Bad Neighbors

The disruption of a keystone interaction erodes pollination and seed dispersal networks, Vitali et al., 2021 Ecology. https://doi.org/10.1002/ecy.3547

Image credit: Ennio Nasi, CC BY 4.0

The Crux

Ecological communities are incredibly complex networks, made up of interactions between the species that reside in them. To properly understand how these interactions shape a community, researchers have to employ a variety of analytical methods and modelling approaches. This was something that I had to learn to appreciate in my work, because I always thought that studying ecology would involve a lot of time outdoors working with animals. While that does happen (and I spent months outside during my PhD), most of the ecological research I’m familiar with centers on math and statistics.

Using math and statistics to model ecological communities helps us to break down how various organisms are connected with one another. For example, keystone species are organisms that are connected to so many others within a given ecosystem such that any change to their populations will have consequences for the entire community. Understanding the processes that affect these keystone individuals (and all of the organisms linked to them) is vital to predicting how processes such as climate change and invasive species will affect natural communities in the future.

Today’s authors investigated how disruption of an important species interaction affected pollination and seed dispersal networks in Patagonia. A hummingbird species (Sephanoides sephaniodes) is the main pollinator for a mistletoe species (Tristerix corym-bosus), while the mistletoe provides the hummingbird with nectar in the winter. The colocolo opossum (Dromiciops gliroides) is a small marsupial that is vital for the mistletoe, as mistletoe seeds must pass through the opossum’s gut to trigger their germination. Additionally, the opossums defecate many seeds on branches in a “necklace” arrangement, which likely helps the mistletoe to parasitize their plant hosts. These three species are tightly connected to one another, and any reduction in abundance for one species may affect the other two, and even destroy the entire food web.

What They Did

The authors sought to answer two questions. First, is the hummingbird-mistletoe-opossum relationship (also known as a mutualism, whereby the species all benefit each other) crucial to maintaining the structure and diversity of the local ecological networks? Second, does disruption of this network alter network structure?

To answer the first question, the authors compared pollination and seed dispersal network structure between areas differing in mistletoe density by counting the number of pollinator and seed disperser species in each area. High mistletoe-density areas had over 400 mistletoe per hectare, while low density areas had less than 20 per hectare. If pollination and seed dispersal networks were reduced in the low density areas, this would indicate an effect of the mutualism on these networks.

To answer the second question, the authors once again compared pollination and seed dispersal networks, but this time they made comparisons between areas with an intact mutualism network, and those areas where the mutualism had been destroyed by non-native ungulates (red deer, dama deer, and domestic cattle). If network complexity was reduced in areas with non-native ungulates, this would indicate an effect of the non-native ungulates on network complexity via their effects on the mutualism.

What They Found

The number of pollinator and seed dispersal species in areas with an intact mutualism was higher than in areas without it (i.e., in areas with high density vs areas with low density). In addition, the networks themselves were more complex, as areas with intact mutualisms had a greater number of unique motifs. Motifs are smaller subnetworks of species combinations that are connected with one another. This suggests that the intact mutualism is an essential component of network complexity in these communities.

In areas where the mutualism had been destroyed by the non-native ungulates the authors found that the network complexity was reduced, with a smaller number of unique motifs and less species interactions. Additionally, the authors did not find a single colocolo opossum in the invaded sites, which may point to its local extinction in these areas.

Fig. 4 from today’s paper. a) and c) show the number of motifs within a given site (i.e., how many motifs with X number of species there were) for pollinator and seed dispersal networks, respectively. b) and d) show the relative frequency of a given motif within a given site (i.e., a value of 0.8 for 6-species motifs means that 80% of all motifs within that site were 6-species motifs). Differences between the intact sites (blue bars) and invaded sites (yellow bars) show that the mutualism increases network complexity. (Image credit: Fig. 4, Vitali et al. 2021, Ecology).

Problems?

I know that I’m fairly biased when I say this, but I wish that the authors had included parasites in their analyses. Previous work has shown that ~75% of the connections between species within a community involved parasites, so by not including parasites I think that the authors may be severely underestimating how much these species interaction networks are being affected by the loss of this mutualism.

So What?

To me, the authors have demonstrated that this three species mutualism is a keystone interaction for both pollinator and seed disperser networks, and because these species depend on one another to thrive, I’d say that that makes them keystone species by default. Communities around the world are experiencing changing conditions due to climate change, pollution, and pressure from non-native species. This paper is not only a great illustration of how a complex and crucial keystone interaction affects other species, but it shows what outcome we can expect when such a keystone interaction fails. Identifying these interactions will be crucial for protecting and preserving natural systems in the future.


Dr. Adam Hasik is an evolutionary ecologist interested in the ecological and evolutionary dynamics of host-parasite interactions who really wants a colocolo opossum of his own. 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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