ecology of phasmids - KLUEDO - Universität Kaiserslautern
ecology of phasmids - KLUEDO - Universität Kaiserslautern
ecology of phasmids - KLUEDO - Universität Kaiserslautern
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Community structure & host range 14<br />
where H’MAX = Maximum possible value <strong>of</strong> the Shannon-Wiener Function<br />
S = Number <strong>of</strong> species in the community<br />
Eveness ranges from 0 to 1. Generally all eveness measures are overestimated (see Krebs 1989) as they<br />
assume that the total number <strong>of</strong> species in the community is known. However, in the presented study<br />
this problem may be <strong>of</strong> minor importance as the phasmid community in forest understory and at forest<br />
edges was sampled excessively.<br />
On the base <strong>of</strong> line-transect data I calculated population density <strong>of</strong> the phasmid community and in<br />
particular population density <strong>of</strong> M. diocles for forest understory and forest edges separately. Generally<br />
population densities from line-transect data can be calculated as described by Krebs (1989):<br />
Equation 2-4<br />
n<br />
D =<br />
2La<br />
where D = Density <strong>of</strong> animals per unit area<br />
n = Number <strong>of</strong> animals seen on transect<br />
L = Length <strong>of</strong> transect<br />
a = Constant that describes the probability <strong>of</strong> detecting an animal<br />
The estimation <strong>of</strong> the detection probability a affords knowledge <strong>of</strong> sighting distance and angle for each<br />
animal measured from the transect line. These parameters could not be assessed because for detection <strong>of</strong><br />
nymphs the undersides <strong>of</strong> leaves had to be searched and therefore I had to leave the central transect<br />
lines. Consequently, I simplified the calculation <strong>of</strong> density by assuming that detection probability was 1<br />
(i.e., all animals were found) and thereby that ‘a’ was negligible (density estimates based on numbers <strong>of</strong><br />
observed animals solely are sometimes referred to as apparent density; Krebs 1989). This assumption<br />
may have lead to an underestimation <strong>of</strong> densities, as it was likely that animals were missed. Further, I<br />
accounted for the fact that in the understory line-transects were searched on both sides while at forest<br />
edges they were sampled one-sided facing the forest (width <strong>of</strong> 2 m). Accordingly, in transects <strong>of</strong> 80 m<br />
length, the corresponding searching area in the understory was 320 m 2 versus 160 m 2 at forest edges.<br />
These modifications led to the calculation <strong>of</strong> population densities by:<br />
n<br />
n<br />
D = for forest understory, and D = for forest edges,<br />
2Lw<br />
1Lw<br />
where w is the one-sided monitored width <strong>of</strong> the transect.<br />
Resulting phasmid densities (ind.*m -2 ) for transects were averaged on habitat level per monitor date (28<br />
monitoring dates and three transects per habitat). To allow for comparison I extrapolated densities to the<br />
base <strong>of</strong> number <strong>of</strong> individuals per hectare searching area (ind.*ha -1 ).<br />
Under the assumption that <strong>phasmids</strong> would show a pronounced seasonal pattern in densities, I expected<br />
higher densities <strong>of</strong> <strong>phasmids</strong> (and particularly <strong>of</strong> nymphs) in the early rainy season. Seasonal variation<br />
in abundances for nymphs and adults was compared on the base <strong>of</strong> cumulative absolute abundances on a<br />
monthly scale. As data for the forest understory were few, I only analyzed abundances in forest edges.<br />
Data for November 2000 and July 2001 were excluded from the analysis, because only one monitor date