ecology of phasmids - KLUEDO - Universität Kaiserslautern
ecology of phasmids - KLUEDO - Universität Kaiserslautern
ecology of phasmids - KLUEDO - Universität Kaiserslautern
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Predation mediated mortality & migratory behavior <strong>of</strong> nymphs 81<br />
mesh). Mortality comparisons <strong>of</strong> nymphs in screened greenhouses and in exclosures can also<br />
help to uncover microclimatic influences by exclosure design.<br />
I installed plots <strong>of</strong> 1 m 2 size. An individual <strong>of</strong> P. marginatum was planted in the center <strong>of</strong> a plot and<br />
eight individuals at the outer borders. Nymphs were prevented from leaving a plot by an adhesive<br />
barrier (Tanglefoot – Insect Trap Coating, The Tanglefoot Co., USA). A single nymph was then set on<br />
the central plant and checked twice daily. Residence, migration and remigration were checked<br />
accordingly. I used the same criteria for nymphs and host plants as described above. In total, I followed<br />
53 nymphs in this experiment. Occasionally, workers <strong>of</strong> Ectatomma ruidum ROGER, a ponerine ant,<br />
passed the adhesive barrier and entered the plots. As these were rare events, predation events can largely<br />
be ruled out.<br />
5.2.4 Data analysis<br />
Survival times were analyzed by the Kaplan-Meier estimate (also known as the product-limit estimate;<br />
Kaplan & Meier 1958). The survival function gives the probability that an individual will survive past a<br />
given time. Differences in residence times were analyzed by the Mantel-Cox log-rank test (Mantel<br />
1966; Cox 1972). Under the assumption that deaths or disappearances <strong>of</strong> nymphs were independent, the<br />
Mantel-Cox test proved suitable for the dataset as it gives equal weight to all events.<br />
Migration distances were calculated as mean migration distance per nymph because some<br />
nymphs moved multiple times.<br />
Plant size was expressed as total number <strong>of</strong> healthy leaves per host plant after death or disappearance <strong>of</strong><br />
a nymph or at day 14 in case the nymph was still present. As data were not normally distributed, I<br />
compared median plant size via Mann-Whitney U-tests.<br />
5.3 Results<br />
5.3.1 Predation-mediated patterns in M. diocles nymph mortality<br />
Survival times <strong>of</strong> M. diocles nymphs differed significantly between exclosures and controls (Figure 5-<br />
1). In total, nymph survival was three times higher on caged plants than on exposed plants (81% vs<br />
27%; Table 5-1). In the exclosures, survival times varied neither spatially among the three forest sites,<br />
nor temporally throughout the study period. In contrast, spatio-temporal variation increased in the<br />
controls where nymphs were exposed to natural levels <strong>of</strong> predation. Most notably, survival times<br />
differed significantly among runs in the controls (Mantel-Cox Test; X 2 = 23.62, df = 3, P < 0.0001).<br />
In controls, significantly more nymphs disappeared at night as compared to the daytime (Figure 5-2). In<br />
contrast, there was no diurnal variation in mortality in the exclosures, where predation was absent<br />
(Mantel-Cox Test; X 2 = 1.96, df = 1, P < 0.16).