Tag Archives: Southwood

Entomological classics – the pitfall trap

Pitfall arghh I would be amazed if there are any entomologists who have not deployed a pitfall trap or two at some stage in their career. I would also hazard a guess that quite a few non-entomological ecologists have come across the joys of pitfall trap setting and catch sorting as part of their undergraduate training; most field courses seem to include a pitfall trap day, and rightly so.  Pitfall trapping is after all, probably the simplest and most efficient way of collecting data, and not always insects 😉 Pitfall - tapir

Tapir pitfall trap

More seriously though, pitfall traps are a remarkably simple and incredibly versatile way of sampling insects, particularly those that are active on the soil surface (epigeal) e.g carabid beetles. Pitfall forest They can be used in most habitats where you are able to dig into the soil,

Pitfall traps cheap

are very cheap as they can be made from easily obtainable household materials Pitfall traps and can be modified easily depending on your objectives and sampling conditions.  It is very important however, that the lip of the trap is either flush with or below the soil surface.  Not very many beetles or other invertebrates,  are willing to climb up the steep sides  to allow you to capture them. Pitfall - spatial patterns They are also amenable to being deployed in a variety of statistically meaningful ways. (Figure ‘borrowed’ from Woodcock (2005)). Pitfall traps - catch a lot They are of course not perfect.   Some of my students complain that they catch too much!

There has been, and continues to be, much debate about what the catch actually represents.  Are they a measure of activity or of density, i.e. do the trap catches represent the most active and careless beetles, rather than the most abundant?  Southwood (1966) in the first edition of Ecological Methods is fairly dismissive of their use except as a way of studying the activity, seasonal incidence and dispersion of single species and considered them to be of no use whatsoever in comparing communities.  Other authors argue however, that if the trapping is carried out over a long period of time then the data collected can be representative of actual abundance (e.g. Gist & Crossley, 1973; Baars, 1979) and despite Southwood’s comments, they are probably most often used to compare communities (e.g. Rich et al., 2013; Zmihorski et al., 2013;  Wang et al., 2014) For a very thorough account of the use and abuse of pitfall traps see Ben Woodcock’s excellent 2005 article (and I am not just saying that because he is one of my former students). You might expect, given the fact that pitfalls were used by our remote ancestors to trap their vertebrate prey, that entomologists would have adopted this method of trapping very early on, especially given the fact that nature got there first, e.g. as used by larvae of the antlion. Antlion trap

Antlion ‘pitfall traps’.

I was therefore surprised when I started researching this article to find that the earliest reference I could find in the scientific literature was Barber (1931).  I found this very hard to believe so resorted to Twitter.  Richard Jones suggested that a sentence in Pitfall silver sand reference

Notes on Collecting and Preserving Natural History Objects

referring to silver sand pits might be a reference to an early form of pitfall trap.  On further research however, it turned out that sand pits were the results of sand mining operations and were used opportunistically by entomologists.  They worked in a very similar way to Pitfall - St Austell

St Austell Ruddle Moor Sand Pit http://www.cornwall-opc.org/Par_new/a_d/austell_st.php

intercept traps (the subject of a future post).   Interestingly, in some parts of the world, sand pits are now being restored in some places as conservation tools for digger wasp sand bees. Pitfall Bohemia

Sand pit restoration – Bohemia.  http://www.outdoorconservation.eu/project-detail.cfm?projectid=17

  But, I digress.  My next port of call was The Insect Hunter’s Companion (Greene, 1880) which I felt certain would mention pitfall traps.  To my surprise, in the 1880s, entomologists intent on capturing beetles, either pursued them with nets, turned over stones and logs, removed bark from trees, used beating trays or even dug holes in the ground, but never used pitfall traps!  So all very active and energetic methods – no sit and wait in those days 😉 So it seems that Barber’s 1931 description of a pitfall trap does indeed commemorate the first scientific use of a pitfall trap. Barber trap

The Barber trap (Barber, 1931).

Despite their late addition to the entomological armoury and despite the many criticisms levelled at their use, they continue to be perhaps the most widely used method of insect sampling ever; for example if you enter Beetle* AND pitfall* AND trap*  into the Web of Science you will return 1168 hits since 2000, which is more than one a week.  If you further refine your search to exclude beetle but add insect* you can add another 320 hits. If by some chance you have never used a pitfall trap, then I heartily recommend that you set one or two up in a convenient flower bed or even your lawn, and then sit back and wait and see what exciting beasties are roaming your garden.

Post script

Since this post was published I have discovered an earlier reference to the use of pitfall traps (Hertz, 1927).  Many thanks to Jari Niemelä  of Helsinki University for sending me a copy of the reference and many thanks to my eldest daughter for translating the relevant bit, which follows –  “The traps were made of meticulously cleaned tin cans (the rectangle ones used for e.g.  sardines) dug into the ground so deep that the top of the tin was absolutely level with the ground…… it is an ideal way to catch the beetles; with their careless way of running around, they easily fell into the deathtraps, and had no time to use their wings (if they have any)”.  The phrase deathtraps is particularly fine.  The majority of the paper is about the species he caught in different locations and he highlights the fact that he caught seven very rare species using this method.

So this is now the oldest known reference to the use of pitfall traps in the literature, although he does mention that he was using this method to catch beetles in 1914.  But if anyone comes across an earlier reference do let me know.

 

References

Baars, M.A. (1979) Catches in pitfall traps in relation to mean densities of carabid beetles. Oecologia, 41, 25-46.

Barber, H.S. (1931) Traps for cave inhabiting insects.  Journal of the Elisha Mitchell Scientific Society, 46, 259-266.

Gist, C.S. & Crossley, J.D.A. (1973) A method for quantifying pitfall trapsEnvironmental Entomology, 2, 951-952.

Greene, J. (1880) The Insect Hunter’s Companion: Being Instructions for Collecting and Describing Butterflies, Moths, Beetles, Bees, Flies, Etc.  

Hertz, M. (1927) Huomioita petokuoriaisten olinpaikoista.  Luonnon Ystävä, 31, 218-222

Rich, M.C., Gough, L., & Boelman, N.T. (2013) Arctic arthropod assemblages in habitats of differing shrub dominance. Ecography, 36, 994-1003.

Southwood, T.R.E. (1966) Ecological Methods, Chapman & Hall, London.

Wang, X.P., Müller, J., An, L., Ji, L., Liu, Y., Wang, X., & Hao, Z. (2014) Intra-annual variations in abundance and speceis composition of carabid beetles in a temperate forest in Northeast China. Journal of Insect Conservation, 18, 85-98.

Woodcock, B.A. (2005) Pitfall trapping in ecological studies.  Pp 37-57 [In] Insect Sampling in Forest Ecosystems, ed S.R. Leather, Blackwell Publishing, Oxford.

Zmihorski, M., Sienkiewicz, P., & Tryjanowski, P. (2013) Neverending story: a lesson in using sampling efficieny methods with ground beetles. Journal of Insect Conservation, 17, 333-337.

 

Post post script

Pitfall traps are even more versatile than you might think. Mark Telfer has developed a nifty subterranean version http://markgtelfer.co.uk/beetles/techniques-for-studying-beetles/subterranean-pitfall-traps-for-beetles/  and at the opposite end of the spectrum, pitfall traps have also been used in trees to sample spiders (Pinzon & Spence, 2008).

Reference Pinzon, J. & Spence, J. (2008) Performance of two arboreal pitfall trap designs in sampling cursorial spiders from tree trunks.  Journal of Arachnology, 36, 280-286

 

Post post script And for those of you who have had to suffer sitting through the Pokémon movie as I did many years ago, there is also a Pokémon version of the antlion! Pitfall Pokemon

http://bulbapedia.bulbagarden.net/wiki/Trapinch_(Pok%C3%A9mon)

 and don’t forget Winnie the Pooh and his heffalump trap 😉  Hopefully you will use them more carefully than he did. Pitfall trap - Heffalump

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Entomological Classics – Southwood 1961 – The number of insect species associated with various trees

 

Nineteen-Sixty-One  was a momentous  year for entomology and ecology, although at the time I suspect few realised it.  Skip forward to 2013 when The British Ecological Society published a slim volume celebrating  the 100 most influential papers published in the Society’s journals.  The papers included in the booklet were selected based on the opinions of 113 ecologists from around the world, who were then asked to write a short account of why they thought that paper influential.  I was disappointed not to be asked to write about my nomination but instead asked to write about Maurice Solomon’s 1949 paper in which he formalised the term functional response.

The paper I had wanted to write about was included, but John Lawton had the privilege of extolling its virtues, and given the word limits did a pretty good job.  I do, however, feel that given its importance to ecology and entomology it deserves a bit more exposure, so I am taking the opportunity to write about it here.  I could have included this post in a series I have planned, called Ten Papers that Shook My World, but given the impact that this paper has had on entomologists I felt it deserved an entry in my Entomological Classics series.

For those of you who haven’t come across this paper before, this was an astonishingly influential paper.  Basically, Southwood, who despite his later reputation as one of the ecological greats, was an excellent entomologist, (in fact he was a Hemipterist), wanted to explain why some tree species had more insect species associated with them than others.  He made comparisons between trees in Britain, Russia and Cyprus and demonstrated that those trees that were more common and had a wider range had more insect species associated with them (Figure 1).

Southwood 1961 Fig 1

From Southwood 1961.  I was surprised to see that he had committed the cardinal error in his Figure caption of describing it as Graph and also including the regression equation in the figure pane; two things that I constantly reprimand students about!

Importantly he also showed that introduced trees tended to have fewer insects than native species.  He thus hypothesised that the number of insects associated with a tree species was proportional to its recent history and abundance and was a result of encounter rates and evolutionary adaptation.  He then tested this hypothesis using data on the Quaternary records of plant remains from Godwin (1956) making the assumption that these were a proxy for range as well as evolutionary age.

He commented on the outliers above and below the line suggesting that those above the line were a result of having a large number of congeners and those below the line either as being taxonomically isolated and/or very well defended.

He then went on to test his ideas about the evolutionary nature of the relationship by looking at trees and insects in Hawaii, (ironically this appeared in print (Southwood, 1960), before the earlier piece of work (Journal of Animal Ecology obviously had a slower turnaround time in those days than they do now).

Hawaiin figure

Figure 2.  Relationship between tree abundance and number of insect species associated with them (drawn using data from Southwood 1960).

Considering the research that these two papers stimulated over the next couple of decades, what I find really odd, is that Southwood, despite the fact that he was dealing with data from islands and that Darlington (1943) had published a paper on carabids on islands and mountains in which he discussed species-area relationships and further elaborated on in his fantastic book (Darlington, 1957), did not seem to see the possibility of using the species-area concept to explain his results.  It was left to Dan Janzen who in 1968 wrote

It is unfortunate that the data on insect-host plant relationships have not in general been collected in a manner facilitating analysis by MacArthur and Wilson’s methods (as is the case as well with most island biogeographical data). What we seem to need are lists of the insect species on various related and unrelated host plants, similarity measures between these lists (just as in Holloway and Jardine’s 1968 numerical taxonomic study of Indo- Australian islands), knowledge of the rates of buildup of all phytophagous insect species on a host plant new to a region, where these species come from, etc. Obviously, the insect fauna must be well known for such an activity. The English countryside might be such a place; it has few “islands” (making replication difficult) but a very interesting “island” diversity, with such plants as oaks being like very large islands and beeches being like very small ones, if the equilibrium number of species on a host plant (Elton, 1966; Southwood, 1960) is any measure of island size.”

 

In 1973 Dan Janzen  returned to the subject of trees as islands and cited Paul Opler’s 1974 paper in relation to the fact that the number of  herbivorous insects associated with a plant increases with the size of the host plant population (Figure 3), and further reiterated

Opler Figure

Figure 3.  Opler’s 1974 graph showing relationship between range of oak trees in the USA and the number of herbivorous insect species associated with them.

 his point about being able to consider trees as ecological islands.  Opler’s 1974 paper is also interesting in that he suggested that this approach could be used for predicting pest problems in agricultural systems, something that Don Strong and colleagues did indeed do (Strong et al., 1977; Rey et al., 1981), and that the concept of habitat islands and the species-area relationship could be used when designing and evaluating nature reserves, something which indeed has come to pass.

Again in 1974 but I think that Strong has precedence because Opler cites him in his 1974 paper, Don Strong reanalysed Southwood’s 1961 data using tree range (based on the Atlas of the British Flora)  as the explanatory variable  (figure 4) to explain the patterns seen.

Strong Figure

Figure 4Strong’s reworking of Southwood’s 1961 insect data using the distribution of British trees as shown in Perring & Walters1 (1962).

The publication of this paper opened the floodgates, and papers examining the species-area relationships of different insect groups and plant communities proliferated (e.g  leafhoppers (Claridge & Wilson, 1976); bracken (Rigby & Lawton, 1981); leaf miners (Claridge & Wilson, 1982); rosebay willow herb (McGarvin, 1982), with even me making my own modest contribution in relation to Rosaceous plants   (Leather, 1985, 1986).

Although not nearly as popular a subject as it was in the 1980s, people are still extending and refining the concept  (e.g. Brändle & Brandl, 2001; Sugiura, 2010; Baje et al., 2014).

Southwood (1961) inspired at least two generations of entomologists and ecologists, including me, and is still relevant today.  It is truly an entomological (and ecological) classic.

References

Baje, L., Stewart, A.J.A. & Novotny, V. (2014)  Mesophyll cell-sucking herbivores (Cicadellidae: Typhlocybinae) on rainforest trees in Papua New Guinea: local and regional diversity of a taxonomically unexplored guild.  Ecological Entomology 39: 325-333

Brändle, M. &Brandl, R. (2001). Species richness of insects and mites on trees: expanding Southwood. Journal of Animal Ecology 70: 491-504.

Claridge, M. F. &Wilson, M. R. (1976). Diversity and distribution patterns of some mesophyll-feeding leafhoppers of temperate trees. Ecological Entomology 1: 231-250.

Claridge, M. F. &Wilson, M. R. (1982). Insect herbivore guilds and species-area relationships: leafminers on British trees. Ecological Entomology 7: 19-30.

Darlington, P. J. (1943). Carabidae of mountains and islands: data on the evolution of isolated faunas and on atrophy of wings. Ecological Monographs 13: 37-61.

Darlington, P. J. (1957). Zoogeography: The Geographical Distribution of Animals. New York: John Wiley & Sons Inc.

Elton, C. S. (1966). The Pattern of Animal Communities. Wiley, New York.

Holloway, J. D., & Jardine, N. (1968). Two approaches to zoogeography: a study based on the distributions of butterflies, birds and bats in the Indo-Australian area. Proceedings of the Linnaean Society. (London) 179:153-188.

MacArthur, R. H. & Wilson, E.O. (1967). The Theory of Island Biogeography. Princeton University Press, Princeton, N. J

Janzen, D. H. (1968). Host plants as islands in evolutionary and contemporary time. American Naturalist 102: 592-595.

Janzen, D. H. (1973). Host plants as islands II.  Competitive in evolutionary and contemporary time. American Naturalist 107: 786-790.

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.

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 hostrange, plant architecture, age of establishment, taxonomic isolation and species-area relationships. Journal of Animal Ecology 55: 841-860.

Macgarvin, M. (1982). Species-area relationships of insects on host plants: herbivores on rosebay willowherbs. Journal of Animal Ecology 51: 207-223.

Opler, P. A. (1974). Oaks as evolutionary islands for leaf-mining insects. American Scientist 62: 67-73.

Perring, F.J. & Walters, S.M. (1962) Atlas of the British Flora BSBI Nelson, London & Edinburgh.

Preston,  C.D., Pearman, D.A. & Tines, T.D. (2002) New Atlas of the British and Irish Flora: An Atlas of the Vascular Plants of Britain, Ireland, The Isle of Man and the Channel Islands. BSBI, Oxford University Press

Rigby, C. & Lawton, J. H. (1981). Species-area relationships of arthropods on host plants: herbivores on bracken. Journal of Biogeography 8: 125-133.

Solomon, M. E. (1949). The natural control of animal populations. Journal of Animal Ecology 18: 1-35

Southwood, T. R. E. (1960). The abundance of the Hawaiian trees and the number of their associated insect species. Proceedings of the Hawaiian Entomological Society 17: 299-303.

Southwood, T. R. E. (1961). The number of species of insect associated with various trees. Journal of Animal Ecology 30: 1-8.

Sugiura, S. (2010). Associations of leaf miners and leaf gallers with island plants of different residency histories.  Journal of Biogeograpgy 37: 237-244

Rey, J.R.M.E.D. & Strong, D.R. (1981) Herbivore pests, habitat islands, and the species area relation. American Naturalist 117: 611-622.

Strong, D. R. (1974). The insects of British trees: community equilibrium in ecological time. Annals of the Missouri Botanical Gardens 61: 692-701.

Strong, D.R., D., M.E., & Rey, J.R. (1977) Time and the number of herbivore species: the pests of sugarcane. Ecology 58: 167-175

 

1Postscript

The Atlas of the British Flora by Perring and Walters (1962) was an iconic piece of work, although not without its flaws.  As with many distribution atlases it is based on a pence or absence score of plant species within one kilometre squares.  So although it is a good proxy or range it does not necessarily give you an entirely reliable figure for abundance.  A dot could represent a single specimen or several thousand specimens.   Later authors attempted to correct for this by using more detailed local surveys e.g. tetrads.  It must have been particularly galling for  Southwood that the Atlas didn’t appear until after he had published his seminal papers, but he later made up for it by reanalysing and extending his data from that original 1961 paper (Kennedy et al., 1984).

Those of us working in this area using the original Atlas had to count the dots by hand, a real labour of love especially for those widely distributed species; the new edition (Preston et al., 2002) actually tells you how many dots there are so the task for the modern-day insect-plant species-area relationship worker is much easier 😉

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