An inordinate fondness for beetlesFebruary 1999

Figure 1 Amphistomus pectoralis. To date, A. pectoralis has neverbeen observed to roll a ball of dung. This observation is surprisinggiven the curved posterior legs of these beetles, particularlymarked in A. pectoralis.

Figure 2 Temnoplectron involucre. Although similar in size to A.pectoralis, this species is black and shiny and is seen here next toa ball of dung which it has rolled (photo: R. Yeldham)

In 1987, Edward Wilson, a notedrainforest ecologist described insects as“the little things that run the world”.Among them, beetles represent the mostspectacular and diverse group, and asHaldane remarked “The creator, if Heexists, must have an inordinate fond-ness for beetles”. In Ancient Egypt,dung beetles were sacred because theywere believed to transform death intolife by rolling and burying balls ofdecayed matter into the ground fromwhich life emerged. Scientists haveconfirmed that the Egyptians’ respectfor dung beetles is well founded; dungbeetles play crucial roles in the ecosys-tem by recycling the soil nutrients,controlling parasites and dispersingseeds.

Dung beetles are strongly influenced byvariations in their habitat, and by thepresence of vertebrates on whom theyrely for food (dung). Scientists haveoften investigated how variations in oneenvironmental factor affect a particularspecies of dung beetle, but some factorsare likely to be more important thanothers, and also, it is difficult to knowhow beetles are affected by the com-plexity of habitat factors (soil features,temperature, vegetation, other animals,etc.). I wanted to understand how someof these factors interact to determinewhere dung beetles are found. I selecteda site where conditions graduallychanged from one type of vegetation toanother. Of course, what I also neededwere two organisms which are easy tosample and to identify.......Welcome tothe world of Amphistomus pectoralis andTemnoplectron involucre!

These two dung beetles are far from“sexy”; at first sight people oftenconfuse them for ticks. Both species aretiny (2-3mm), flightless (see figures 1and 2), nocturnal and generally mostactive during the wet season. They arefound only at Mt Spec (975m above sealevel), a small area of the Wet TropicsWorld Heritage Area, in the Palumarange. I set pitfall traps across a naturalboundary of rainforest and wet euca-

lypt forest (also called sclerophyll) tocollect and observe both species, and torecord other details about the habitat(see figure 3).

An important aspect of this study wasto assess the influence of variations invegetation structure and soil texture onthe distribution of the beetles. On a 2metre square area around each trap, Irecorded the number of trees and Iestimated the coverage of canopy,shrubs, grass, ferns, leaf litter, bare soiland dead wood. I also analysed soiltexture (amount of clay, sand, and silt)with a laser particle sizer.The results show that A. pectoralis is rareand its distribution is patchy, whereas T.involucre is abundant and widelydistributed across the boundary.Moreover, A. pectoralis never occurredfurther than 50 metres into the eucalyptforest, whereas T. involucre occurred asfar as 150m into this habitat. Bothspecies seem to prefer habitat with a tallforest canopy and lots of tall shrubs.Also, A. pectoralis was more likely to befound in silty soils.

The investigation continuesDifferences in vegetation and soiltexture, therefore, only partly explainthe pattern of distribution of these twospecies. The reality is that there arepossibly more important ecological andbehavioural factors determining what ishappening across the boundary. Forexample, dung beetles live and repro-duce within the soil, and therefore, soilmoisture and temperature are likely tobe important factors influencing dungbeetle survival. A. pectoralis, which is

Figure 4 Researcher Agnes Rortais collecting specimens

Figure 3 The survey area at Mt Spec. Pitfall traps were laid out in 3 paths (300 metres long) across therainforest-wet ecucalypt forest boundary. In each path, 5 pitfall traps were set 5m apart, at 150m -100m -50m -20m -10m from the boundary, and on the boundary itself.

mostly restricted to the rainforesthabitat, may have a low tolerance forthe drier soil conditions or temperaturefluctuations outside the rainforest. I amaiming to test this by comparing thephysiological tolerance of each species,in the lab, to different conditions of soilmoisture and temperature.

The amount and type of dungIn 1996, another Rainforest CRCresearcher, Dr Chris Hill, found that A.pectoralis and T. involucre are preferen-tially attracted to the dung of omnivores(for example, pig) and herbivores (forexample, wallaby), and to larger ratherthan smaller dung piles. If the amountand type of dung determines thedistribution and abundance of thebeetles, we would expect more omnivo-rous and herbivorous dung, and inlarger amount, where the beetles are themost abundant. Steve Williams(Rainforest CRC researcher) has studiedsmall mammal abundance at therainforest—wet eucalypt boundary.Steve found the biomass of smallmammals decreased from the rainforestto the sclerophyll. So too do the dungbeetles! The type and quantity of dung

may, therefore, also explain the distribu-tion of the two beetle species.

How to be a good competitorFor dung beetles, the more dung theyget, the more competitive they are.There are many ways to be a goodcompetitor such as being first on thedung pad and rolling balls away. Thisrolling behaviour allows dung beetles totake away, store, hide, and protect foodfrom competitors. Beetles which rolldung often bury the balls in the groundor store them under leaves where themoisture level is more favourable.Observations in the lab and in the forestshow that T. involucre is more mobilethan A. pectoralis and that T. involucrearrives first on dung. I have alsoobserved that T. involucre feed eitherdirectly on the dung or roll balls,whereas A. pectoralis does not roll, itonly feeds directly on the dung pad.Why is this important? The sclerophyllis drier and has less dung, and A.pectoralis might find itself outcompetedby the faster actions of T. involucre.Further research and observations are

required to evaluate whether competi-tion for dung is an important factordetermining where these two speciesare found.

Dung beetles respond differently tochanges in the environment. Theseresponses, when fully understood, mayindicate different things about theenvironment, depending on the speciesand the habitat. For example,•A. pectoralis may indicate the presenceof an important ecological boundarybetween the rainforest and the eucalyptforest •T.involucre, in the eucalypt forest, may bean indicator of vertebrate diversity •A.pectoralis and T. involucre, in therainforest, may be correlated to thebiomass of vertebrates.

For more informationPlease call Agnes Rortais directly at theDepartment of Zoology and TropicalEcology, James Cook University, QLD4811, Australia.Phone: (07) 4781 5768Facsimile: (07) 4725 1570Email: agnes.rortais@jcu.edu.au