ecology of phasmids - KLUEDO - Universität Kaiserslautern
ecology of phasmids - KLUEDO - Universität Kaiserslautern
ecology of phasmids - KLUEDO - Universität Kaiserslautern
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Adult female feeding preference & nymph performance 54<br />
attained in all cases. However, failure to meet the assumption <strong>of</strong> homogeneity <strong>of</strong> variances is not fatal to<br />
ANOVA. Analysis <strong>of</strong> Variance still remains robust, even when groups are <strong>of</strong> different sample sizes (Zar<br />
1999, p. 185 and references therein).<br />
Data on leaf toughness, specific leaf weight and total phenol content were log-transformed before<br />
statistical analysis. As water content and MAI were expressed on a relative scale data were arcsine<br />
transformed. Interspecific variation in leaf characters was then analyzed in ANOVA.<br />
4.3.1 Analysis <strong>of</strong> preference<br />
In relation to P. marginatum preference <strong>of</strong> M. diocles females for 12 Piper and two Philodendron<br />
species was tested. On the base <strong>of</strong> consumed leaf dry material I calculated a mean acceptability index<br />
(MAI) (Richardson & Whittaker 1982) from the ratios <strong>of</strong> consumed dry weight (dw) <strong>of</strong> test material to<br />
the total <strong>of</strong> consumed dry weight in each replicate:<br />
(Equation 4-1)<br />
[consumed dw] test<br />
MAI =<br />
[consumed dw] + [consumed dw]<br />
test<br />
reference<br />
The MAI allows to group plant species to their ranking as acceptable food source <strong>of</strong> a distinct phasmid<br />
species and ranges from cero to one. MAI-values below 0.5 and down to cero thereby indicate<br />
preference for P. marginatum whereas values above 0.5 represent preference towards the test plant<br />
species.<br />
Consumption ratios from feeding trials were not normally distributed. The distribution <strong>of</strong> MAI values<br />
was two-tailed as 0 and 1 ratios derived frequently resulting from females eating only one food source.<br />
Consequently differences in preference among food plant species were tested by nonparametric<br />
Kruskal-Wallis ANOVA. Preference is expressed as median MAI.<br />
The median <strong>of</strong> the MAI corresponded significantly with its mean (Product Moment Correlation r = 0.98,<br />
df = 12, P < 0.01) and median MAI values were normally distributed. Thus the relationships between<br />
each leaf trait and the established preference pattern were analyzed in parametric Product Moment<br />
Correlation Analysis.<br />
A test series <strong>of</strong> dual-choice feeding trials was conducted where nymphs were <strong>of</strong>fered leaf discs with<br />
artificially increased phenol and tannin contents. Resulting preferences, expressed as MAI, were<br />
compared in Mann-Whitney U-tests. The calculation <strong>of</strong> the MAI is described above.<br />
4.3.2 Analysis <strong>of</strong> performance<br />
Survival <strong>of</strong> nymphs was analyzed by the Kaplan-Meier estimate (also known as the product-limit<br />
estimate). The survival function gives the probability that an individual survives past a given time.<br />
Differences in survival times were analyzed by the Mantel-Cox log-rank test (Mantel 1966; Cox 1972).<br />
Under the assumption that deaths <strong>of</strong> nymphs were independent the Mantel-Cox test proved suitable for<br />
the dataset as it gives equal weight to all events.