2.2. Sampling techniques
We used a paired design, with 25 pitfall traps inside each garden
and 25 outside. Fifteen gardens were selected from four valleys
surrounding the town of St Katherine (Shraij, Tuffaha, Itlah and
Abu Fraish). Garden choice was based on permission from garden
owners and whether suitable trapping sites were available in
the surrounding vicinity. External traps were placed a minimum of
15 m from their associated gardens, with approximately half placed
along the base of the wadi and half placed higher up, upon the wadi
slopes. It was not possible to place traps on inaccessible steep slopes
or busy paths where they were frequently disturbed.
Traps were laid out in 5
×
5 grids, with 2 m between each trap.
The simple trap design consisted of a plastic cup flush with the
ground and filled to one-third with a solution of water and detergent.
Traps were set between 9 am and 11 am in the mornings and
emptied 24 h later. The contents of the traps were collected and
stored in alcohol. Insects were grouped initially by family and then
sorted into morpho-species based on visual characteristics. Representative
individuals of beetle, spider and ant morpho-species
were stored for identification by Dr Mahmoud Abdel-Dayem of
Cairo University (beetles), Hisham El-Hennawy of Cairo (spiders)
and Dr Brian Taylor of Nottingham (ants).
Pitfall traps do have limitations and cannot provide accurate
estimates of true abundance, nor the relative abundance of the
different functional groups, because certain groups tend to be overrepresented
(Lang, 2000). However, they can still provide relative
numbers for a comparison between the two habitat types. Other
methods of sampling were ruled out due to impracticality of the
landscape–a high proportion of plants inside and outside of the
gardens have thorns and spines making sweep-netting unfeasible.
Although vegetation within the gardens is denser than outside,
there are still large areas of exposed sandy soil with a similar
composition to that found outside. Traps were all set in these
open areas and not within areas of higher leaf litter (such as
flower-beds), allowing for a comparable trapping effort inside and
outside.