https://phys.org/news/2018-10-k-core-predictor-collapse-mutualistic-ecosystems.html

The idea applies to any network—from species interacting in ecosystems, like plant-pollinators or predator-prey—to financial markets where brokers interact in a financial network to determine the prices of stocks and products.

In all these networks a hierarchical structure emerges: each species in the ecosystem belong to a given shell in the network: the so called K-shells. In the periphery of the network is where the commensalists live. These are species that mainly receive the benefits from the core of the network but give nothing back (not to be confused with parasites which benefit from but at the same time harm the network core).