ecology of phasmids - KLUEDO - Universität Kaiserslautern
ecology of phasmids - KLUEDO - Universität Kaiserslautern
ecology of phasmids - KLUEDO - Universität Kaiserslautern
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Life history & potential population growth 43<br />
Some minor restrictions <strong>of</strong> this study need comment. In general, the presented results are based on<br />
extensive data collection, and hence are considered safe from negative influence from lab conditions.<br />
Fecundity and adult lifetime <strong>of</strong> <strong>phasmids</strong> can be negatively affected by crowding (stress and epidemic<br />
diseases) and by poor quality <strong>of</strong> food plants (cf. Joern & Gaines 1990). I compensated for low quality<br />
food by providing animals with a range <strong>of</strong> fresh food plant species planted in pots. Diseases most likely<br />
played a role in the lab population because two major die-<strong>of</strong>fs <strong>of</strong> nymphs were observed during the<br />
study period. As stated in the introduction, negative density effects due to intraspecific competition<br />
cannot be avoided and epidemic diseases as competition-free situations are almost non-existent (Begon<br />
et al. 1996). Still, if these factors influenced the presented results, then this would weaken only my<br />
conclusion regarding the role <strong>of</strong> the ‘low’ biotic potential as one factor partially explaining low<br />
M. diocles density.<br />
3.4.4 Conclusions<br />
Clearly the presented model <strong>of</strong> population growth represents a synthetic situation while natural<br />
populations are exposed to an interplay between biotic and abiotic factors leading to fluctuating<br />
population densities as suggested for grasshoppers (Berryman et al. 1987, Joern & Gaines 1990). The<br />
comparably low biotic potential and the reduction <strong>of</strong> population growth by failed hatching <strong>of</strong> eggs are<br />
the first steps leading towards the understanding <strong>of</strong> why natural M. diocles population densities are low.<br />
Clearly, this approach lacks any explanation for population fluctuations. Reproductive output <strong>of</strong> females<br />
and hatching success vary depending on factors like the females physiological state, pathogens and egg<br />
predators, humidity and temperature (e.g., Bedford 1978; Chapman & Joern 1990; Bernays & Chapman<br />
1994). Nevertheless, I demonstrated that M. diocles, despite its comparably low reproductive output, has<br />
the potential to reach much higher densities than found in its natural setting (cf. Chapter 1).