Species’ roles in food webs show fidelity across a highly variable oak forest

Silk button galls formed by the gall wasp (Neuroterus numismalis) located on the leaves of European Oak. Photo by Riikka Kaartinen.

by Nick Baker

Ecological communities are under constant pressure from a variety of natural and human-caused disturbances. As communities are made up of multiple interacting species, both the species identity and the way the species interact may be important to understanding how communities change due to disturbance. However, relatively little it is known about how important individual species are to the overall structure and function of a community.

To answer these questions, we explored species-interaction networks in a host-parasitoid community that has been shaped by considerable habitat modification. This system is a fragmented range of European oaks in southern Finland, where each tree in the system behaves as a habitat island supporting a high diversity of Hymenopteran and Lepidopteran species and their associated parasitoids.

Field collection of leaf miners and gallers using a pole pruner from an isolated European oak tree in Southern Finland. Photo by Riikka Kaartinen.

When working with data such as these, taking a network approach can prove quite useful. On the other hand, networks can become overly complex. One way to overcome this complexity is to "disassemble" networks into their constituent parts by building on the concept of network motifs. The general idea is that any network can be decomposed into a smaller set of motifs that act as the building blocks of the network which, when reassembled, form the original network. Furthermore, motifs allow us to identify the role that each species plays in the community which can bridge the gap between a list of the species in the community and the way in which those species contribute to community structure.

We applied the concept of network motifs to study variation in the host and parasitoid roles within our fragmented oak community through space and time. To do this, we first quantified the network motifs of each network and determined the role of each species in the network. We then tested whether species' roles are an intrinsic species property, whether the role of a species depends on the network in which it is found, and whether species' roles are consistent over time. Our results showed that both host and parasitoid roles were clustered by species identity, suggesting that species' roles may indeed be an intrinsic species characteristic. Furthermore, we found that community networks were characterized by a high degree of role overlap, and that species' roles were remarkably consistent between years.

After leaf miners and gallers are collected, individuals are placed into containers and reared until hatching to determine if they were attacked by a parasitoid species. Photo by Riikka Kaartinen.

Collectively, we were quite excited by our results because of the very reasons we started this collaboration in the first place. You see, in a paper published in 2012, Lewinsohn and Cagnolo (Science 2012) suggested that this system might be too ephemeral for a pattern like this to hold given the extreme turnover of species between years at each site. Instead, we found that the role structure of these networks was remarkably resilient to changes in species composition, reinforcing the idea that species' roles may provide an ideal tool for forecasting changes in a community. As a result, we believe that structural descriptions of species' roles might serve as a surprisingly useful jumping-off point for predicting future changes in ecological community structure due to disturbance.

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