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 32<br />
because restrictions in the biotic and abiotic setting <strong>of</strong> an organism’s environment almost always affect<br />
fecundity and survivorship (e.g., Joern & Gaines 1990, and references therein). In practice, an<br />
approximation <strong>of</strong> the biotic potential can be obtained from laboratory populations where most <strong>of</strong> these<br />
restrictions can be cut <strong>of</strong>f or minimized (e.g., populations under laboratory conditions are not<br />
necessarily competition-free but experience the least competition, cf. Begon et al. 1996).<br />
Data on demographic population parameters and life history traits assessed from laboratory populations<br />
can help in understanding an organism’s realized density in its natural setting and provide the basis for<br />
estimating potential population growth. Under the assumption that density estimates obtained from field<br />
records are derived from a stable population oscillating below K (not being in colonization or in<br />
extinction), the difference between potential and realized population densities then reflects the impact <strong>of</strong><br />
control.<br />
Generally, <strong>phasmids</strong> on BCI occurred in low abundances in both the understory and at forest edges as I<br />
showed in Chapter 2. Although being rather common in comparison to the majority <strong>of</strong> phasmid species,<br />
M. diocles density was low in both habitats. Most other phasmid species are rare in tropical forests<br />
(Bedford 1978, Willig & Camilo 1991, Novotny & Basset 2000) while they occasionally reach pest<br />
status in temperate ecosystems (e.g., Campbell 1961, 1974).<br />
Low densities <strong>of</strong> a species may at least partially result from a low biotic potential, whereas low<br />
density opposed to high biotic potential is indicative <strong>of</strong> high levels <strong>of</strong> control. Among phasmid<br />
species biotic potentials seem to be highly variable as indicated by great differences in<br />
generation times and fecundity (available data do not include these parameters but report<br />
developmental times <strong>of</strong> eggs and nymphs as well as maximum adult lifetimes and maximum<br />
egg production; cf. Bedford 1978).<br />
In this chapter, I present data on demographic population parameters and life history traits from a<br />
laboratory population <strong>of</strong> Metriophasma diocles. In particular, I assessed mean values for (1) individual<br />
fecundity (birthrate), sex ratio and generation time. Based on these parameters (2) I developed a model<br />
for the potential population growth <strong>of</strong> M. diocles (i.e., the biotic potential). This model established the<br />
base for conclusions on the impact <strong>of</strong> population control factors, namely the effects <strong>of</strong> (3) egg mortality<br />
(included here), food quality (Chapter 4) and predation (Chapter 5).<br />
Sex ratio and birth rate represent principle demographic population parameters. The sex ratio provides<br />
information on the proportion <strong>of</strong> females in following generations and approximates equality in most<br />
insects (Wrensch & Ebbert 1993). The birthrate describes the number <strong>of</strong> <strong>of</strong>fspring produced per unit<br />
time, also referred to as mean individual fecundity (Begon et al. 1996). Individual fecundity <strong>of</strong> insects is<br />
known to be influenced by female body size (or weight) with smaller females laying fewer eggs (e.g.,<br />
Price 1984, Wiklund et al. 2001, Zanuncio et al. 2002). In the natural setting <strong>of</strong> an organism, female<br />
body size or weight may reflect impact from different factors, such as food quality and availability,