Some years ago I wrote about how one of my ecological heroes, Sir Richard Southwood (later Lord Southwood), influenced my research and stimulated what has become a lifelong interest of mine, island biogeography, in particular the iconic species-area relationship. Apropos of this it seems apposite to write about another huge influence on my research, Sir John Lawton. I first encountered John*, as he was then, at the tender age of 17, when our Sixth Form Science class were bussed from Ripon Grammar School to York University to hear a very enthusiastic arm-waving young ecologist, yes John Lawton, talking about food webs. Excellent as it was, it wasn’t, however, this talk that inspired me :-), but a paper that he and Dieter Schroeder wrote a few years later (Lawton & Schroeder, 1977), in which they showed that structurally more diverse plants potentially hosted more insect species per unit range than those plants with less complex architecture. A couple of years later Strong & Levin (1979) showed that this also applied to fungal parasites in the USA. The mechanism behind the finding was hypothesised to be based on apparency – the bigger you are the easier you are to find, the bigger you are, the more niches you can provide to be colonised, pretty much the same reasoning used to explain geographic island biogeography and species-area accumulation curves (Simberloff & Wilson, 1969). John Lawton, Don Strong and Sir Richard Southwood also highlighted this in their wonderful little book (Strong et al, 1984) which has provided excellent material for my lectures over the years.
As someone who is writing a book, theirs is an excellent example of how you can improve on other people’s offerings. Staying with the theme of plant architectural complexity, Strong et al (1984) brilliantly reported on Vic Moran’s masterly study on the relationship between Opuntia growth forms and the number of insects associated with them (Moran, 1980). Vic’s study was an advance on the previous studies because he examined one family of plants, rather than across families, so reducing the variance seen in other studies caused by phylogenetic effects. I should also point out that this paper was also an inspiration to me.
The figure as shown in Victor Moran’s paper.
The revamped Moran as shown in Strong & Lawton (1984).
Okay, so how did this shake my world? As I have mentioned before, my PhD and first two post-docs were on the bird cherry-oat aphid, Rhopalosiphum padi, a host-alternating aphid that uses bird cherry, Prunus padus, as its primary host. Never being one to stick to one thing, I inevitably got interested in bird cherry in general and as well as eventually writing a paper about it (Leather, 1996) (my only publication in Journal of Ecology), I also, in due course, set up a long term experiment on it, the outcome of which I have written about previously. But, I digress, the first world shaking outcome of reading Lawton & Schroeder, was published in Ecological Entomology (incidentally edited by John Lawton at the time), in which I analysed the relationships between the insects associated with UK Prunus species and their distribution and evolutionary history, and showed that bird cherry had a depauperate insect fauna compared with other Prunus species (Leather, 1985).
I’m not working with very many points, but you get the picture (from Leather, 1985). Bird cherry (and also Gean, the common wild cherry. Prunus avium) hosts fewer insect species than would be expected from its range and history.
This in turn led me on to an even more ambitious project. Inspired by a comment in Kennedy & Southwood (1984) that a better resolution of the species-plant range relationship would result if the analysis was done on a taxonomically restricted group of plants and by the comment in Southwood (1961) that the Rosaceae were a very special plant family, I spent several months wading through insect host lists to compile a data set of the insects associated with all the British Rosaceae. Once analysed I submitted the results as two linked papers to the Journal of Animal Ecology. Having responded to Southwood’s demand that “this manuscript be flensed of its too corpulent flesh” it was eventually published (Leather, 1996). My somewhat pompous introduction to the paper is shown below.
“This relationship is modified by the structure or complexity of the plant, i.e. trees support more insect species than shrubs, which in turn support more species than herbs (Lawton & Schroder 1977; Strong & Levin 1979; Lawton 1983).”
“Kennedy & Southwood (1984) postulated that if taxonomically restricted groups of insects and/or plants were considered, the importance of many of these variables would increase. Few families of plants cover a sufficiently wide range of different growth forms ranging from small herbs to trees in large enough numbers to give statistically meaningful results. The Rosaceae are a notable exception and Southwood (1961) commented on the extraordinary number of insects associated with Rosaceous trees. It would thus appear that the Rosaceae and their associated insect fauna provide an unparalleled opportunity to test many of the current hypotheses put forward in recent years concerning insect host-plant relationships.”
Cutting the long story short (I am much better at flensing nowadays), I found that Rosaceous trees had longer species lists than Rosaceous shrubs, which in turn had longer lists than herbaceous Rosaceae.
Rather messy, but does show that the more architecturally complex the plant, the more insect species it can potentially host (from Leather, 1986).
Flushed by the success of my Prunus based paper, I started to collect data on Finnish Macrolepidoptera feeding on Prunus to compare and contrast with my UK data (I can’t actually remember why this seemed a good idea). Even if I say so myself, the results were intriguing (to me at any rate, the fact that only 19 people have cited it, would seem to suggest that others found it less so), in that host plant utilisation by the same species of Macrolepidoptera was different between island Britain and continental Finland (Leather, 1991).
From Leather (1991) Classic species-area graph from both countries but some intriguing differences in feeding specialisation.
Despite the less than impressive citation index for the UK-Finland comparison paper (Leather, 1991), I would like to extend the analysis to the whole of Europe, or at least to those countries that have comprehensive published distributions of their Flora. I offer this as a project to our Entomology MSc students, every year, but so far, no luck ☹
Although only four of my papers can be directly attributed to the Lawton & Schroeder paper, and taking into account that the insect species richness of Rosacea paper, is number 13 in my all-time citation list, I feel justified in counting it as one of the papers that shook my World.
Kennedy, C.E.J. & Southwood, T.R.E. (1984) The number of species of insects associated with British trees: a re-analysis. Journal of Animal Ecology, 53, 455-478.
Lawton, J.H. & Schroder, D. (1977) Effects of plant type, size of geographical range and taxonomic isolation on numbers of insect species associated with British plants. Nature, 265, 137-140.
Leather, S.R. (1985) Does the bird cherry have its ‘fair share’ of insect pests ? An appraisal of the species-area relationships of the phytophagous insects associated with British Prunus species. Ecological Entomology, 10, 43-56.
Leather, S.R. (1986) Insect species richness of the British Rosaceae: the importance of host range, plant architecture, age of establishment, taxonomic isolation and species-area relationships. Journal of Animal Ecology, 55, 841-860.
Leather, S.R. (1991) Feeding specialisation and host distribution of British and Finnish Prunus feeding macrolepidoptera. Oikos, 60, 40-48.
Leather, S.R. (1996) Biological flora of the British Isles Prunus padus L. Journal of Ecology, 84, 125-132.
Moran, V.C. (1980) Interactions between phytophagous insects and their Opuntia hosts. Ecological Entomology, 5, 153-164.
Simberloff, D. & Wilson, E.O. (1969) Experimental zoogeography of islands: the colonization of empty islands. Ecology, 50, 278-296.
Southwood, T.R.E. (1961) The number of species of insect associated with various trees. Journal of Animal Ecology, 30, 1-8.
Strong, D.R. & Levin, D.A. (1979) Species richness of plant parasites and growth form of their hosts. American Naturalist, 114, 1-22.
Strong, D.R., Lawton, J.H. & Southwood, T.R.E. (1984) Insects on Plants – Community Patterns and Mechanisms. Blackwell Scientific Publication, Oxford.