> (At the risk of opening this can of worms again...) Well, Mr Majerus
> opinion is in. Just exactly how much data does he have that birds are the
> principal predators? Check his data.
Perhaps it is necessary to keep opening this can of worms until there
is a concession that the textbooks need revision. I have Majerus'
book in front of me. This post is to transcribe one relevant
paragraph.
> >Of all the people I know, including both amateur and
> >professional entomologists who have experience of this moth, I know of none
> >who doubts that differential bird predation is of primary importance in the
> >spread and decline of melanism in the peppered moth."
>
> That settles it. *All * agree that birds are the major predators. Where
> are the data? If there were data, you can bet he would not be citing so
> many opinions....
Majerus on "Melanism". On page 125 is a short section entitled:
"Birds find and eat peppered moths".
"One of the main reasons why Tutt's (1896) suggestion that bird
predation was influential in the evolution of industrial melanism was
initially ignored, was that lepidopterists and ornithologists tended
to agree that birds were not major predators of tree-resting adult
moths. However, following Kettlewell and Tinbergen's production of a
filmed record of wild birds taking live peppered moths off tree
trunks, the evidence that birds are major predators of adult moths
has slowly increased. Many ornithologists and entomologists have now
shown by experiment and observation, that birds do eat significant
numbers of moths, and it is now the accepted view that bird predation
is one of the main causes of mortality in many species of adult moth.
Although observations of peppered moths being taken from natural
resting positions are still lacking and are urgently needed, it is
highly probable that predation levels are significant."
my comments:
1. Kettlewell and Tinbergen's film was not "natural" predation.
My understanding is that the moths were disoriented - having been
released in the day time in a sluggish state. (If this unnatural
situation has been the trigger for an increasing perception that bird
predation is important, it would be wise for lepidopterists and
ornithologists to check that the mistake has not been repeated in
later "observations and experiments").
2. I am entirely comfortable with the thought that birds are the
major predators of many species of adult moth. Perhaps we ought to
be enquiring about the relative importance of other predators - such
as bats.
3. We have almost no information about the peppered moth
predators in the natural state, nor which is the most important.
4. Majerus's view is that significant bird predation levels are
"highly probable" - but this is based on the consensus that bird
predation is important in many species. Majerus points to the lack
of data relating to the Peppered Moth.
It MAY BE that the story about bird predation will turn out to be
true. But our present state of knowledge is inadequate for this to
be the textbook example of natural selection changing gene
frequencies in a population of organisms.
Best regards,
David J. Tyler.