I have long held Dan Janzen in high regard, and not just because he wrote a paper with the memorable title “What are dandelions and aphids?” (Janzen, 1977). I have always found his writing enjoyable, and was, and still am, in awe of his ability to straddle whole swathes of ecology, both practically and conceptually. The paper that has, however, had the most impact on me, and perhaps the concept that Janzen is most renowned for, is the one that gave rise to the Janzen-Connell hypothesis/effect (Janzen, 1970). Janzen was addressing the question of why tropical forests, while generally species rich, have a low density of adult trees of each species when compared with temperate forests (Black et al., 1950). Janzen states “I believe that a third generalization is possible about tropical tree species as contrasted with temperate ones: for most species of lowland tropical trees, adults do not produce new adults in their immediate vicinity (where most seeds fall).” He based this statement on his own personal observations, discussions with tropical foresters and on discussions with Joseph Connell. He then works through several models testing different scenarios, from allelopathy*, different modes of seed dispersal and seed predation. Although allelopathy has been shown to affect seedling recruitment in several tree species (e.g. Webb et al., 1967), his conclusion was that the efficiency of seed/seedling specific predators was the main factor causing the patterns seen in tropical forest structure. The simple take-home message, and one that I often say to the three of my grown-up sons who still live with us, is that it pays to move away from home; if your parents don’t kill you, then something else will 😊 Easy to remember and understand.
The Janzen model – the further away you are from your parent, the greater the probability that you will survive (Janzen, 1970).
The user-friendly version I use in lectures https://agoutienterprise.files.wordpress.com/2012/09/j-c-diagram1.jpg
So, given that the first mention in print is the Janzen 1970 paper, why is it the Janzen-Connell model/hypothesis/effect and why was a marine biologist, Joseph Connell, writing about tropical forest diversity (Connell, 1971)? It could so easily have been a repeat of the Wallace and Darwin contretemps. If you read both papers, it is obvious to see that both men had discusd the subject with each other, and saw their hypothesis as an extension of an earlier paper by the great ecologist, Robert Paine (Paine, 1966). Connell refers to the Janzen paper as in press, but his ideas saw the light of day in 1970, albeit not analysed fully and referred to as in preparation, at a conference, the proceedings of which did not appear until the following year (Connell, 1971). The actual data he referred to in his paper, did not appear in journal format until 1984, perhaps one of the longest in preps** ever (Connell et al., 1984).
Although Connell and Janzen continued to address the subject e.g. (Janzen, 1971; Connell, 1978), their names were not linked until Steve Hubbell did so in 1979 (Hubbell, 1979). This linking of the two names seems to have been the fuse that set off the citation rocket. As of now, it has been cited over 15 000 times and shows no signs of slowing down.
The Janzen-Connell citation rocket; 15 286 citations to date
So, apart from using it in teaching, how has the Janzen-Connell hypothesis shaken my world? Although I had used the concept in my teaching since the mid-1990s, it was my weekly walk round my 52 sycamore tree transect that got me thinking about it as research topic. Field work is a great way to keep in touch with your study organism, things go one outside that don’t happen in the lab. My sycamore transect was set up to monitor the insect herbivores and their natural enemies, but after a few years something else struck me, particularly, during high seed production years (another twenty-year data set for my never going to publish series). I noticed that although lots of sycamore seedlings emerged underneath my study trees in the spring, by mid-summer hardly any were left; underneath other tree species, they were however, much more common, especially under oak trees. My first thought was allelopathy, but a quick test using potted sycamore seedling in soil from underneath oak and sycamore trees with standard compost as a control, quickly showed this not to be the case.
Effects of soil type on growth of sycamore seedlings (F = 1.68 2/33 df P =NS).
I then used an undergraduate student assistant (paid I hasten to add) to do a couple of surveys, counting the incidence of sycamore seedlings and saplings underneath different tree species. This convinced me that there was something going on and I set up twenty permanent plots in 2005, which I monitored until I left Silwood in 2012 (another set of data unlikely to be published), ten under mature sycamore and ten underneath mature oak trees, counting the number of sycamore seedlings that merged every spring and survived or not. After a couple of years I was convinced that there was every possibility of a Janzen-Connell effect going on and persuaded Alex Pigot, then a MSc student that it would be a great project. To cut a long story short, Alex demonstrated that sycamore seedling survival, was as with tropical tree seedlings, dependent on predation pressure and that this was mainly due to invertebrate herbivores and was greatest underneath their parent trees.
Sycamore seedling mortality highest under sycamore and oak when exposed to invertebrates, vertebrates or both (Pigot & Leather, 2008).
Before anyone accuses me of taking credit for being the first person to demonstrate that the Janzen-Connell effect was also applicable to temperate forests, let me point you at a paper by Douglas Gill (Gill, 1975) who suggested that the spatial patterns of pines and oaks in the New Jersey Pine Barrens might be a result of differential seed predation as suggested by Janzen and Connell.
Despite the undoubted popularity of the Janzen-Connell Hypothesis in ecology, it is still not entirely clear cut; as my colleagues and I pointed out recently “What is clear, is that more studies targeting closed tall forests, and trees from other plant families and their seedlings are urgently needed before we can make sweeping conclusions about the generality of Janzen–Connell effects induced specifically by insects” (Basett et al., 2019), but nevertheless this is a paper that shook my world and one that is definitely worth reading if you haven’t come across it before or just taken the concept it as gospel.
Basset, Y., Miller, S.E., Gripenberg, S., Ctvrtecka, R., Dahl, C., Leather, S.R. & Didham, R.K. (2019) An entomocentric view of the Janzen–Connell hypothesis. Insect Conservation and Diversity, 12, 1-8.
Black, G.A., Dobzhansky, T. & Pavan, C. (1950) Some attempts to estimate species diversity and population density of trees in Amazonian forests. Botanical Gazette, 111, 413-425.
Connell, J. H. (1971) On the role of natural enemies in preventing competitive exclusion in some marine animals and forest trees. In: den Boer, P. J. and Gradwell, G. R. (eds), Dynamics of Populations. Centre for Agricultural Publications and Documentation, Wageningen, the Netherlands, pp. 298-312.
Connell, J.H. (1978) Diversity in tropical rain forests and coral reefs. Science, 199, 1302-1310.
Connell, J.H., Tracey, J.G. & Webb, L.J. (1984) Compensatory recruitment, growth, and mortality as factors maintaining rain forest tree diversity. Ecological Monographs, 54, 141-164.
Gill, D.E. (1975) Spatial patterning of pines and oaks in the New Jersey Pine Barrens. Journal of Ecology, 63, 291-298.
Hille Ris Lambers, J., Clark, J.S. & Beckage, B. (2002) Density-dependent mortality and the latitudinal gradient in species diversity. Nature, 417, 232-235.
Hubbell, S.P. (1979) Tree dispersion, abundance, and diversity in a tropical dry forest. Science, 203, 1299-1309.
Janzen, D.H. (1970) Herbivores and the numbers of tree species in tropical forests. American Naturalist, 104, 501-528.
Janzen, D.H. (1971) Escape of juvenile Dioclea megacarpa (Leguminosae) vines from predators in a deciduous tropical forest. American Naturalist, 105, 97-112.
Janzen, D.H. (1977) What are dandelions and aphids? American Naturalist, 111, 586-589.
Paine, R.T. (1966) Food web complexity and species diversity. American Naturalist, 100, 65-75.
Pigot, A.L. & Leather, S.R. (2008) Invertebrate predators drive distance‐dependent patterns of seedling mortality in a temperate tree Acer pseudoplatanus. Oikos, 117, 521-530.
Webb, L.J., Tracey, J.G. & Haydock, K.P. (1967) A factor toxic to seedlings of the same species associated with living roots of the non-gregarious subtropical rain forest tree Grevillea robusta. Journal of Applied Ecology, 4, 13-25.