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The CRC for Tropical Rainforest Ecology and Management
An inordinate fondness for beetles
In 1987, Edward Wilson, a noted rainforest ecologist described insects as "the little things that run the world". Among them, beetles
represent the most spectacular and diverse group, and as Haldane remarked "The creator, if He exists, must have an inordinate
fondness for beetles". In Ancient Egypt, dung beetles were sacred because they were believed to transform death into life by rolling
and burying balls of decayed matter into the ground from which life emerged. Scientists have confirmed that the Egyptians' respect for
dung beetles is well founded; dung beetles play crucial roles in the ecosystem by recycling the soil nutrients, controlling parasites and
dispersing seeds.
Dung beetles are strongly influenced by variations in their habitat, and by the presence of vertebrates on whom they rely for food
(dung). Scientists have often investigated how variations in one environmental factor affect a particular species of dung beetle, but
some factors are likely to be more important than others, and also, it is difficult to know how beetles are affected by the complexity of
habitat factors (soil features, temperature, vegetation, other animals, etc.). I wanted to understand how some of these factors interact to
determine where dung beetles are found. I selected a site where conditions gradually changed from one type of vegetation to another.
Of course, what I also needed were two organisms which are easy to sample and to identify.... Welcome to the world of. Amphistomus
pectoralis and Temnoplectron involucre! These two dung beetles are far from
"sexy"; at first sight people often confuse them for ticks. Both species are tiny (2-3mm), flightless (see figures 1 and 2), nocturnal and
generally most active during the wet season. They are found only at Mt Spec (975m above sea level), a small area of the Wet Tropics
World Heritage Area, in the Paluma range. 1 set pitfall traps across a natural boundary of rainforest and wet eucalypt forest (also called
sclerophyll) to collect and observe both species, and to record other details about the habitat (see figure 3).
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Amphistomus pectoralis.
Has never been observed to roll a ball of dung. This observation is
surprising given the curved posterior legs of these beetles. |
Temnoplectron involucre
Although similar in size
to A.pectoralis, this species is shiny black
and is seen next to a ball of dung it rolled. |
The survey area at Mt Spec. Pitfall traps were laid out in 3 paths (300 metres long) across the rainforest-wet
eucalypt forest boundary. In each path, 5 pitfall traps were set 5m apart, at 150m -100m -50m - 20m -10mfrom the
boundary, and on the boundary itself.
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An important aspect of this study was to assess the influence of variations in vegetation structure and soil texture on the distribution
of the beetles. On a 2 metre square area around each trap, I recorded the number of trees and I estimated the coverage of canopy,
shrubs, grass, ferns, leaf litter, bare soil and dead wood. 1 also analysed soil texture (amount of clay, sand, and silt) with a laser particle
sizer.
The results show that A. pectoralis is rare and its distribution is patchy, whereas T involucre is abundant and widely distributed
across the boundary. Moreover, A. pectoralis never occurred further than 50 metres into the eucalypt forest, whereas T
involucres
occurred as far as 150m into this habitat. Both species seem to prefer habitat with a tall forest canopy and lots of tall shrubs. Also, A.
pectoralis was more likely to be found in silty soils.
The investigation continues
Differences in vegetation and soil texture, therefore, only partly explain the pattern of distribution of
these two species. The reality is that there are possibly more important ecological and behavioural factors determining what is
happening across the boundary For example, dung beetles live and reproduce within the soil, and therefore, soil moisture and
temperature are likely to be important factors influencing dung beetle survival. A. pectoralis, which is
mostly restricted to the rainforest habitat, may have a low tolerance for the drier soil conditions or temperature fluctuations outside the rainforest. I am aiming
to test this by comparing the physiological tolerance of each species, in the lab, to different conditions of soil moisture and temperature.
The amount and type of dung
In 1996, another Rainforest CRC researcher, Dr Chris Hill, found that A. pectoralis and T. involucre are preferentially attracted to the dung of omnivores (for
example, pig) and herbivores (for example, wallaby), and to larger rather than smaller dung piles. If the amount and type of dung determines the distribution and
abundance of the beetles, we would expect more omnivorous and herbivorous dung, and in larger amount, where the beetles are the most abundant. Steve
Williams (Rainforest CRC researcher) has studied small mammal abundance at the rainforest-wet eucalypt boundary. Steve found the biomass of small
mammals decreased from the rainforest to the sclerophyll. So too do the dung beetles! The type and quantity of dung
may, therefore, also explain the distribution of the two beetle species.
How to be a good competitor
For dung beetles, the more dung they get, the more competitive they are. There are many ways to be a good competitor such as being first on the dung pad and
rolling balls away. This rolling behaviour allows dung beetles to take away, store, hide, and protect food from competitors. Beetles which roll dung often bury
the balls in the ground or store them under leaves where the moisture level is more
favorable. Observations in the lab and in the forest show that T involucre is
more mobile than A. pectoralis and that T involucre arrives first on dung. I have also observed that T involucre feed either directly on the dung or roll balls,
whereas A. pectoralis does not roll, it only feeds directly on the dung pad. Why is this important? The sclerophyll is drier and has less dung, and A. pectoralis
might find itself out competed by the faster actions of T involucre. Further research and observations are
required to evaluate whether competition for dung is an important factor determining where these two species are found.
Dung beetles respond differently to changes in the environment. These responses, when fully understood, may indicate different things about the environment,
depending on the species and the habitat. For example, *A. pectoralis may indicate the presence of an important ecological boundary between the rainforest and
the eucalypt forest - I involucre, in the eucalypt forest, may be an indicator of vertebrate diversity
*A. pectoralis and T. involucre, in the rainforest, may be
correlated to the biomass of vertebrates.
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For more information
Please call Agnes Rortais directly at the
Department of Zoology and Tropical
Ecology, James Cook University, QLD
4811, Australia.
Phone: (07) 47815768
Facsimile: (07) 4725 1570
Email: agnes.rortais@icu.edu.au |
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The CRC for Tropical Rainforest Ecology and Management is a formal partnership between the
CSIRO,
James Cook University of North Queensland, The University of Queensland, Griffith University and the Wet Tropics Management Authority.
P 0 Box 6811
Cairns Qld 4870 tel: (07) 4042 1246
fax : (07) 4042 1247
Web
site: http://www.crctrem.edu.au
Providing science for the conservation and management of Australia's World Heritage tropical rainforests.
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