Recently the wonderful Linden Hawthorne (@Haggewoods) sent me a paper entitled ‘The Number of Species of Insect Associated with Various Trees’ by T.R.E Southwood. The paper was published in 1961 and has influenced a number of eminent entomologists including Professor Simon Leather and was recently recognised by the British Entomological Society in their 100 influential papers series in which John Lawton praises Southwood’s vision for an approach to ecology which didn’t become fashionable for another 30 years. This approach was macroecology – looking for patterns in nature without experimentation.
I have to admit that I have never read this paper before. However I rely on the thought pathways and knowledge that this paper set into motion all the time and think it is especially important for current conservation measures to take the concepts that this paper unravels, into account. Therefore in this blog post I aim to quickly sum up this paper and show why the paper is as relevant, if not more so, to conservation today than it was over 60 years ago.
The premise of this paper is quite simple but had never been thought about in this way before. Firstly this paper showed that native trees have more associated invertebrates associated with them. Secondly it shows that trees which are more abundant in a country, have more insect species associated with these trees. I have always thought that native trees would have more associated insects and generally be better for wildlife, however it didn’t cross my mind that the more abundant a tree is, the more associated invertebrates it would have too. This paper compares tree and insect associations in Britain and Russia and has found that coniferous species, which are more abundant in Russia, have more associated insects there too. They also compare trees in Britain and Cypress and note that more recently introduced species, although often abundant, support lower numbers of associated insects than less dominant but native tree species. Southwood suggests that this is because the longer a tree is in the landscape, the longer species have to co-evolve with the tree and become more specialised. So not only do you get more species living on trees which are more abundant and have been in the landscape for longer, you also get more sympatric speciation and more highly specialised species. Interestingly he also notes that when trees become less abundant in an area, so do the associated insect faunas. This is would be what we would expect but this paper proved it for the first time statistically.
But why should this matter to a 21st century conservationist in Britain? Well according to the recent State of Nature Report between 1900 and 1970 an estimated 90% of our traditionally coppiced woodlands have been lost. These woods contain a variety of native species at different stages including old veteran trees hundreds of years old. Associated with these woodlands are invertebrates reliant on both the live and dead wood and the associated habitats that they provide. Southwood’s paper is just as relevant now as it was in 1961 and shows us just how important it is that we preserve out ancient woodlands and the native trees that they contain. Commercially often monocultures of conifers are planted instead of native species in order to generate higher profits. Southwood’s paper shows the detrimental effect this has on invertebrates!
More generally other flowering plant species also have highly associated insect faunas. The area of lowland meadow in England and Wales has declined by 97% between the 1930s and 1984– a total loss of 64,000 sq km, just imagine the effect that this is having not only on our plants, but also our invertebrates. The very invertebrates that are neglected in many conservation measures but are keystone species in supporting the more charismatic species that we all love such as mammals and birds. N.b. I think invertebrates can be just as charismatic, if not more so than birds and mammals.
In conclusion, I wholeheartedly agree with Professor Simon Leather, who more eloquently summarises this paper , that we need a database of which species are associated with which plants, so we can conserve the two in tandem, they have co-evolved other thousands of years and it is essential for us to conserve them both, for their sake and ours.
Southwood, T. R. E. (1961). The number of species of insect associated with various trees. Journal of Animal Ecology 30: 1-8.