Ecosystems are a paradigmatic example of a complex system: a large number of species, both animal and plant, which interact in a variety of ways. These interactions can be positive like mutualism, negative like predation or neutral like neutralism, and are key to maintaining the balance of ecosystems in nature. However, this balance can be affected by a multitude of causes: changes in climate, the introduction of exotic species, the very action of human beings... Disturbances that can lead to the extinction of one or more species, thus altering the state of the ecosystem. The loss of one species can lead to the extinction of others that are dependent. Thus, understanding how ecological networks respond to disturbances or disruptive events is key to anticipating biodiversity loss and possible cascading extinctions.
A multidisciplinary team of scientists, including researchers from IFISC (UIB-CSIC), has published an article in Scientific Reports in which they analyze how plasticity in an ecological network with mutualistic relationships causes its topology to change in response to biodiversity loss. In ecology, mutualism is that association between two (or more) organisms of different species that is beneficial to both, such as the relationship between a pollinator and a plant. Pollination networks have proven to be quite robust in coping with species extinction as they tend to be asymmetric bipartite networks, i.e. each flower has several pollinators with similar roles. In this type of network, it is assumed that there is only a pollinator-plant interaction and not a plant-plant or pollinator-pollinator interaction.
The researchers analyzed 130 real mutualistic networks, including plant-pollinator pairs, seed-dispersers, plant-ants and host-symbiont. These types of networks have two types of species, represented with nodes, depending on their role: those that act as a resource (plants) and those that act as consumers (pollinators, for example). To simulate the extinction of a species, one node is removed from the network and, with some probability, each relationship that connected the extinct species goes on to connect another pair of nodes. In this way, different properties of the network can be analyzed, such as its stability, its robustness or its modular structure.
Among the results obtained, the researchers found that redundancy in the network is correlated with the robustness of the ecosystem, since the role of one species can be supplanted by another if the latter becomes extinct. The study concludes that those ecosystems that, if the reconnection between species that are not extinct takes into account the affinity with resources, show greater modularity, robustness and stability, i.e. more likely to recover from disruptive events. This may imply that this mechanism was favored by evolution to prevent cascades of extinctions.
Sheykhali, Somaye; Fernández-Gracia, Juan; Traveset, Anna; Ziegler, Maren; Voolstra, Christian R.; Duarte, Carles M.; Eguíluz, Víctor M., Robustness to extinction and plasticity derived from mutualistic bipartite ecological networks. Scientific Reports, 10 (1). doi: 10.1038/s41598-020-66131-5