ecology of phasmids - KLUEDO - Universität Kaiserslautern

Life history & potential population growth 40
3.4 Discussion
This study is the first to model potential population growth of a tropical phasmid and to give a first
estimate of the impact of hatching failure of eggs on population increase. M. diocles disposed of a
comparably low biotic potential that may partially explain the low density of this species documented
on BCI (cf. Chapter 2). As expected, generations of this phasmid did not overlap and the according
model of potential discrete stepwise population growth resulted in markedly higher population size than
realized in the field. Hatching failure of eggs caused a marked reduction of population increase by 10 %
suggesting that infertility and fungal infestations of eggs may be important but not prominent factors in
the maintenance of low population density of M. diocles.
3.4.1 Demographic population parameters and life history traits
Presented data for M. diocles demographic and life history parameters largely corresponded with data
compiled by Bedford (1978), who provided a helpful and capacious source of information on phasmid
biology. Unless otherwise cited the following comparisons are based on Bedford (1978).
Sex ratio of the M. diocles lab population equaled 1:1 and is in accordance with sex ratio derived from
many phasmid (Bedford 1978) and insect species (Wrensch & Ebbert 1993). Female to male ratios of
sexually reproducing animals generally are expected to near equality when sons and daughters are
equally costly to produce (Fisher 1930). For phasmids, sex ratio is sometimes female-biased, indicating
that a species reproduces at least partially parthenogenetic. For instance, the sperm supply of a
spermatophore may not last the whole adult life of a female. Some phasmid species then lay unfertilized
eggs that develop into females. In contrast, M. diocles seemed to reproduce sexually resulting in a
balanced sex ratio (even though copula could never be observed).
As for all phasmid species, sexes in M. diocles differed in size and weight with females being bigger
and heavier than males. Sexual size dimorphism is widespread in animals with bigger females in most
invertebrate species (Andersson 1994; Fairbairn 1997). Female-biased dimorphism is often attributed to
a fecundity-advantage of large female size (Shine 1989). Larger female body size positively relates to
longevity and to energy resources for egg production, both favoring higher reproductive success
(Fairbairn 1997). Positive relationships of female body size to reproductive success of variable strengths
were demonstrated for several insect taxa (e.g., Fox et al. 1995a; Zanuncio et al. 2002) and proofed to
hold proved to be true for M. diocles on a modest level. Fox et al. (1995a) found a similar weak relation
of reproductive success to female weight in a seed beetle, but they provided evidence that larger females
more often mated with larger males than their smaller conspecifics. Thereby larger females received
more male-derived nutrients via extragametic substances of the spermatophores (that are bigger in
bigger males; Fox et al. 1995b) and increased their egg production. Similarly larger M. diocles females
may gain additional reproductive benefit via intrasexual competition for bigger sized males.