Harri Roblin

The ‘evolutionary arms race’ was a concept relating to coevolving species introduced by Richard Dawkins. It is a cyclic theory where animals constantly adapt and counter-adapt their genes to gain an edge over their competition. This can vary between animals of the same species or predator versus prey. This idea centralises around the Red Queen hypothesis which suggests that animals have to constantly adapt and evolve in order to survive alongside coexisting species. One commonly occurring example of the ‘evolutionary arms race’ is plants adapting in order to defend themselves against herbivory and disease. They have several mechanisms which can increase their chances of survival against these organisms.

The first defence against parasites is plants grow physical barriers to make them harder to consume. Examples of this include thorns, trichomes and toughened leaves (Abdul Rashid War et al., 2012). These adaptations make these plants more dangerous or energy-inefficient to consume, so herbivores start to look for alternative plants without these adaptations. These animals then adapt in order to be able to consume these plants. For example, a giraffe’s favourite food is a thorny acacia tree. The inside of their mouth is therefore layered with thickened papillae which means thorns cannot cut the inside of it. They also have a very thick saliva. In the case they consume a thorn, it will be coated by this thick saliva which will protect the digestive system. This increases the amount of food available to these animals and gives them a better chance at survival due to the counter-adaptation to the acacia trees’ adaptation.

The second plant defence against parasites and disease is chemical defence. These can be anywhere from emitting a repellent scent to deter herbivores to the plant’s inducible defence being activated due to the detection of a pathogen-associated molecular pattern (PAMP). Like the physical barriers, these make the plant less likely to be eaten and infected. The optimal chemical defence is one that minimises the damage caused by the parasite and the costs caused by the defensive activities (Emi Shudo and Iwasa, 2001). One of the key defences against pathogens is antimicrobial peptides. These are released as a general defence and are used throughout the plant kingdom. They are key for signalling in plants and can be used to inhibit the release of enzymes. However, several pathogens have developed methods to neutralise and bypass these peptides. The gram-positive bacteria’s method is to partially neutralise their cell wall. This changes the membrane composition as one of the key components that makes antimicrobial peptides effective is their interaction with the negatively charged membrane (Assoni et al., 2020). This is an example of a plant developing a chemical defence to reduce disease and the bacteria evolving to bypass this mechanism.

Another plant defence used is morphological defences. Plants use mimicry to protect themselves from herbivores. This involves changing the colour of the plant to make it seem like it has already suffered from herbivory. This is effective as herbivores assume one of two things: either a plant already has induced defence, so it is better protected or there’s the presence of herbivores’ natural enemies. An example of this is an Alcea setosa. Its stem is covered by dark flecks which look like hundreds of aphids. This discourages other aphids from trying to predate on these plants due to the reasons stated above. There is no real way for parasites to counter-adapt to this apart from learning the pattern of the stem. Once a herbivore discovers that it is just the pattern of the stem, they will learn and be more inclined to eat that specific plant. However, this phenomenon will only really occur in the absence of other viable food options. This is because aphids will still be cautious in eating these plants just in case the plant is not using mimicry and it is aphids.

The final defence is plants recruit herbivore’s natural enemies. They do this through several mechanisms and it is very effective at deterring herbivores due to fear of getting predated on themselves. An example of this is ants providing protection for a Neotropical rainforest tree. A paper by (Ann and Marquis, 1999) explored the effect of these trees attracting ants using their extrafloral nectaries. The results showed that the presence of the ants not only increased protection but also reduced the incidence of pathogen attacks. They achieved this by swarming the herbivores. This study shows the effectiveness of plants recruiting ‘bodyguards’ in order to protect themselves from herbivores. However, a paper by (Cong van Doan et al., 2021) talked about the numbers of these ‘natural enemies’ dwindling due to climate change. This calls into repute the effectiveness of this defensive method in the modern ecosystem.

To conclude, there are several mechanisms by which plants defend themselves against herbivory and disease. Herbivores have developed counters to some of these techniques such as changing their anatomy (giraffe) or manipulating their cell structure to ‘trick’ the plant. However, methods such as recruiting herbivore’s natural enemies and mimicry are challenging to counter. As more plants survive with these adaptations, they will reproduce and pass them down. This will mean more and more of these alleles will be in the population, so herbivores will have to discover a solution to overcome them.