Like a House of Cards

An empirical attack tolerance test alters the structure and species richness of plant–pollinator networks (2020) Biella et al., Functional Ecology, https://doi.org/10.1111/1365-2435.13642

Image Credit: Adamantios, CC BY-SA 3.0, Image Cropped

The Crux

Put simply, ecosystem function is the process that control how nutrients, energy, and organic matter move through an environment. Think about a forest. You have small plants that are eaten by small animals, small animals that are eaten by larger animals, and those larger animals are eaten by even larger animals. When those animals die, they are broken down and consumed by scavengers, fungi, and bacteria. These processes result in a continuous flow of nutrients and energy through the ecosystem. However, if one link (organism) in this chain breaks (goes extinct), the ecosystem could lose its function, and other species that depend on this cycle could go extinct as well.

The way in which a given ecosystem reacts to or recovers from any negative impact that it sustains is key to understanding how ecosystems function. Classically, this is tested with attack tolerance tests, in which all species on a given trophic level are removed and the ecosystem is then monitored to see how/if it maintains its function. In studies of plant-pollinator networks, this is usually modeled with computers, but studies which use natural systems are lacking. Today’s authors wanted to use a natural plant-pollinator system to see what happens.

What They Did

The authors used four different sites, three of which were manipulated and one which was left alone as a control. Each site consisted of a small grassland area surrounded by trees, which would limit the movement of pollinators from the experimental sites. The experiment consisted of four rounds of plant removal. For each round, the authors would go to the experimental sites and remove one generalist plant species from the grassland, such that by the end of the experiment four species had been removed from the experimental plots. Before each removal the authors collected all insects visiting flowers, and monitored which plant-insect associations were lost between rounds of plant species removal. They termed this loss of the plant-insect association as an “extinction”.

Grassland such as this provided the perfect habitat for the authors to test their ideas. (Image credit: Andrew Smith, CC BY-SA 2.0)

The data from the experimental plots was compared to that of the control plot, which was sampled for plant-insect associations in the same way as the experimental plots, but without the plants being removed in each sampling round. The authors wanted to compare their experimental data to two expectations: rewiring or modularization. Rewiring would mean that the network of species interactions would continue to exist, but with new connections between plants and insects. Modularization would indicate that parts of the existing network broke down in such a way that the original network was now broken up into two or more subnetworks that are no longer connected.

Did You Know: Species Interactions

This paper focused on the function of an ecosystem and how it changed after some species and their associated interactions were lost. Species interactions, like the name suggests, are interactions between species. These can be direct, such as a shark eating a smaller fish, or two male moose fighting for territory. Those are called “direct interactions”, as they involve one organism interacting directly with another. Indirect interactions, however, are those in which an interaction still occurs, but it is mediated by a third party. Apparent competition is a good example of this, as this is where two prey species compete with one another for resources (direct competition), but they also compete indirectly  via a shared predator. If species A and species B share a predator, and the predator population increases as a result, then species B suffers not only from direct competition with species A, but also from the apparent competition with species A via the increased predator population. 

What They Found

As plant species were removed from the experimental plot, more and more species interactions were lost, and more species went “extinct”. As a result, despite some pollinator species visiting new plant species (i.e. rewiring) the ecosystem tended to become more modular as the total network broke down into smaller, more isolated networks. Interestingly, theoretical models used to predict species extinctions and the loss of interactions between species in this system not only underestimated the amount of species that would go extinct, but also overestimated the amount of species interactions that would persist.

Problems?

The only “issue” I have with this experiment is that the treatments they compared to one another were not true replicates. The authors could not set up smaller subplot within each grassland used, because any pollinator could simply move from the area where the plants were removed to an area where they were not. As such, they had to set up three different grasslands and use them as their experimental replicates. I don’t think that this would have affected their results in a way that would significantly change anything, but it would have been better to have comparisons made within the grasslands, rather than between them.

So What?

I really like studies like this one, as I am a big fan of experimental data being used to back up a theoretical idea. This experiment highlighted the importance of these types of studies, as they found that the natural system of plants and pollinators behaved much more differently than the theoretical models predicted that they would. In addition, the damage to ecosystem function that was caused by the removal of generalist plant species shows that conservation efforts may do better focusing on a core group of generalist species, instead of working to save the entire ecosystem itself.

Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions. You can read more about his research and his work for Ecology for the Masses here, see his personal website here, or follow him on Twitter here.

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