64
Life Cycle Tactics and Development
The above scheme fits well to Mediterranean arthropods living in habitats
with predictable seasonal and interannual fluctuations of environmental
variables. It is also evident that in Mediterranean regions climatic predictability is accompanied by hazardous anthropogenic disasters resulting in
spatial heterogeneity. Thus, unlike the circumstances of arthropods from
extreme environments where parthenogenesis is a key factor, obligatory or
facultative thelytocous parthenogenesis or even female dominance are of
minor importance in Mediterranean habitats. Likewise, temperate and Mediterranean environments hosting complex communities seem to favour sexual
reproduction.
The advantage of sexual over asexual reproduction lies in the re-combination of genetic material that may be more favourable for survival in spatially
heterogeneous habitats (e.g. SiepeI1994). In seasonally predictable, spatially
heterogeneous environments such as Mediterranean ones, greater genetic
variation may ensure establishment of offspring at particular micro sites.
Thus, sexual reproduction in Mediterranean regions can be viewed as a selfmaintaining adaptation to spatially uncertain habitats. As a matter of fact,
only three out of 26 oribatid species recorded in the evergreen-sclerophyllous
formation on Hortiatis can reproduce parthenogenetically. Moreover, when
studying reproductive tactics in oribatids along a huge Mediterranean gradient, Sgardelis (1995) recorded a sex ratio of approximately 1:1 in larger species, while female-biased ratios were recorded in some smaller ones. Most of
these species inhabit protected sites where a few parthenogenetic species
were also found.
A recorded bias in sex ratio towards females is frequently attributed to the
mass death of male animals soon after mating. For example, Warburg and
Cohen (1992) recorded a sex ratio approximating unity in the isopod Armadillo officinalis with a bias towards females. This was probably due to the
death of males after mating. Similarly, adult males of the oniscid Schizidium
tiberianum constituted less than 10% of total adults, while in juveniles the sex
ratio was 1:1 (Warburg 1994). According to Warburg et al. (1993), sex ratio is
a dynamic rather than constant characteristic and changes during the life
history of isopods.
At first glance, the lack of amphitocous species combining both arrhenotoky (which favours rapid selection) and apomictic thelytoky (favouring
rapid numerical recovery) appears surprising in ecosystems suffering from
destructive human impacts such as fire. Nevertheless, in ecosystems where a
modest recovery time results in progressive restoration of the initial ecosystem type, neither rapid numerical recovery nor rapid selection are advantageous. Consequently, amphitoky does not serve Mediterranean arthropods
well.
Life Cycle Tactics and Development
The above scheme fits well to Mediterranean arthropods living in habitats
with predictable seasonal and interannual fluctuations of environmental
variables. It is also evident that in Mediterranean regions climatic predictability is accompanied by hazardous anthropogenic disasters resulting in
spatial heterogeneity. Thus, unlike the circumstances of arthropods from
extreme environments where parthenogenesis is a key factor, obligatory or
facultative thelytocous parthenogenesis or even female dominance are of
minor importance in Mediterranean habitats. Likewise, temperate and Mediterranean environments hosting complex communities seem to favour sexual
reproduction.
The advantage of sexual over asexual reproduction lies in the re-combination of genetic material that may be more favourable for survival in spatially
heterogeneous habitats (e.g. SiepeI1994). In seasonally predictable, spatially
heterogeneous environments such as Mediterranean ones, greater genetic
variation may ensure establishment of offspring at particular micro sites.
Thus, sexual reproduction in Mediterranean regions can be viewed as a selfmaintaining adaptation to spatially uncertain habitats. As a matter of fact,
only three out of 26 oribatid species recorded in the evergreen-sclerophyllous
formation on Hortiatis can reproduce parthenogenetically. Moreover, when
studying reproductive tactics in oribatids along a huge Mediterranean gradient, Sgardelis (1995) recorded a sex ratio of approximately 1:1 in larger species, while female-biased ratios were recorded in some smaller ones. Most of
these species inhabit protected sites where a few parthenogenetic species
were also found.
A recorded bias in sex ratio towards females is frequently attributed to the
mass death of male animals soon after mating. For example, Warburg and
Cohen (1992) recorded a sex ratio approximating unity in the isopod Armadillo officinalis with a bias towards females. This was probably due to the
death of males after mating. Similarly, adult males of the oniscid Schizidium
tiberianum constituted less than 10% of total adults, while in juveniles the sex
ratio was 1:1 (Warburg 1994). According to Warburg et al. (1993), sex ratio is
a dynamic rather than constant characteristic and changes during the life
history of isopods.
At first glance, the lack of amphitocous species combining both arrhenotoky (which favours rapid selection) and apomictic thelytoky (favouring
rapid numerical recovery) appears surprising in ecosystems suffering from
destructive human impacts such as fire. Nevertheless, in ecosystems where a
modest recovery time results in progressive restoration of the initial ecosystem type, neither rapid numerical recovery nor rapid selection are advantageous. Consequently, amphitoky does not serve Mediterranean arthropods
well.