In the Wirral Peninsula, East Anglia and Michigan, the proportions of
light moths increased before any lichens had reappeared on the trees.
This is a serious problem for the theory that industrial melanism is due
to cryptic coloration and selective predation.
The conclusion of the claim is a non sequitur for at least two
reasons.
The dark moths were better camouflaged in industrial areas not
merely because the surfaces of the trees were lichen-free but also
because they had been darkened by soot and smoke. When pollution
control was introduced, the surfaces of the trees in heavily
polluted industrial areas soon became lighter (Bishop and Cook
1975, 92-93). Even though the black moths would still have
enjoyed a selective advantage over the light ones in these areas,
it would nevertheless have been smaller than it had been earlier,
so the proportion of black moths would be expected to
decrease, because their now smaller advantage would no longer be
sufficient to counterbalance the effects of migration of the light
moths from neighboring areas where they were more common.
Thus, even if the disappearance of lichens does
turn
out to have
been an important factor in the original spread of the dark moths,
there seems to be no good reason why their their reappearance
should have preceded the recovery in the numbers of the light
moths.
After pollution controls were introduced, lichens would be expected
to first recolonize those areas straddling the borders between the
countryside where they had remained abundant and industrial centers
where they were absent. The proportion of light moths would
therefore be expected to increase in these areas first, and then in
areas closer to the industrial centers a little later, as the
influence of migration began to take effect.
Thus, it is quite possible for the recovery of lichens in one area to
have been indirectly responsible for an increase in the proportion of
light moths in another area where lichens had remained absent. Indeed,
there is observational evidence to indicate that this may well have
been a contributing factor to the increase in the proportion of the
light moths in the Wirral during the late 1960s and early 1970s (see
below).
Claims that the proportion of light moths began to increase before
lichens recovered are based on anecdotal evidence which is either
largely irrelevant because it refers to only a single location (Clarke
et al. 1985, 191-193), or is so lacking in detail that its
significance is difficult to assess (Kettlewell 1973, 151; Clarke et
al. 1985, 193; Grant and Howlett 1988, 231; Grant et al. 1995, 1996,
355-356). Moreover, for the Wirral and East Anglia, there appears
to be direct observational evidence that contradicts it.
The first indication of an increase in the proportion of light peppered
moths in Britain were obtained by Clarke and Sheppard (1966) at Caldy
on the Wirral peninsula. The increase appears to have started between
1962 and 1963, some six or seven years after Britain had passed the
Clean Air Act of 1956.
Henderson-Sellers and Seaward (1979) recorded the recolonization of a
large urban area in West Yorkshire by the crustose lichen Lecanora
muralis over the period 1969 to 1977. They found that the
recolonization of a given location occurred about 5 years after the
atmospheric concentration of sulfur dioxide had fallen below 120 μg
m-3 there, and progressed at an average rate of about 23
km2 per year over the period of the study. At the start of
their study, the lichen was already well established over an area of
about 100 km2, indicating that the recolonization could well
have been occurring since the early 1960s. While L. muralis is a
species of little importance for the protection of the peppered moth
(because it inhabits surfaces of stone), closely related species of
pollution-resistant lichens, such as L. conizaeoides, which may well
be of such importance, are likely to have behaved similarly. Indeed
there is some corroborating evidence for this.
The areas where various bryophytes and lichens appeared on trees in
southern Britain in the late 1960s are recorded on a map of Hawksworth
and Rose (1970). This map shows that over the Wirral peninsula, and a
strip of land about 10km wide on the other side (i.e. the south-western
bank) of the river Dee delta, the lichen cover scored 0 to 2 on their
classification scale. Roughly speaking, a higher score on the scale
corresponds to a greater abundance of the more pollution-sensitive
lichens. The categories 0 to 2 correspond to the absence of any
lichens except for the grey-green L. conizaeoides occurring at the
bases of trees in zones of classification 2. However, measurements by
Bishop et al. (1975), showed that by 1975 the land south-west of the
Dee delta had been sufficiently recolonized by lichens to be
reclassified into category 3 in the south-east and 4 in the north-west.
Zones of category 3 are characterized by the trees having
L. conizaeoides extending up their trunks, and another crustose
species, Lepraria incana, frequent at their bases. Zones of
category
4 are characterized by the occurrence of some of the more hardy foliose
species of lichen at the bases of trees.
While these observations were only made after the
numbers
of light
moths had started to increase on the Wirral peninsula, they
nevertheless seem to contradict the impression of Wynne (Clarke et al.
1985, 193) that "there has been little change in the lichen
situation in N Wales in the last 30 years". Thus, there seems to be no
good reason for supposing that the changes in lichen cover recorded by
Bishop et al. (1975) had only started to occur some time after
Hawksworth and Rose had drawn up their map.
Kettlewell (1973, 124) stated that in 1954 he had examined large
areas of countryside over the eastern half of England, including parts
of East Anglia, without finding any lichened woods south of Yorkshire.
Yet Hawksworth and Rose's (1970) map shows that by 1970 most of East
Anglia scored 6 to 7 on their scale, and a substantial area on the
north-eastern coast scored 8. Zones in these categories are
characterized by trees bearing a wide range of both foliose and
crustose lichens on their trunks. This would appear to contradict
Howlett's observation that the recovery of the light moths in East
Anglia occurred "in the virtual absence of these lichens". (Grant and
Howlett 1988, 231).
Until the apparent contradictions between these accounts can be
resolved by further investigations, categorical declarations that that
the proportion of light peppered moths started to increase before the
reappearance of any lichens would seem to be unwarranted.
References:
Bishop, J. A. and L. M. Cook, 1975. Moths, melanism and clean
air. Scientific American 232: 90-99.
Bishop, J. A., L. M. Cook, J. Muggleton and M. R. D. Seaward, 1975.
Moths Lichens and Air Pollution along a Transect from Manchester
to North Wales. J. App. Ecol. 12: 83-98.
Clarke, C. A., G. S. Mani and G. Wynne, 1985. Evolution in reverse:
clean air and the peppered moth. Biol. J. Linn. Soc. 26: 189-199.
Clarke, C. A. and P. M. Sheppard, 1966. A local survey of the industrial
melanic forms in the moth Biston betularia and estimates of the
selective values of these in an industrial environment. Proc. R. Soc.
Lond.(B) 165: 424-439.
Grant, Bruce and Rory J. Howlett, 1988. Background selection by the
peppered moth (Biston betularia Linn.): Individual differences.
Biol. J. Linn. Soc. 33: 217-232.
Grant, Bruce, Denis F. Owen and Cyril A. Clarke, 1995. Decline of
melanic moths. Nature 373: 565.
Grant, B.S., D.F. Owen and C. A. Clarke, 1996. Parallel rise and fall
of melanic peppered moths in America and Britain. J. Hered. 87:
351-357.
Hawksworth, D. L. and F. Rose, 1970. Qualitative scale for estimating
sulphur dioxide air pollution in England and Wales using epiphytic
lichens. Nature 227: 145-148.
Henderson-Sellers, A. and M. R. D. Seaward, 1979. Monitoring lichen
reinvasion of ameliorating environments. Environ. Pollut. 19:
207-213.
Kettlewell, Bernard, 1973. The Evolution of Melanism.
Oxford University Press, London.