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 89<br />
found spiders, bugs (Reduviidae) and ants <strong>of</strong> the genus Ectatomma preying upon nymphs. The relevance<br />
<strong>of</strong> Ectatomma ants as predators <strong>of</strong> M. diocles is supported by high densities <strong>of</strong> these ants in the BCI<br />
forest (Levings & Franks 1982), and by the fact that E. ruidum workers preyed upon nymphs in lab<br />
experiments (data not shown). E. ruidum is mainly foraging in the litter and on the soil whereas<br />
E. tuberculatum searches different plant strata (Levings & Franks 1982). Ectatomma spp., like many<br />
other ant species show no expressed pattern in diurnal activity (Kaspari & Weiser 2000). In contrast,<br />
Pheidole radowskowskii FOREL (Formicidae) was shown to preferentially forage at night (Kaspari &<br />
Weis 2000) and to forage in high frequencies on leaves <strong>of</strong> Piper marginatum (Dyer et al. 1999).<br />
5.4.4 Conclusions<br />
M. diocles and many other <strong>phasmids</strong> occur in particularly low densities in humid tropical forests (cf.<br />
Chapter 2; Novotny & Basset 2000). Yet, the factors influencing population densities in <strong>phasmids</strong> have<br />
never been addressed in an empirical study. My findings support the top-down view <strong>of</strong> population<br />
regulation in M. diocles. While my data cannot uncover any impacts on population dynamics, for<br />
example because predation rates alone fall short to including density-dependence <strong>of</strong> predator-prey<br />
interactions (Sih et al. 1985), the high rates <strong>of</strong> predator-induced mortality I found may directly translate<br />
into a reduction <strong>of</strong> the intrinsic rate <strong>of</strong> population increase and hence explain the low abundances <strong>of</strong> this<br />
stick insect.<br />
Contrary to my predictions, emigration <strong>of</strong> nymphs was not related to top-down pressure. Hence, the<br />
function <strong>of</strong> observed behavior in one prey may not be assignable to another. In accordance with Witz<br />
(1990), I emphasize experimental confirmation before assuming a behavior as predator avoidance. My<br />
results suggest host plant size as an alternative factor influencing the movement patterns <strong>of</strong> herbivores.<br />
Host plant size may be directly related to suitability as food source. Herbivore survivorship and<br />
fecundity can then be affected directly by plant size related leaf traits reducing quality and indirectly by<br />
increasing migratory behavior thereby increasing exposure to predators.