Plants and the animals that eat them comprise more than 90 per cent of the Earth’s biomass. Most of the biodiversity we see around us is shaped by this critical interaction between plants and herbivores. Herbivores take many forms, from the large 70-tonne sauropod dinosaurs of the Cretaceous to tiny insects that live inside plant stems and leaves.
Herbivores shape nature by eating and damaging plant material, reducing plant survival and reproduction, which in turn triggers evolutionary change in their plant food species. Herbivores and plants are coupled together in a millennial-scale co-evolutionary dance as herbivores evolve more effective ways of accessing food and plants resist, tolerate and defend themselves against the onslaught.
Plants are far from passive in the face of ravenous animals seeking to gorge themselves on leaves, flowers, seeds and even wood. Some of the most potent chemicals in nature are produced by plants to defend themselves and repel herbivores.
Every part of the native Irish yew tree is highly toxic to animals, except for the red fleshy fruit. Nicotine, produced in tobacco plants, is a very effective insect repellent. Humans exploit this pharmaceutical cornucopia to produce drugs for treating diseases and to use as natural insecticides.
Many plants have harnessed herbivory to provide themselves with survival and reproduction benefits. Sugar-rich nectar and protein-rich pollen attract pollinating insects which, in return for the dietary boost, transport pollen to fertilise seeds on neighbouring plants. Foxes at this time of year gorge on blackberries and rosehips from hedgerows, depositing the seeds in new locations together with a convenient nutrient-rich manure enabling the plants to germinate and grow.
We can taste chemical plant defences. The hot taste of the green wasabi paste that goes so well with sushi comes from chemicals called glucosinolates
The extinction of fruit-eating animals in the tropics has marooned plants which relied on large animals to disperse their big seeds. Without their dispersers the seedlings germinate close to their parents and suffer from competition with their siblings. These clusters of closely related individuals are vulnerable to the same diseases and herbivores, reducing their survival compared to seedlings that germinate further from their relatives.
The tropics are a hotbed of plant-herbivore interactions with many fruit-eating birds and mammals as well as vast numbers of herbivorous insects. As part of a large collaborative group of scientists worldwide we recently tested whether average rates of herbivory and plant to plant variation in herbivory are different in the tropics compared to temperate areas of the world. We collected data on more than 500 species of plants at more than 790 sites, including Ireland.
While average rates of herbivory are slightly higher in the tropics, the variation in herbivory is lower in the tropics. This means that an individual plant in a population in the tropics is unlikely to escape from herbivory, most plants will experience the same level of herbivory. In temperate areas, however, the variation from plant to plant is higher, with some plants experiencing high amounts of herbivory and others escaping entirely.
The consequences of this variation in herbivory are that evolution of plant defences against herbivory will be different in the tropics compared to temperate areas. If a plant has a chance of escaping herbivory then it may not be advantageous to increase its defences until it actually needs them. If all plants in a population are likely to be attacked, then it might make sense to always have defences on stand-by. We might therefore expect different kinds of physical and chemical defences in different parts of the world.
We can taste chemical plant defences. The hot taste of the green wasabi paste that goes so well with sushi comes from chemicals called glucosinolates. When a caterpillar chomps on the wasabi leaf it brings the glucosinolates into contact with an enzyme called myrosinase with this chemical reaction producing isothiocyanates. It is the isothiocyanates which give us a more or less pleasurable burning sensation and which effectively deter the caterpillar from coming back for seconds.
Humans benefit from this herbivory as it is the isothiocyanates in cabbage and Brussels sprouts which give these green leafy vegetables their cancer fighting properties. When you next eat your greens, enjoy their slightly bitter, mustardy taste. The caterpillar’s loss is our gain.
Yvonne Buckley is an ecologist and professor of zoology at Trinity College Dublin