ecology of phasmids - KLUEDO - Universität Kaiserslautern

Community structure & host range 27
However, in the presented study, seasonal differences in phasmid abundances may have been concealed
by high variation of overall abundances throughout the study period (cf. Figure 2-3). The magnitude of
variation could reflect deficiencies of the sampling method. For example, insect activity is often
restricted to particular times of day (e.g., Willig et al. 1993; Basset 2000) and phasmids, when resting,
are highly camouflaged (Bedford 1978; pers. obs.). Thus alternating sampling times may have resulted
in time-dependant sample size (e.g., heavy rains often did not allow to follow a strict monitor plan)
indicated by heterogeneity of phasmid adult abundances throughout the study period. In contrast,
sample time may affect the sighting of nymphs to a lower extent. Phasmid nymphs have small home
ranges and were shown to display patch fidelity (Willig et al. 1993). Consistent with these findings,
high occurrence of O. martini on two Vismia baccifera individuals was mainly based on nymphs (data
not shown).
2.4.3 Niche breadth and habitat choice
Phasmids on BCI were restricted in the use of their resources. Values for Levin’s standardized niche
breadth assessed from records of potential food plants along line-transects ranged from 0 to 0.4
(representing the realized niche). Niche breadths resulted both from a low number of potential host plant
families (Otocrania sp., I. dentipes) and from contagious distribution of phasmid individuals among
resources (all focal phasmid species). For example, 55 % of all O. martini and 91 % of all I. dentipes
specimens shared two plant individuals of Vismia baccifera. Such clumped distribution was shown
before to partially reflect habitat characteristics like plant composition (Willig et al. 1993) and may
result from preference for and/or availability of a resource (Bernays & Chapman 1994). Here
contagiousness reflected ‘true’ food resources: In feeding trials all the plant families turned out to be
edible, where phasmid individuals were aggregated on in he field. (Unfortunately I. dentipes could not
be established in lab populations. But field observations confirmed Vismia baccifera as food plant.) In
contrast, other potential host plant families, that were included in the realized niches were rejected in
feeding trials. Partial overlaps of realized niches, assessed from phasmid presence on plants in the field,
and absolute food niches, assessed in feeding trials (as depicted by Figure 2-4), underline that field data
represent multidimensional niches, including resources such as mating, oviposition, transience and the
realized food niche (Krebs 1989; Bernays & Chapman 1994).
When discussing the factors that may have led to the observed restriction in using food resources
evolutionary and ecological processes have to be considered. A first insight into host range evolution
can be gained by applying conventional definitions that refer to taxonomic relationships of host plant
range on the results of feeding trials (representing the absolute food niche). Utilizing this concept, the
user can conclude the most probable behavioral response (Bernays & Chapman 1994). To exemplify
with my data, Otocrania sp. that fed on two plant species out of the Sapindaceae (and hence is
monophagous) may use some characteristic of the plants they have in common in determining their