Monthly Archives: January 2019

Ten more papers that shook my world – It pays to move away from home – Janzen (1970) & Connell (1971)

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

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.

* the chemical inhibition of one plant (or other organism) by another, due to the release into the environment of substances acting as germination or growth inhibitors

**Let me know if you know of a longer one.  I don’t count Darwin, as he didn’t, as far as I know, actually refer to his theory in print before publication was forced upon him by Wallace.


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Pick and Mix 26 – more gleanings from around the world

How dinosaurs got their name

It seems that most scientists only have temporary careers 😦

Yes, as we suspected, competitive grant writing is inefficient and wastes scientist’s time

Stephen Heard explains how William Caxton inlfuenced how we report statistics

You don’t need fossil fuels to keep the economy running

Drawing specimens (rather than taking a photograph) is the best way to learn about morphology and taxonomy

Continuing with the botanical theme, plant blindness, yes it is a thing, probably worse than insect blindness which I have written about in the past

Incredible blueness – Ray Cannon on butterfly wings

Electrifying  – flying spiders

Earwig wings – real life origami



Filed under Pick and mix

British Ecological Society Annual Meeting 2018 – representing ecologists but not ecology?

I managed to get to the BES annual meeting this year.  I hadn’t been since 2014 as I boycotted the 2015 meeting*  and the timing of the 2016 and 2017 meetings meant I couldn’t attend those due to teaching commitments.  This time the meeting was in Birmingham and term had ended so there was nothing to get in the way of reconnecting with the annual meetings, the first of which I attended in 1977.  I arrived, soaked to the skin, at the International Conference Centre on a very rainy Sunday afternoon.  Despite the inauspicious start, I was heartened to have a reminder of the BES Undergraduate Summer School; one of my fluorescent beetles from the evening “track a beetle” exercise was on display 😊

Fluorescent carabid beetle, the star of the evening at the Malham BES Summer School 2018

In general, despite the sad memories the pre-Christmas period carries with it, It was good to catch up with old friends and former students.  As a bonus there were some fantastic plenaries; I particularly enjoyed Sam M Gon III’s talk on The Hawaiian Islands as a Model for Biocultural Conservation, which opened with a traditional Hawaiian chant.

A most unusual and very enjoyable plenary

Great to see lots of very special insects

Another great plenary was Danielle Lee’s on science communication and the importance of getting local non-scientists involved in one’s research programmes.

Danielle Lee – On the importance of science communication, a subject close to my heart

There were a lot of great talks, but as is often the case with large meetings, a lot of clashes and hard decisions to make about which talks to miss.  As a member of the Twitterati I was made very aware of this by seeing the Tweets about talks I was missing 😊

Alistair Seddon – a Doctor Who fan

One thing that struck me very forcibly, was that entomology seemed to be very under-represented compared with when I first started attending BES meetings.  There were no specific sessions dedicated to invertebrates; in earlier years it was relatively easy to find insect-themed sessions and talks.  This year, and perhaps this is a modern trend in ecology, even the titles of many of the talks didn’t mention the study organism, the abstract being the only clue about what was being discussed.  I have noticed this trend in paper titles recently too, and will, I am sure, address this in a future blog post 😊 It worries me somewhat that conservation biologists and ecologists have, despite the warnings that a number of eminent ecologists have made in the past, former BES President, Bob May, for example (Clarke & May, 2002) that funding and practical conservation is heavily biased in favour of vertebrate (Seddon et al., 2005), which are hardly representative of global macro-biodiversity. As far as the British Ecological Society goes, one would expect that a Society that has, over the last decade or so, become increasingly politicised, and on the face of it, publicly engaged with climate change and other ecological issues, to actively implement a change in direction of the research supported and showcased.

I have previously taken the Journal of Animal Ecology to task for ignoring most of the world’s animal life, yes you guessed it, invertebrates 😊 Their cover images are similarly biased.  Sadly, I am now going to have to take the British Ecological Society to task. I mentioned earlier that I felt the general content of the talks and posters was not representative of the world we live in and on leaving the conference decided to see if my gut feeling was a true reflection of the event.  Amy Everard of the British Ecological Society, kindly supplied me with the abstracts of the talks and posters which I then categorised according to the study organism(s) covered.  Some were a bit difficult, as even with the abstract it was difficult to decide where the focus was, so fungi and microbes may be a little more under-represented than they were in reality, particularly where the talk was on the interactions between fungi, microbes, insects and plants and in some cases, vertebrates.  I lumped all invertebrates together, although as you might expect, most invertebrates were arthropods and those were mainly insects. Plants included trees and forests where the focus was on the role the plant component played and general includes models and multi-organismal studies.  Vertebrates, which were largely birds and mammals, also includes fish, and the very few studies on amphibians and reptiles. Crude, but I feel it gives the overall picture.

First, just to remind you how life on the planet is divided up between the various taxa based on species described to date (Figure 1).

Figure 1. Relative proportions of plant, animal, fungi and microbial species described to date.

So how does this compare with what attendees at BES2018 saw and heard about? As you can see, my gut was right, the little things that run the world were under-represented in both the talks (Figure 2) and posters (Figure 3).

Figure 2. Taxa represented in talks at BES2018 (plants 32%, vertebrates 25%, invertebrates 20%, general 19%, fungi and microbes 4%)


Figure 3. Taxa represented in posters at BES2018 (plants 34%, vertebrates 31%, invertebrates 15%, general 13%, fungi and microbes 7%).

Of some comfort to plant scientists is that despite the often cited unpopularity of plants among students, about a third of all the talks and posters were plant-based.   If one goes purely by biomass, then this is an under-representation of the importance of plants.  A recent paper (Bar-On et al., 2018), estimates that plants make up almost 90% of the planet’s biomass, with the animal kingdom making up perhaps as little as 5% (Figure 4). Given that insects and other invertebrates account for perhaps 97% of all animal life, this further emphasises that the time and funding given to vertebrate ecology is totally unjustified.

Figure 4. Biomass of organisms on Earth from Bar-On et al (2018)

Unfortunately, the British Ecological Society is not alone in overemphasising the importance of the tiny number of vertebrates.  Perhaps more disturbingly is the fact that references to insects in introductory biology textbooks have declined hugely over the last century (Figure 5) while those to vertebrates have increased (Gangwani & Landin, 2018).

 Disappearing insect references (Gangwani & Landin, 2018).

This is a serious problem and one that the British Ecological Society for one, should be doing something about.  Yes, the BES might represent ecologists in general, but they certainly don’t represent ecology.  The Trustees of the BES should take note of the following statement from a group of ecological entomologists “the neglect of insects as study organisms has led to serious bias in our understanding of the functional ecology of ecosystems” (Basset et al., 2019) and the concerns echoed by conservation practitioners (Figure 6) and if that isn’t enough, then perhaps this will “a broader taxonomic base for threatened species assessments, adequately representing invertebrates, will facilitate more profound conservation and policy decisions” (Eisenhauer et al., 2019).

Figure 6. What people on the ground say; a haphazard selection from Twitter

I’ll just leave you with this thought, there are as many aphid species in the world as there are mammal species, just over 5000, but you wouldn’t know it from the number of PhD and post-doctoral positions that are advertised annually, and as for Tipulids (craneflies), a similar sized family….



Bar-On, Y.M., Philips, R. & Milo, R.  (2018) The biomass distribution on Earth. Proceedings of the National Academy of Sciences, 115, 6506-6511.

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 & Diversity, 12, 1-8.

Clarke, J.A. & May, R.M. (2002) Taxonomic bias in conservation research. Science, 297, 191-192.

Eisenehauer, N, Bonn, A. & Guerra, C.A. (2019) Recognizing the quiet extinction of invertebrates. Nature Communications, 10, 50

Gangwani, K. & Landin, J. (2018) The decline of insect representation in biology textbooks over time. American Entomologist, 64, 252-257.

Seddon, P.J., Soorae, P.S. & Launay, F. (2005) Taxonomic bias in reintroduction projects. Animal Conservation, 8, 51-58.


Filed under Bugbears

The Roundabout Review 2018

Welcome to my, now very definitely, traditional review of the past year.

Enjoying ENTO18 at Edgehill University


Impact and reach

Brigitte Nerlich warns against measuring the impact of science communication but as far as I can tell she is not against reporting the following type of data, although if I was looking for impact these annual review posts would not be the best way to go*.

I have continued to post at about ten-day intervals; this is my 229th post.  As I wrote last year, there never seems to any difficulty in coming up with ideas to write about; the problem is more in deciding which one to use and when.  As happened last year, some of my blogs have made it, albeit in slightly modified forms, into print (Cherrill & Leather, 2018; Leather, 2018).

For those of you who remain lukewarm about the idea that social media has a place in science, I wold ask you to think again and if you need any more convincing, refer you to a recent paper that very clearly demonstrates the benefits arising from such interactions (Côté & Darling, 2018); evidence that science communication via social media is a very worthwhile use of our time. Semi-related to my Blogging and Tweeting are my other forms of science communication, giving talks and helping at outreach events, such as the Big Bang Fair and EntoSci18.  I also had three Skype a Scientist dates this year, one with a school in the USA and two with schools in the UK.  I really enjoyed them and hope that the pupils were equally pleased. If you have not come across this scheme, check them out here.

My blog had visitors from 181 countries (165 last year, 174 in 2016 and 150 in 2015), so only another 14 to go to achieve total global domination 😊  My blog received 54 300 views (40 682 last year, 34 036 in 2016; 29 385 in 2015).  This year, as last year, most of my readers came from the USA, with views from India moving from 5th to 4th place and Brazil being replaced by South Africa.

Top reads

My top post (excluding my home page) in 2018 was the same as last year, one of my aphid posts,  A Winter’s Tale – Aphid Overwintering, although there may have been some disappointment felt by those who were hoping to find a reference to Shakespeare’s play or the song by Queen. It is now my all-time winner with just over 9000 views, knocking Not All Aphids are Vegans with over 8 000 views into an honourable second place.  My top ten posts tend to be either about aphids or entomological techniques/equipment which I guess means that I am filling an entomological niche.

A Winter’s Tale – aphid overwintering 3,941
Not all aphids are vegans 2,113
Not Jiminy Cricket but Gregory Grasshopper – someone ought to tell Walt 1,338
Ten papers that shook my world – watching empty islands fill up – Simberloff & Wilson (1969) 1,225
Entomological classics – The Moericke (Yellow) Pan Trap 1,105
Entomological Classics – The Pooter or Insect Aspirator 1,097
Aphid life cycles – bizaare, complex or what? 1,051
Entomological classics – The Tullgren (Berlese) Funnel 948
Entomological classics – the Window (pane) Flight Intercept Trap 871
Not all aphids have wings  769



There still seems to be no signs of the number of people viewing my site reaching an asymptote or for that matter, taking off exponentially; just a straightforward linear relationship.

Still no signs of slowing down?

Tweeting for entomology

I still find my interactions on Twitter very rewarding, although this past year I have become somewhat more political; Brexit and Trump, need I say more?  Most of my tweets are, however, still entomological and ecological and the increase in political comment has not stopped my followers from growing.  I finished 2017 with 5860 followers and begin 2019 with just over a thousand more, 6884.   It would have been nice to have hit the 7 000-follower milestone before the end of the year but many thanks to all my readers and especially to those who take the time to comment as well as pressing the like button.  My top commenters, as indeed they were last year, were fellow bloggers, Emma Maund, Emily Scott, Jeff Ollerton, Amelia from A French Garden and Philip Strange.  I look forward to interacting with you all in 2019.

This past year marked my partial retirement from academia but I hasten to add, not from entomology.  I have, as planned, spent more time doing the things I enjoy and finally got some of my book projects off the ground.  My co-authors Tilly Collins and Tricia Reader and I spent a week together at our house in France writing a book outline and in December signed a contract with Oxford University Press for our provisionally titled ‘Field Course Handbook’.

Authors at work 😊

I have also submitted an outline for a semi-popular book about insects which I have great hopes will appeal to the Commissioning Editor’s choice of reviewers.

And if anyone is worried that this means that the entomological provision at Harper Adams University will be diminished, rest assured.  Not only did we appoint a very talented junior member of faculty, Heather Campbell (@ScienceHeather) we also appointed another talented entomologists (whom I taught some years ago), Simon Segar (@simonsegar); both are proving very popular with the students and staff. I am doing pretty much the same teaching as I have always done, so our entomology provision has actually increased, which is just as well as we have now started an undergraduate degree in entomology, the only one in the UK.

A Happy and Prosperous New Year to you all.


Cherrill, A.J. & Leather, S.R. (2018) Predatory journals a growing threat to scientific integrity?  In Practice, 102, 38-40.

Côté, I.M. & Darling, E.S. (2018) Scientists on Twitter: preaching to the choir or singing from the rooftops?  Facets, 3, 682-694.

Leather, S.R. (2018) “Ecological Armageddon” – more evidence for the drastic decline in insect numbers.  Annals of Applied Biology, 172, 1-3.




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