BIOS 6150: Ecology - Western Michigan...

BIOS 6150: Ecology - Dr. S. Malcolm. Week 2: Birth, Death: Varley, Gradwell & Hassell, 1973. Slide - 1

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.


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.


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


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


5. The winter moth, Operophtera brumata (L.):

Winged male O. brumata

All images (except Fig. 7.2) http://www.inra.fr/Internet/Produits/HYPPZ/

RAVAGEUR/6opebru.htm Wingless female O. brumata on cherry blossom bud

O. brumata larva on apple leaf

Fig. 7.2


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.


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.


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.


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.

•  Predators of pupae •  e.g. mice, shrews and beetles (Fig. 7.5).

Figure 7.5. Common predators of winter moth pupae. Feronia is a carabid beetle and Philonthus is a staphylinid beetle.


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).


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.


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.


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 anti-clockwise 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.


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.


Figure 7.1: Census routine for the winter moth and its parasites.


Table 7.1:


Table 7.2:


Figure 7.6: Values for k5 against time joined in a time sequence show delayed density dependence.


Figure 7.7: Densities of winter moth larvae and their natural enemies.


Figure 7.8: Observed data and model predictions for winter moth and parasite densities.


Figure 7.9: Consequences of variation in model parameters used to generate predictions of Figure 7.8.

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