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 78<br />
migration should be displayed under accordingly high predation pressure (Snyder & Wise 2000, Venzon<br />
et al. 2000, Magalhães et al. 2002).<br />
However, whether an observed behavior does reflect predator avoidance <strong>of</strong>ten remains speculative as<br />
many studies assume but do not test an anti-predator function (Witz 1990). Predation avoidance may not<br />
be the ultimate motivation <strong>of</strong> small-scale migratory movement patterns <strong>of</strong> herbivores. Most ecologists<br />
now agree that both bottom-up and top-down forces directly or indirectly influence populations and<br />
communities <strong>of</strong> herbivores (e.g. Price et al. 1980, Hunter & Price 1992, Hunter et al. 1997, Walker &<br />
Jones 2001, but see Hassell et al. 1998, Hunter et al. 2000 and references therein). Resource limitations,<br />
such as the depletion <strong>of</strong> a food source or reduced digestibility <strong>of</strong> plant tissue, are bottom-up forces that<br />
influence herbivore performance and distribution (e.g. Häggstrom & Larsson 1995, Burghardt & Fiedler<br />
1996, Barker & Maczka 1996). Larval performance <strong>of</strong> insect herbivores can also be affected by plant<br />
size (Price 1991, Teder & Tammaru 2002) and according to Price (1991) herbivores should prefer<br />
bigger and more vigorous plants or plant modules. Depletion or low suitability <strong>of</strong> a host plant can then<br />
result in larvae moving more frequently between feeding sites (Bergelson & Lawton 1988, Loader &<br />
Damman 1991, but see Häggstrom & Larsson 1995). Another reason why herbivores move away from<br />
their host plants is that many herbivores perform best with a mixture <strong>of</strong> food plants, either to complete<br />
their diet or to dilute toxins (Bernays & Chapman 1994, Hägele & RoIll-Rahier 1999).<br />
When explaining herbivore migration by bottom-up forces one has to differentiate between generalists<br />
and specialists. Bottom-up forces, such as chemical plant defenses, should be more effective against<br />
generalist herbivores (Feeny 1976). Specialized herbivores (mono- or oligophagous) that overcome such<br />
plant defenses by detoxifying or even sequestering plant compounds should be less affected (Ehrlich &<br />
Raven 1964).<br />
Concluding from the above, I hypothesized that nymphs <strong>of</strong> an exophytic specialist herbivore should be<br />
particularly regulated by the third trophic level. If these nymphs show migration behavior, I expected<br />
predation avoidance to cause these movement patterns as bottom-up forces were expected to be <strong>of</strong><br />
minor importance for this specialized herbivore (see Chapter 4).<br />
The stick insect Metriophasma diocles (Phasmatodea) is an exophytic herbivore that occurs in low<br />
abundances in the understory <strong>of</strong> neotropical rainforest and feeds exclusively on plants from two<br />
different plant families, the Piperaceae and Araceae (see Chapter 2; Berger & Wirth 2001). As these<br />
families are not closely related, the degree <strong>of</strong> specialization <strong>of</strong> M. diocles is best described as disjunct<br />
oligophagous (see Bernays & Chapman 1994).<br />
I quantified the predation-related mortality <strong>of</strong> M. diocles nymphs <strong>of</strong> by (1) exposing nymphs to natural<br />
levels <strong>of</strong> predation on uncovered host plants (controls), and simultaneously (2) estimating the proportion<br />
<strong>of</strong> intrinsic mortality (not related to natural enemies) through the exclusion <strong>of</strong> predators by covering<br />
host plants with mesh cages (treatment). For the description <strong>of</strong> natural enemies and the temporal pattern