BIOS 6150: Ecology
Dr. Stephen Malcolm, Department of Biological Sciences
•  Week 2: Birth and Death:
•  Varley, G.C., G.R. Gradwell & M.P. Hassell. 1973.
Insect Population Ecology: An analytical approach.
Oxford: Blackwell (chapter 7).
•  The winter moth, Operophtera brumata, is a common insect
herbivore of oak trees in western Europe.
•  O. brumata is also a serious pest of fruit trees, such as apple
and cherry.
•  It was introduced into North America and occurs throughout
Europe and Asia as far east as China.
•  Chapter 7 describes:
•  Analysis of winter moth life tables.
•  Interpretation of population census data.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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2. Researchers:
•  George Varley:
•  Hope Professor Zoology (Entomology) at Oxford
University until his retirement in 1981 (deceased).
•  George Gradwell:
•  Forest entomologist/statistician in the Forestry
Department at Oxford (deceased).
•  Mike Hassell:
•  George Varley’s doctoral student. He worked on
tachinid fly parasitism of winter moth larvae and the
dynamics of host-parasitoid interactions. Formerly head
of Silwood Park, Imperial College, University of London.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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3. Location of research:
•  The research was
conducted in Wytham
Woods near Oxford, in
southern England.
•  20 individuals of the
common oak tree,
Quercus robur.
•  Census data collected
from 1950 to the
1970s.
Wytham Wood in summer - coppiced trees
Http://nolimits.nmw.ac.uk/IEN/Wytham.html
Wytham Wood on 2 January 2001
Http://www.ecn.ac.uk/photos/Wytham_photo.htm
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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4. Wytham Wood and “The Woodman’s Daughter”:
•
Sir John Everett Millais painted the background
to this picture in Wytham Wood in 1850 to
illustrate the poem by his friend Coventry
Patmore:
•  “She went merely to think she helped;
And, whilst he hack’d and saw’d,
The rich squire’s son, a young boy then,
For whole days, as if awed,
Stood by, and gazed alternately
At Gerald, and at Maud.
He sometimes, in a sullen tone,
Would offer fruits, and she
Always received his gifts with an air
So unreserved and free,
That half-feigned distance soon became
Familiarity.”
Http://www.corpoflondon.gov.uk/gag_july/
victorian_woodman.htm
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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5. The winter moth, Operophtera brumata (L.):
Fig. 7.2
Winged male O. brumata
All images (except Fig. 7.2) http://
www.inra.fr/Internet/Produits/HYPPZ/
RAVAGEUR/6opebru.htm
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
O. brumata larva
on apple leaf
Wingless female
O. brumata on cherry
blossom bud
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6. Life history of the winter moth:
•  November/December:
•  Adult moths emerge from the soil under oak
trees in November and December (early winter)
and flightless females climb up trees.
•  Winged males fly at dusk and at night and mate with
females on oak tree trunks.
•  Females lay eggs on bark.
•  Data collection (sampling): females were collected in
traps and used to estimate moth density as N/m2 of
tree canopy (Fig. 7.1 & Table 7.1).
•  Total adults = 2N, because male N = female N.
•  Average egg N/female = 150.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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7. Life history of the winter moth:
•  April to November:
•  At bud burst in April, eggs hatch and new first
instar larvae feed on emerging buds.
•  End of May, larval development is complete and
larvae drop on silk threads to ground where they
pupate.
•  N sampled with trays (Fig. 7.1 & Table 7.1).
•  Surviving, overwintered pupae emerge as adults
in November/December.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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8. Winter moth life table
•  Collected data are highlighted in Table 7.2:
•  Remaining numbers are calculated.
•  Females were estimated at 5.85/m2 of which
25% were trapped to leave 4.39/m2.
•  k-values for mortality between each life history
stage are calculated in Table 7.2:
•  Based on the numbers observed or calculated from
the highlighted field data.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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9. Natural enemies include:
•  Cyzenis albicans
•  a tachinid fly parasite of
larvae
•  Cratichneumon culex
•  an ichneumonid wasp
parasite of pupae
•  Plistophora operophterae
•  a microsporidian protozoan
parasite of larvae
•  like malaria, in larval salivary
glands.
Figure 7.5. Common predators of winter
moth pupae. Feronia is a carabid beetle
and Philonthus is a staphylinid beetle.
•  Predators of pupae
•  e.g. mice, shrews and beetles (Fig. 7.5).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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10. Changes with time:
•  Collection of
successive life table
data allowed the
researchers to
examine changes in
mortality from year to
year (Fig. 7.3).
Figure 7.3: Winter moth population
change with time as, (A) generation
curves, (B) mortality (k-values).
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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11. Population regulation
and density dependence:
•  Plotting the k-values for
different winter moth
mortalities against the
logarithm of population
densities on which the
mortality factor acted
provides an objective
criterion for density
dependent or density
independent effects (Fig.
7.4).
Figure 7.4: k-values for
winter mortalities plotted
against population densities.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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12. Key mortality factors:
•
•
•
•
k5 is strongly density dependent.
k3 is weakly inversely density dependent.
k1 and k6 are density independent and variable.
k2 and k4 are also density independent but
constant.
•  The regression method is useful but has a statistical
problem in that the k-values are not independent of
LogN – the “independent” variable.
•  LogN should also be measured without error:
•  But sampling errors are expected.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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13. Delayed density dependence:
•  If the density dependent mortalities for k5 are
replotted as in Fig. 7.6,
•  And the data points can be joined by a line in
chronological sequence to generate an anticlockwise spiral;
•  Then this indicates delayed density dependence.
•  Data such as those in Fig. 7.7 might also show
cycle lags between moth prey and various
predators.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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14. Population model:
•  The data collected for the winter moth were used to
build a descriptive model of the major mortality
factors and their intensity.
•  Power of this model is indicated in Fig. 7.8 and shows
that the field observations could be used to build an
accurate description of population changes in the winter
moth and two of its parasitoids.
•  Stability characteristics of the descriptive model are also
examined in Figure 7.9 for a range of intensities of different
mortality factors:
•  This provides parameters for use in the model as the intensity of
mortality changes with density.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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Figure 7.1: Census routine for the
winter moth and its parasites.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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Table 7.1:
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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Table 7.2:
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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Figure 7.6: Values for k5 against time joined in a
time sequence show delayed density dependence.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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Figure 7.7: Densities of winter moth
larvae and their natural enemies.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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Figure 7.8: Observed data and model predictions
for winter moth and parasite densities.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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Figure 7.9: Consequences of variation in model
parameters used to generate predictions of Figure 7.8.
BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973.
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