The dead hand of Malthus...

Michael Gochfeld gochfeld at
Wed Apr 24 17:17:55 EDT 2002

Ken is of course right about the limitations of applying Malthusian
principles universally. 
But Malthus was writing primarily about humans, where in most places
predation is not a limiting factor.  

Where I grew up in Westchester County, NY, our acre and surrounding
woodland contained many Grey Birches (Betula populifolia). In the 1950's
they were attacked by a Saw Fly known as the Birch Leaf Miner (probably
Fenusa pusilla, but maybe more than one species involved).  Surface
sprays failed to reach the larvae between the leaf surfaces, and the
applicators we knew did not know how to deal with it.  In short, there
was essentially 100% elimination of the Grey Birch in our area within
3-4 years.  Complete defoliation occurred in June, followed by a new
waves of leaves which mostly survived to fall.  But the trees couldn't
tolerate continued assault.  So the Birch Leaf Miner became locally
extinct (pretty Malthusian if you ask me).  Although other species of
birches were said to be susceptible I never found affected leaves on
nearby Black, Yellow or White Birches.  


Mike Gochfeld

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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