ecology of phasmids - KLUEDO - Universität Kaiserslautern
ecology of phasmids - KLUEDO - Universität Kaiserslautern
ecology of phasmids - KLUEDO - Universität Kaiserslautern
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Introduction 3<br />
agents in maintaining forest dynamics and ecosystem stability (Lowman 1984; Brokaw 1985;<br />
Schowalter et al. 1985; Brown et al. 1987). Their relevance as herbivores in tropical forests was<br />
confirmed in studies on the abundant endemic Puerto Rican species Lamponius portoricensis REHN.<br />
Population density <strong>of</strong> this polyphagous species seems to temporally vary to an enormous extent (Willig<br />
et al. 1986, 1993; Willig & Camilo 1991). Yet, besides drastic reductions in population densities<br />
following natural disturbance <strong>of</strong> high intensity (Willig & Camilo 1991) the causal factors <strong>of</strong> these<br />
fluctuations largely remain unknown. Host plant densities partly explained distribution patterns and<br />
patch densities <strong>of</strong> L. portoricensis (Willig et al. 1993), and individuals displayed differential preference<br />
among hosts depending on age, sex and prior experience (Sandlin & Willig 1993). This preference may<br />
be the result <strong>of</strong> specific nutritional constraints but could also reflect selection by predation pressure:<br />
Sandoval (1994) showed that color morphs <strong>of</strong> an endemic Californian species suffered differential<br />
predation on varying host plants thereby propagating specialization and speciation.<br />
A variety <strong>of</strong> behavioral and morphological adaptations underline the importance <strong>of</strong> predation-related<br />
selection on phasmid evolution. Most <strong>phasmids</strong> species exhibit a repertory <strong>of</strong> predation-avoidance<br />
behaviors (Robinson 1968a, 1968b, 1969; Bedford 1978). They live cryptic, resembling sticks and<br />
leaves, and the majority <strong>of</strong> species is active at nights (Bedford 1978). Most phasmid species dispose <strong>of</strong><br />
glands (Bedford 1978) whose secretions were shown to deter predators (Eisner 1965; Eisner et al.<br />
1997). Many phasmid eggs attract ants with a protein rich cap (Capitulum) and may thereby be<br />
protected from parasitoid wasps (Compton & Ware 1991; Hughes & Westoby 1992; Windsor et al.<br />
1996).<br />
Information on phasmid species <strong>of</strong> Panamá is restricted to the early taxonomic work <strong>of</strong> Hebard (1923,<br />
1929, 1933) and the studies about the defensive behavior <strong>of</strong> several <strong>phasmids</strong> by Robinson (1968a,<br />
1968b, 1969). Panamanian <strong>phasmids</strong> have been observed feeding on plant species <strong>of</strong> the family Araceae<br />
(Robinson 1969). And <strong>phasmids</strong> are reported to feed on Piper spp. (Piperaceae) in Costa Rica (Marquis<br />
1991) and Puerto Rico (Willig et al. 1986, 1993; Sandlin & Willig 1993).<br />
1.2.1 Design and aims <strong>of</strong> the study<br />
In this dissertation, I take an approach to contribute to the understanding <strong>of</strong> the ecological processes<br />
affecting distribution and density <strong>of</strong> tropical insect herbivores. Starting from descriptive information on<br />
the phasmid community on Barro Colorado Island (BCI) (Chapter 2), the study then focuses on<br />
population parameters <strong>of</strong> the phasmid Metriophasma diocles WESTWOOD (Chapter 2&3) and on<br />
experiments on potential control factors related to the bottom-up (Chapter 4) and top-down (Chapter 5)<br />
views <strong>of</strong> population regulation (Figure 1-1).