The dead hand of Malthus...

[Stan Gorodenski]

Not being an expert on the Malthusian principle, maybe the worst that
can be said about Malthus was that he was not very learned about ecology
in the 1800's, or that the study of population dynamics was not at that
advanced. Certainly it is now know that other population regulating
mechanisms exist besides food supply. E.g., it is recognized the role
mountain lions play in regulating deer populations. In their absence,
the deer population explodes resulting in a crash (was this seen in the
1920's?  I don't remember). It seems to me that that modern day version
of the Malthusian principle would be that in the _absence_ of other
regulating mechanism, food will be the limiting resource, as was seen
regarding the deer populations I just mentioned.

With respect to _Phyllocnistis populiella_, from the description below
it does not appear the population was food limited, i.e., "...nearly
every aspen..." but not all. Also, is it possible for more than one
miner to exist on one side of a leaf? 

_Nymphalis antiopa_ also must be regulated by other factors besides food
supply. It seems the Malthusian principle has been heavily relied on in
early studies regarding population growth models with Tribolium beetles,
for example. Are we not taking the Malthusian principle out of context
in these discussion when it has previously been applied, in a scientic
sense, to closely regulated population studies?
Stan

Kenelm Philip wrote:
> 
> > If you can take habitat size as a constant - and not as a shrinking
> > factor - natural systems do work as Mathus postulated.
> 
> I don't claim Malthus was wrong--he just didn't cover the whole story in
> many cases. Let's consider two species of leps:
> 
> 1) _Phyllocnistis populiella_ (aspen serpentine leaf miner). A few years
> ago, nearly every aspen in the Fairbanks area had two mines (one on the
> top surface, one on the bottom) on almost every leaf. Clearly, the pop-
> ulation was being limited by the amount of food--that year. In other years,
> the numbers have been much smaller--showing that one or more other factors
> were at work. The number of aspen leaves stays roughly constant from
> year to year...
> 
> 2) Nymphalis antiopa_ (mourning cloak). The foodplant is willow, and there
> is _lots_ of willow here. If the population were limited by food, we would
> see the sky darkened by mourning cloaks (which would be an interesting
> sight). Instead this species' numbers fluctuate about some much lower
> level such that you don't normally see totally defoliated willows (the
> usual result of their colonial feeding behavior) in the wild. In this
> case I don't think Malthusian considerations apply to the _antiopa_ it-
> self. They may apply to its parasites, however.  :-)
> 
>                                                         Ken Philip
> 
> 
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