82
Phenological Patterns
Table 6.1. Life history traits of collembolans and oribatids displaying left- and right-skewed
asymmetric as well as symmetric phenologies. (Stamou et al. 1993a)
Left-skewed phenology
Precocity
Iteroparity (circumscribed
oviposition)
High rate of juvenile
developmet into subsequent
stadia during the favourable
period
Developmet of juveniles into
subsequent stadia during the
unfavourable period
Small number of eggs
overcoming adversity
More evenly distributed
reproductive effort during
adulthood
Right-skewed phenology
Precocity
Iteroparity (circumscribed
oviposition)
High rate of juvenile
developmet into subsequent
stadia during the favourable
period
No developmet of juveniles
into subsequent stadia during
the unfavourable period
Large number of eggs
overcoming adversity
Less evenly distributed
reproductive effort during
adulthood
Left-convex adult survivorship Right-convex or linear adult
curve
survivorship curve
Symmetrical phenology
Precocity
Iteroparity (either circumscribed or uncircumscribed
oviposition)
High rate of juvenile
developmet into subsequent
stadia during the favourable
period
Development of juveniles into
subsequent stadia during the
unfavourable period
Either small or large number
of eggs overcoming adversity
Reproductive effort evenly
distributed during adulthood
or confined to short periods
Right-convex or S-type adult
survivorship curve
According to Stamou et al. (1993a), on account of the capacity of most
immature oribatids to develop even during adverse periods, the cohort that
overcomes a time of drought is age structured. By contrast, collembolan
cohorts overcoming adversity consist either of aestivating eggs which were
deposited at the beginning of drought or of adults in anhydrobiosis. As a
result, the number of oribatids can recover rapidly after summer drought,
and right-skewed phenologies are displayed. In contrast, collembolans exhibit left-skewed phenologies and density peaks occur later in the year. In oribatids, the energy-consuming capacity to withstand summer drought is distributed over the various the life stages and is compensated for by relatively
low mortality rates. The reverse holds for Collembola.
The life history tactics of species displaying skewed phenologies are shown
in Table 6.1. The capacity of most oribatid populations to recover rapidly
during autumn results from the hatching of eggs which were deposited in
large numbers before the beginning of the adverse period and from the rapid
development of immature individuals which attain adulthood within short
periods of time. In turn, the newly emerged adults produce and deposit eggs
rapidly. The smooth decline in population size corresponds with a lower
reproductive value among subsequent adult stages which is coupled with a
left-convex or linear survivorship curve for these adults.
Condensed ecological time coincides with decreasing population size in
most collembolan species, and phenologies are right-skewed. These species
are adapted to respond rapidly to adversity. The smooth increase in popula-
Phenological Patterns
Table 6.1. Life history traits of collembolans and oribatids displaying left- and right-skewed
asymmetric as well as symmetric phenologies. (Stamou et al. 1993a)
Left-skewed phenology
Precocity
Iteroparity (circumscribed
oviposition)
High rate of juvenile
developmet into subsequent
stadia during the favourable
period
Developmet of juveniles into
subsequent stadia during the
unfavourable period
Small number of eggs
overcoming adversity
More evenly distributed
reproductive effort during
adulthood
Right-skewed phenology
Precocity
Iteroparity (circumscribed
oviposition)
High rate of juvenile
developmet into subsequent
stadia during the favourable
period
No developmet of juveniles
into subsequent stadia during
the unfavourable period
Large number of eggs
overcoming adversity
Less evenly distributed
reproductive effort during
adulthood
Left-convex adult survivorship Right-convex or linear adult
curve
survivorship curve
Symmetrical phenology
Precocity
Iteroparity (either circumscribed or uncircumscribed
oviposition)
High rate of juvenile
developmet into subsequent
stadia during the favourable
period
Development of juveniles into
subsequent stadia during the
unfavourable period
Either small or large number
of eggs overcoming adversity
Reproductive effort evenly
distributed during adulthood
or confined to short periods
Right-convex or S-type adult
survivorship curve
According to Stamou et al. (1993a), on account of the capacity of most
immature oribatids to develop even during adverse periods, the cohort that
overcomes a time of drought is age structured. By contrast, collembolan
cohorts overcoming adversity consist either of aestivating eggs which were
deposited at the beginning of drought or of adults in anhydrobiosis. As a
result, the number of oribatids can recover rapidly after summer drought,
and right-skewed phenologies are displayed. In contrast, collembolans exhibit left-skewed phenologies and density peaks occur later in the year. In oribatids, the energy-consuming capacity to withstand summer drought is distributed over the various the life stages and is compensated for by relatively
low mortality rates. The reverse holds for Collembola.
The life history tactics of species displaying skewed phenologies are shown
in Table 6.1. The capacity of most oribatid populations to recover rapidly
during autumn results from the hatching of eggs which were deposited in
large numbers before the beginning of the adverse period and from the rapid
development of immature individuals which attain adulthood within short
periods of time. In turn, the newly emerged adults produce and deposit eggs
rapidly. The smooth decline in population size corresponds with a lower
reproductive value among subsequent adult stages which is coupled with a
left-convex or linear survivorship curve for these adults.
Condensed ecological time coincides with decreasing population size in
most collembolan species, and phenologies are right-skewed. These species
are adapted to respond rapidly to adversity. The smooth increase in popula-