ecology of phasmids - KLUEDO - Universität Kaiserslautern

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