I love plants but something that I especially love about plants is how they defend themselves! Plants, unlike us humans and other animals can not simply run away from their problems. They are (in most cases) literally rooted to the ground and face all their troubles with clever evolutionary and adaptive mechanisms for survival.
One mechanism that plants use to ensure their survival are plant defences, physical or chemical mechanism to prevent herbivores from munching on their tasty leaves. Although it doesn’t always stop us humans. Caffeine, something we may love to enjoy in our daily cup of tea or coffee, evolved as an insecticide paralyse insects!
There was an idea in the plant ecology literature where plants would probably do better in the warm tropical regions of the world, at lower latitudes. The weather is always nice and warm, allowing plants to grow happily and in huge numbers. However, having more plants in one space means that plants compete with one another, not only for resources but to prevent herbivory – from being eaten – all year round. So, to prevent and minimise herbivory, it would seem like plants in tropical regions on earth such as rainforests would have stronger defences than plants growing in higher latitudes like cold tundra environments.
The dominant theory (1, 2) was that plants at lower latitudes faced a higher level of animal-plant interactions like herbivory than at higher latitudes. Facing a higher level of herbivory suggested that lower latitude growing plants would have evolved to have greater plant defences. Having more defences may indicate that more plants in the tropics would have more biologically active compounds to prevent herbivory than plants at higher latitudes. Consequently, pharmaceutical companies were inspired to look for plants with biologically active compounds in the tropics, as tropical forests tend to have a large diversity of species that all compete with one another for survival and defend themselves from being invaded or eaten.
This idea of plants being more well defended in the tropics than in higher latitudes is one of ecology’s zombie ideas. Old theoretical ideas that should be dead due, being proven wrong by new data and theory, but somehow continue to persist or continue to be accepted in the literature. Professor Angela Moles and her fellow colleagues tested this idea to find that plants tend to have higher levels of chemical defences at higher latitudes. They also found that herbivory is not as intense at low latitudes and plants from low latitudes don’t really have greater levels of defence against herbivores (3).
Her work is a great example about re-examining ideas that are fundamental to our understanding of ecology. We should never stop testing and questioning theories and ideas with new data and evidence. Testing old ideas helps us in furthering our collective knowledge of nature for both science and conservation.
“The good thing about science is that it’s true whether or not you believe in it.” – Neil DeGrasse Tyson
Dobzhansky, T., 1950. Evolution in the tropics. American scientist, 38(2), pp.209-221.
MacArthur, R.H., 1984. Geographical ecology: patterns in the distribution of species. Princeton University Press.
Moles, A.T., Bonser, S.P., Poore, A.G., Wallis, I.R. and Foley, W.J., 2011. Assessing the evidence for latitudinal gradients in plant defence and herbivory. Functional Ecology, 25(2), pp.380-388. Moles papers