Finding out when rare and common species change their interactions using multi-site interaction turnover

This blog post is provided by Marie V. Henriksen and tells the #StoryBehindThePaper for the paper “A multi-site method to capture turnover in rare to common interactions in bipartite species networks”, which was recently published in the Journal of Animal Ecology.

In ecological networks, species are linked by their interactions to form complex interaction networks. How species interact in these networks reveals what role they play in maintaining the function and stability of the system. Ecological networks are usually made up of many rare species that have few interactions and a few common generalists that interact with many species. Because of their many interactions, generalists are particularly important for maintaining the overall structure of the system. On the other hand, rare species are found in the periphery of the network and can be lost without large impact on other species. Due to their high numbers, rare specialists are, however, important for overall species richness. To conserve both species and their ecological networks, it is therefore necessary to know how both rare and common species respond to changes in their environment.

In the last decade, different measures of interaction turnover have been used to show when, and why, species change their interactions across space and time. However, due to how they are calculated, these methods are dominated by the interactions of species that are rare across networks and largely overlook the response of common species. Interaction turnover is adapted from the species turnover literature and a focus on rare species is recognised as a general challenge for calculating turnover in ecology. In 2014, Melodie McGeoch and Cang Hui therefore proposed zeta diversity as a multi-site measure of species turnover that describes change in both rare and common species. Even though zeta diversity was developed for species communities, it has the potential to be used as a comprehensive measure of change in many types of systems and contexts, including ecological networks.

While zeta diversity was being developed, I was working on my PhD at Monash University in Australia with Melodie McGeoch as my supervisor. For this work, I was studying species interactions in an ecological network of gall wasps and their natural enemies on Acacia trees. As part of my PhD, along with our co-authors, we applied the concept of zeta diversity to ecological networks to develop a measure of interaction turnover that calculates change in the interactions of both rare and common species.

Galls made by gall wasps on Acacia trees (left) attract a range of different natural enemy wasps that use their ovipositors to lay eggs inside the gall chambers (right). Photo credit: Marie V. Henriksen.

To understand the mechanisms driving network change, we also expanded on two components of interactions turnover that have previously been proposed – species turnover and interaction rewiring. Interactions can change due to species turnover when species disappear from the network, for example, because the environment is no longer suitable for them. Alternatively, interactions can change due to rewiring when species switch, or rewire, their interactions to feed on a different resource species. Since rare species are typically less flexible in their food choice than common species, rare and common species are expected to contribute differently to species turnover and interaction rewiring. We therefore developed this multi-site method to reflect these potential differences between rare and common species.

When multiple ecological networks are compared (M1, M2 and M3), interactions between species (black bars) can be divided into interactions that are shared (grey) and interactions that change because species disappear (green), species change their resource (orange) or a mixture of both (purple). When many networks are compared, it is the common interactions that dominate interaction turnover.

With this method we could then calculate several factors that influence species interactions, including environment, commonness, species turnover and interaction rewiring. To illustrate this, we used the gall wasp-natural enemy system which was sampled across locations in Melbourne. This landscape is highly impacted by human activity and there is large variation in the amount and fragmentation of available insect habitat such as host plants and urban green space. Over a few weeks in early summer 2014, with help from two great research assistants, we collected 10,000 galls on Acacia trees along roads and in city parks and stored them in the laboratory at the university.

The gall wasp larvae feed on plant tissue in the gall where they also encounter larvae of natural enemy wasps. The natural enemies eventually eat the gall wasp larvae or compete with them by eating the plant tissue. Both gall wasps and their natural enemies can be identified when they emerge from the galls as adults, weeks to months later. So, for the rest of the summer, we checked the collected galls and identified adult wasps that emerged.

Interactions between wasp larvae in gall chambers are revealed when the surviving adult gall wasps (left) and natural enemies (right) emerge from the galls. Photo credit: Marie V. Henriksen.

From the emerged gall wasps and natural enemies, we constructed 13 interaction networks from sites across the urban landscape and calculated how interactions of rare and common wasps in the system changed with habitat loss and fragmentation. The results showed that different types of habitats were needed to maintain the interactions of rare and common species in the networks. Rare interactions changed in response to urban green space while common interactions were strongly related to host plant availability and habitat fragmentation. Interactions of both rare and common species changed most rapidly when there was little of these habitats left in the surrounding landscape, revealing their different threshold tolerances to habitat loss. These habitat loss thresholds are critical for guiding management actions aimed at conserving ecological networks and their species diversity. Change in interactions of common species is particularly important for the conservation of ecological networks because of their central role in maintaining network structure.

Read the paper

Read the full paper here: Henriksen, M. V., Latombe, G., Chapple, D. G., Chown, S. L., & McGeoch, M. A. (2021). A multi-site method to capture turnover in rare to common interactions in bipartite species networks. Journal of Animal Ecology, 00, 1– 13. https://doi.org/10.1111/1365-2656.13639

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