Tag Archives: diversity

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 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.

References

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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Being inspired by the BES

This week (20th July) I have had the privilege of being able to interact with 50 undergraduates (mainly just finished their first year) under the auspices of the British Ecological Society’s new undergraduate summer school held at the Field Studies Council’s Malham Tarn Centre. The scheme enables aspiring ecologists to have “an opportunity to enhance their existing knowledge with plenary lectures from senior ecologists, fieldwork, workshops, careers mentoring and more at a week-long residential course” This was especially pleasurable for me because as a school boy and student I spent several enjoyable camping holidays at Malham and it gave me an opportunity to take part in a field course again, something I have missed since leaving Silwood Park where I ran the now defunct annual two-week long Biodiversity & Conservation field course. The programme included two ecological luminaries and old friends of mine, Sue Hartley from the University of York and plant scientist and author, Ken Thompson formerly of Sheffield University and also Clare Trinder from the University of Aberdeen.  Also in the programme was conservation biologist, Stephanie Januchowski-Hartley,  and additional input from the Chartered Institute of Ecology & Environmental Management (CIEEM), microbial ecologist, Dr Rob Griffiths from CEH and ecologist Dr Peter Welsh of the National Trust.

I arrived mid-morning of the Tuesday, having driven up from Shropshire to Yorkshire the night before, having taken the opportunity to stay in the old family home in Kirk Hammerton before it is put up for sale. Whilst there I also set a few pitfall traps to collect some insects that we might not catch otherwise. As it happened they were a dismal failure, returning mainly spiders, harvestmen and woodlice, plus one nice carabid beetle, more of which later. The weather didn’t look all that promising for an insect sampling session but I kept my fingers crossed and hoped that it wouldn’t rain as much as it did almost 40 years ago when my best friend from school and I aborted our camping holiday at nearby Malham Cove after three days of solid rain 😉

Malham Tarn

Malham Tarn – not quite raining

  I was greatly amused on arriving to be greeted by a very large arachnid lurking on an outhouse.

Malham spider

We breed them big in Yorkshire!

Malham Tarn FSC

Malham Tarn Field Studies Centre

After checking my equipment and locating suitable sampling sites I joined the students, Karen Devine, the BES External Affairs manager and some of the PhD mentors for lunch. After lunch it was my slot, a chance to infect (sorry, inspire), fifty ecologically included undergraduates with a love of insects. After being introduced by Karen I launched into my talk to a very full room of students.

Karen Devine

Karen instilling order and attention 😉

Ready to be inspired

Ready and waiting to be inspired

The undergraduates came from thirty different UK universities with a strong female bias, 34:16. Exeter University had four representatives, with Reading, Liverpool John Moores, UCL and Bristol with three each. I was sorry to see that there were no students from my Alma mater Leeds, or from my former institution, Imperial College, once regarded as the Ecological Centre of the UK, although UEA where I did my PhD, had two representatives.  There was also one representative from my current place of work, Harper Adams University. Incidentally one of the students turned out to have gone to the same school that I did in Hong Kong, King George V School, albeit almost fifty years apart; a small world indeed.

I set the scene by highlighting how many insect species there are, especially when compared with vertebrates.

The importance of insects

The importance of insects and plants

Number of animal species

Or to put it another way

After a quick dash through the characteristics of insects and the problems with identifying them, exacerbated by the shortage of entomologists compared with the number of people working on charismatic mega-fauna and primates, I posed the question whether it is a sound policy to base conservation decisions on information gained from such a small proportion of the world’s macro-biota.

Then we were of into the field, although not sunny, at least it was not raining so I was able to demonstrate a variety of sampling techniques; sweep netting with the obligatory head in the bag plus Pooter technique, butterfly netting, tree beating and, as a special treat, motorized suction sampling, in this instance a Vortis.

Sampling

With aid of the PhD mentors and Hazel Leeper from the Linnaen Society, the students were soon cacthing interesting things (not all insects) and using the Pooters like experts.

Students sampling

Getting close up with the insects

I also let some of the students experience the joy of the Vortis, suitably ear-protected of course. All good things come to an end and it was then time to hit the microscopes, wash bottles, mounted pins and insect keys.

In teh lab

Getting stuck in – picture courtesy Amy Leedale

Down the microscope

What’s this?

I was very impressed with how well the students did at getting specimens down to orders and families and have every confidence that there are a number of future entomologists among them. After the evening meal, Kate Harrison and Simon Hoggart from the BES Publications Team introduced the students to the tactics of paper writing and publishing which I think they found something of an eye-opener. The students, after a rapid descent on the bar, enjoyed a Pub Quiz whilst I relaxed with a glass of wine until it was dark enough for me to demonstrate the wonders of using fluorescent dust to track our solitary carabid beetle using my UV torch before heading off to bed.

Fluorescent carabid Eloise Wells

Glow in the dark carabid beetle – the bright lights of Malham Tarn – photo courtesy of Eloise Wells

I was sorry to have to leave the next morning, it would have been great fun to have stayed the full week, but next year I do hope to be able to be there for at least two days and nights so that we can do pitfall trapping and light trapping and of course, have more fun with fluorescent insects.

I hope the students found the whole week inspirational and useful, I was certainly inspired by their obvious enjoyment and interest and will be surprised I if do not come across some of them professionally in the future.

Well done BES and congratulations to Karen and her team for providing such a great opportunity for the students. I am really looking forward to next year and being able to see great Yorkshire features like this in the sunshine 😉

Yorkshire grit

 

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