ecology of phasmids - KLUEDO - Universität Kaiserslautern

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