ecology of phasmids - KLUEDO - Universität Kaiserslautern

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