Using information theory to decode network co-evolution of insect-plant ecosystems

This topic is central to the GEITP theme of gene-environment interactions. The “environmental signal” is a chemical(s) emitted (by a plant or an insect), and “genes” in the genome (of the insect or plant) respond to that signal. “Attracting” or “repelling” the signal is regarded as one of the main forces that shape plant-herbivore interaction networks. For example, insects have a large number of olfactory receptors with high sensitivity for chemical signals — which play essential roles in their fundamental activities including foraging (obtaining food) and oviposition (depositing eggs). Chemically-mediated species interactions are based on information transfer, which can be defined simply as “communication,” regardless of the benefit to the emitter or the receiver.

Understanding chemical communication between plants and herbivores (animals that eat plants) has been an active field of research; most work has focused on how a specific plant (or genus of plants) defends against herbivores — directly by using chemical repellents or by producing stronger chemical defenses, or indirectly by emitting signals to attract herbivores’ enemies (e.g. predators or parasites). Little is known about how information transfer shapes these species interactions (e.g. it remains unclear how plants code chemical information to deal with a diversity of potential herbivores; and how herbivores decode such olfactory signals to distinguish among plants and identify potential hosts). Even less is known about whether there is any general information structure of plant-herbivore chemical communication and how such a structure can be maintained during an ongoing chemical “arms race” between plants and herbivores.

Information theory has been discussed before in these GEITP pages; it provides a quantitative and scalable means to measure information transfer, and has already brought key insights about the structure and emergence of human language. Authors [see attached article & editorial] extended information theory to increase our understanding about the “chemical language” in ecological communities. Previously, however, such attempts had been used to study chemical communication of only a single plant species with its interacting insects. In the present study [see attached] authors used information theory to study plant volatile organic compounds (VOCs) as a communication channel, forming plant-insect interaction networks. From an information perspective, the relationship between sender and receiver (i.e. ‘speaker’ and ‘listener’) determines the nature of the signal transmission and evolution of the information structure shaping the communication pattern between individuals (i.e. individuals can provide either clear information that can be decoded easily, or spurious information that can be difficult to decode).

[Classical Information theory considers a sender and a receiver trying to communicate using a given code sent through a noisy channel (Morse code in the old telegraph system would be one example). For each signal sent, there is a probability that it will be misunderstood by the receiver. Not surprisingly, a great deal of standard information theory deals with finding ways of optimizing communication efficiency. However, in an ecological system in which interactions are highly asymmetrical (i.e. one species eats the other),the needs at the ends of the (coevolving) communication channel are in clear conflict: Insects need to faithfully identify what leaves are edible, and plants need to avoid being identified as edible.]

By integrating information theory with ecological and evolutionary theories, authors [see attached article & editorial] therefore demonstrated that a stable information structure of plant VOCs is able to emerge from a conflicting information process between plants and herbivores. Authors corroborated this information “arms race” theory with field data — recording plant-VOC associations and plant-herbivore interactions in a tropical dry forest. Authors discovered that plant VOC redundancy and herbivore specialization can be explained by a conflicting information transfer. Authors conclude that information-based communication approaches should be able to increase our understanding of species interactions across levels of feeding and nutrition.


Science 19 Jun 2020; 368: 1337-1381 & editorial pp 1315-1316

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