BIOS 3010: Ecology
Laboratory 3: Life Tables
The winter
moth, Operophtera brumata
Varley et al. (1973)
In todayÕs lab we will be using data collected on
winter moths, Operophtera brumata,
by George Gradwell and George Varley in Wytham Woods near Oxford in
England. This is a classical
example of how life tables can be used to describe and understand population
fluctuations over time. We will
imagine that we are pest managers acting for a local government and must
implement a strategy to control winter moths according to the evidence found in
our life tables. The biology of
this system is very like that of the gypsy moth, Lymantria dispar, here in Michigan and the north eastern United
States.
From life tables for winter moths, we can identify
the main causes of population change from year to year. Density dependent mortality serves to
regulate the population density and keeps it within limits.
Density dependence: ÒA change in the influence of an environmental factor (a
density dependent factor) that affects population growth as population density
changes, tending to retard population growth (by increasing mortality or
decreasing fecundity) as density increases or to enhance population growth (by
decreasing mortality or increasing fecundity) as density decreasesÓ (Lincoln et
al. 1992).
Density dependent mortality may either act directly,
as in food limitation, or indirectly, through behavioral responses of parasites
and predators to their own and to their hostÕs population densities.
The winter moth is an easy insect to study because it
is very abundant and has an annual life-cycle with each stage concentrated at different times of the
year. The same could be said for
animals with restricted breeding seasons (e.g., partridge, owls, grouse, fish). Because the winter moth has an annual
life-cycle, our life table is called a cohort life table. A cohort life table considers a group of individuals that are born
within the same short interval of time and follows each individual until death.
Life
Table Variables
ax the total number of individuals observed in
the population at each stage (a0 individuals in the initial stage, a1 individuals in the following one, etc.)
lx the proportion of the original cohort
surviving to the start of each stage (age specific survivorship)
dx the proportion of the original cohort dying
during each stage (difference between lx and lx+1) {can be summed}
qx the average probability of an individual dying
in that stage (age-specific mortality) {cannot be summed}
kx Òkilling-powerÓ, reflects the intensity of
mortality and is equivalent to: log10ax - log10ax+1 {can be summed} Also, these values are standardized and can be used to
compare separate studies.
Fx the total number of offspring produced at that
stage
mx individual fecundity or birth rate, i.e., the mean number of eggs produced per surviving
individual
Ro net
reproductive rate, in an annual species it is the overall extent by which
the population has increased (Ro>1) or decreased (Ro<1) over that time, = ·Fx/a0, also = ·lxmx
Tc the cohort generation time, = ·xlxmx/·lx
mx
r the intrinsic rate of natural increase, = lnRo/Tc, the
change in population size per individual per unit time. Populations increase in size for r > 0 and decrease for r < 0.
Life
History of Winter Moths
Larvae of the winter moth are able to feed on a wide
range of trees and shrubs, but they are especially abundant on oaks (Quercus
robur), which they sometimes
defoliate. Winter moth adults
emerge from the soil under the oak trees in November and December. At dusk, the flightless females walk to
the trees and climb up them.
Varley et al. (1973)
The winged males rest by day and fly actively at dusk
or during the night and congregate on the lower part of tree trunks. Here they mate with females (Fig. 7.1
from Varley et al., 1973) which
continue to climb the trees to lay eggs in crevices in bark and lichen high
above the ground. When the oak
buds begin to open in early April, the eggs hatch and the first stage
caterpillars feed on the expanding buds, where they do great damage to the tiny
leaves. By the latter half of May,
feeding is completed and the caterpillars spin down from the trees on silk
threads, burrow into the soil, spin cocoons and pupate. They will reappear again in
November. Larvae of the winter
moths were parasitized by a tachinid fly (Cyzenis) and by a Microsporidian protozoan. Larvae are also prey for numerous bird
species. Pupae are attacked by
wasps (Cratichneumon) and some
soil insects and predators such as mice and shrews. Adults also have birds as their predators.
Experimental
Method
1. Five
oak trees were sampled.
2. One
quarter of the females climbing each of the five trees were caught in traps
like small lobster-pots made of fabric supported with wire. Two traps were placed on opposite sides
of each tree, and each trap was arranged to obstruct 1/8 of the perimeter of
the tree. The total catch of
females multiplied by four and divided by the total canopy area of the five
trees (282 m2) gave the estimate of females.
3. The
numbers of adults per m2 was twice this amount because we knew there
were equal numbers of males to females in the pupae stage.
4. Females
were dissected for counts of the eggs they carried (avg. 150 eggs).
5. From
this count, we could estimate the number of eggs per tree.
6. Larvae
were trapped by placing trays filled with water under the trees and counting
the number trapped.
7. The
larvae were examined for external parasites and dissected for internal
parasites. From these figures, we
could assume the survival of the larvae.
8. Pupae
were trapped while they were emerging from the soil using two trays (each 0.5 m2)
on the ground.
9. Data
were compiled and placed into our life table.
Procedure
1. Calculate the remaining values on the winter moth life table
using a Microsoft Excel
spreadsheet.
2.
Draw Graphs
(using Excel) of:
(A)
the k-values versus time or stages,
(B)
survivorships (lx) versus time or stages, and
(C)
mortalities (qx) versus time or stages.
3.
Determine at which stage the population of winter moths
experiences the most pressure and determine which factors contribute.
4.
Graph a survivorship curve for the winter moths (as in
Begon et al. figure 4.12, plot ax against stage.
5.
List management strategies for the winter moth without
pesticides and minimal disturbance.