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