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Trophic Cascades as a Network Cascade

According to an article about keystone species and trophic cascades, a species is considered “strongly interactive” when its absence leads large changes in the ecosystem. This type of connection can be compared to a strong tie in a network or ecosystem. The presence or absence of a “strongly interactive” species would mimic the effects that strong ties would have in a network when switching to a new technology. A weak tie in the given ecosystem or network would not have a great effect because the nodes or species are not closely intertwined; thus, they do not greatly affect one another.

In an ecosystem, there is a density of different species of animals and plants that are linked. An ecosystem is like a network where certain species are like nodes that are in different positions than others. Some nodes share weak ties, where a change in one individual of a species has no effect on the other. Some nodes have strong ties where the change in one individual for other species can cause a cascading effect in the other species, and eventually the ecosystem network. A keystone species is one that has this “splintering” effect on an ecosystem or network. Removal of keystone species would act like the introduction of a new technology, causing a different species to spread. For example, in Yellowstone, when wolves are missing, coyote numbers increase. This causes a declining cascade in pronghorn numbers because coyotes are major predators of young pronghorns. The presence and absence of wolves, a keystone species, has cascading network effects. It is important to consider that trophic cascades are grounded in opportunity and that “choice” based on incentive is less relevant for the ecosystem networks, since animals do not have the same freedom of choice. Network effects describes how changes in the nodes give connected nodes, or neighbors, different opportunities. Choices dependent on these opportunities have cascading effects. Ecosystems are networks that attempt to remain in balance, despite changes to nodes, so often the cascading effect is in balance. This means that with the addition of something, something else tends to decrease, and vice-versa.

Species interactions in an ecosystem can be viewed on a local level, like introduction of a new technology. When considering the adoption of new technology, it can be difficult to get a new technology to gain standing in a population-level model, even when it is an improvement on the status quo (Ch 19, Cascading behavior in networks). Similarly, with the addition or removal of some individuals of a species it can be hard to determine the effect it may have, or whether there would be a cascading effect.

In a network that is locally grounded, meaning an individual’s decisions are dependent upon the choices of neighbors rather than the entire network, it is possible for a small set of initial adopters, or a small change in species composition to “start a long fuse running that eventually spreads” the technology globally, or in a species context, across the ecosystem. Keystone species and “strongly interactive” species may not always be the most abundant– they just have a large impact. In the population-level view, it can be hard to see the effect addition or removal will have, since these species may not interact with all aspects or nodes in a network or ecosystem. When the keystone species decreases enough though, the other species in the environment are affected. The decrease in the density of keystone species must be below the threshold number of individuals to keep the network as is. From this point of view, introduction or removal of a keystone species can cause cascading effects based on its connection to its neighbor and considering the threshold, q.

 

Sources:

http://www.missionwolf.org/page/trophic-cascade/

https://wildlandsnetwork.org/keystone-species-trophic-cascades/

https://www.cs.cornell.edu/home/kleinber/networks-book/networks-book-ch19.pdf

 

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