ecology of phasmids - KLUEDO - Universität Kaiserslautern

Life history & potential population growth 42
mortality (r = 0.113*d -1 , Chi & Yang 2003), that exceeds maximum r of M. diocles almost tenfold
(0.0125*d -1 ; no mortality until mean generation time). Thus low abundances of M. diocles may at least
partially result from its low biotic potential.
Despite large differences in biotic potentials among species, most insect herbivores in the tropics are
rare (Basset 1996, 1999; Basset et al. 1992, 1996; Barone 1998; Novotny et al. 2002b). This is also true
for phasmids (Bedford 1978, Willig & Camilo 1991, Novotny & Basset 2000) and was shown for
M. diocles in Chapter 2. Regardless of the comparably low reproductive output, potential population
growth of M. diocles described by a model of discrete stepwise growth resulted in exponential
population growth. Already in the first generation, modeled potential population densities were higher
(applying T1 = 40 ind. on a ha base; cf. Figure 3-3) than realized in the forest edge or understory on BCI
(21.5 and 6.7 ind.*ha -1 respectively; cf. Chapter 2). In following generations modeled population sizes
by far exceeded M. diocles realized population densities (Figure 3-3). Consequently M. diocles cannot
reach its potential population size in its natural setting and population control factors must act
continuously to maintain its low realized density.
3.4.3 A first insight into M. diocles population control
Egg mortality (sensu hatching failure or fungal infestation) may be one factor reducing reproductive
output of M. diocles. Mortality of insect eggs can be of major importance in limiting insect populations
(e.g., Parker & Wakeland 1957, Prior & Greathead 1989, Caldas 1994, Dyssart 1995, Phoofolo et al.
2001). Hatching failure may occur due to infertility of eggs (Caldas 1994) or because of fungal or
bacterial infestation (Bedford 1978). However, the impact of hatching failure due to these factors has
reached little attention. Data from Caldas (1994) indicate that infertility of eggs may play a minor role
(0.5 % infertile eggs in a nymphalid butterfly). Here, I observed hatching failure of 10 % with 2 % of
M. diocles embryos being killed by fungi. Hatching failure initially lowered population growth
markedly (cf. Figure 3-3). Nevertheless, this may be of no impact after generation five when population
density had reached legion (only being limited by resource availability). On the other hand, egg
mortality may play an important role when population density is low as shown for M. diocles in
Chapter 2 and when a number of other factors increases mortality levels. For eggs, predators (Parker &
Wakeland 1957, Phoofolo et al. 2001) and parasitoids (Prior & Greathead 1989, Caldas 1994, Dyssart
1995) may play a much more prominent role than infertility or pathogens. For example, predators may
reduce lepidopteran egg densities by 50 % (Phoofolo et al. 2001) and parasitoids were shown to kill
11 % of grasshopper (Dyssart 1995) and up to 38 % of butterfly eggs (Caldas 1994). Likewise it is
known that wasps parasitize phasmid eggs (Chrysididae, Cleptidae ;Bedford 1978). Most likely
M. diocles eggs face a wider array of mortality agents in its natural setting and egg mortality may be
much higher than under lab conditions. Accordingly, egg mortality may be one important factor among
a variety of population control factors acting on later life stages that explain low population densities of
M. diocles.