonomic groupings have more categories but still provide relatively little information.