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Ba Na Nature Reserve 1996
6.0 INVERTEBRATE SURVEYS
6.1 Aims and Methods
The aim of the invertebrate survey was to compare the invertebrate communities of the forest
transects outlined above, and thus to assess relative diversity of invertebrate groups in the habitats
represented in the Ba Na reserve.
Two methods were used in order to quantitatively sample invertebrates in the forest transects:
sweep-netting and pitfall trapping. Both are commonly used methods of insect collection, and the
principles and drawbacks associated with each are discussed by Southwood (1978), Biological
Survey of Canada (1994), and other authors.
6.1.1 Sweep-netting
In each transect, 100 sweeps were made with a sweep-net and the invertebrates captured were
removed using a pooter. Once caught, the animals were preserved in ethanol and sorted into
Recognisible Taxonomic Units (RTUs), or morphospecies (morphologically distinct types). The
efficiency of sorting to RTU varies between different animal groups, depending on the degree of
inter- and intraspecific variation found. However, Oliver and Beattie (1993) have found that the
number of RTUs correlate closely to the number of actual species for most orders, even when
unskilled labour is involved. The use of RTUs allows diversity indices to be calculated, and,
therefore, the faunas of different sites can be compared. Unfortunately, however, the possibility
of comparison to other surveys is limited, as the taxonomic and ecological data conveyed by the
RTU is limited (Biological Survey of Canada, 1994).
Sweep netting is a difficult technique to standardise, and the catch depends on the nature of the
habitat being sampled and the individual carrying out the sampling, as well as many other factors
(Biological Survey of Canada, 1994).
6.1.2 Pitfall trapping
Pitfall traps were laid out for two nights at each of the forest transects. On each occasion, four
traps were placed in an array which covered approximately 3.14m 2 . Salt water was used in the
traps as this does not cause bias in the final catch, as do other preservatives such as ethanol.
Invertebrates were removed from the salt solution and placed in ethanol for preservation, and
sorted to orders, and RTUs, as for sweep net catches.
Two indices were calculated to describe the insect assemblages caught; Fisher's α, a measure of
diversity which takes account of the number of species (here, RTUs) and individuals in a sample
(Fisher et al, 1943), and the dominance measure d (Berger and Parker, 1970), the proportion of
the total number of individuals in the sample which belong to the most abundant species.
Methods follow Magurran (1988).
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