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 74<br />
importance <strong>of</strong> N to reproduction and showed that insects can perceive differences in N-contents. Adult<br />
females may modulate their reproductive output depending on N-content <strong>of</strong> their foods as they decrease<br />
egg production with decreasing N-levels and vice versa (McCaffery 1975). Similarly, host selection (i.e.<br />
preference) and performance <strong>of</strong> M. diocles is likely to depend on nutritious quality, the more as I<br />
showed that defensive leaf traits may be <strong>of</strong> minor importance for this herbivore. I therefore suggest that<br />
analysis <strong>of</strong> leaf nutritional quality should be subject <strong>of</strong> further research on M. diocles diets.<br />
4.5.2 Ecological and evolutionary consequences<br />
The described differential performance <strong>of</strong> M. diocles nymphs and the discriminative preference <strong>of</strong> adults<br />
are <strong>of</strong> significance in several aspects <strong>of</strong> the biology and <strong>ecology</strong> <strong>of</strong> this herbivore.<br />
Slow growth may involve a higher susceptibility to natural enemy attack (Feeny 1976). This slow-<br />
growth high-mortality hypothesis (Feeny 1976; Clancy & Price 1987) suggests that slow growing<br />
herbivores suffer greater mortality from natural enemies, because they remain longer in the more<br />
vulnerable early life stages <strong>of</strong> development than faster growing herbivores. Häggström & Larsson<br />
(1995) provided one <strong>of</strong> the few empirical tests for this hypothesis. They showed that predation on larva<br />
<strong>of</strong> the leaf beetle Galerucella lineola was positively related with longer development that depended on<br />
food quality. Accordingly, in their natural setting differential growth <strong>of</strong> M. diocles nymphs may<br />
translate into differential predation-related mortality. Thereby predation may outbalance high host plant<br />
related mortality <strong>of</strong> fast growing nymphs (e.g,. Piper peltatum) with high host plant related survival <strong>of</strong><br />
slow growing nymphs (e.g,. Phil. inaequilaterum). As a consequence, selection should favor some<br />
intermediate host, i.e. with comparably high growth rate and low host plant related mortality. In the case<br />
<strong>of</strong> M. diocles, the gap associated Piper dilatatum and P. marginatum represent such hosts. Over<br />
evolutionary time one would predict that low suitable plants such as P. grande will be abandoned as<br />
hosts. Together with the fact that M. diocles densities were higher in gaps than in the understory<br />
(Chapter 2) the outlined evolutionary scenario may at least partly explain the distribution <strong>of</strong> this<br />
herbivore.<br />
Still, nymph mortality in a six week period was 50 % or higher for all species but Phil. inaequilaterum<br />
(cf. Table 4-3). This time period reflects less than half the time <strong>of</strong> phasmid nymph development (approx.<br />
100 days, cf. Chapter 3). Such strong constraints in tissue quality (regardless whether related to<br />
defensive or nutritional quality) may motivate an herbivore to leave its host and search for more<br />
adequate food. Leaving the host, the herbivore runs risk not to find a new suitable host. To cite an<br />
example, van Dam et al. (2000) showed that very young caterpillars <strong>of</strong>ten have trouble to find even<br />
neighbouring plants. In addition, when leaving its host the herbivore exposes itself to predation<br />
particularly by arthropods (Haccou & Hemerik 1985). Both risks probably decrease with increasing size<br />
<strong>of</strong> the herbivore, because its action range increases and thereby its searching efficiency (Damman 1991),<br />
while its susceptibility to predators may decrease (Dyer 1995). In addition, the herbivore is deprived <strong>of</strong><br />
food while searching (Schultz 1983). Again all these factors favour selection pressure towards hosts