From: achenbac@epcot.spdc.ti.com (Jeff Achenbach)
Newsgroups: talk.politics.animals
Subject: Re: Deer Population Control Mechanisms
Date: 12 Jan 93 16:28:36 GMT
Organization: TI Semiconductor Process and Design Center

In article <72z502xZ31s101@JUTS.ccc.amdahl.com> tsr00@duts.ccc.amdahl.com (Ted Robinson) writes:
>
> 
>Now let's imagine 2 different perturbations on our system: 
>
>  A. A disease sweeps through the deer, cutting the population in half. 
>  B. An extremely mild winter temporarily reduces the death rate, leaving
>     the herd with an increased population.   
>
>and see how each of the above mechanisms responds to the perturbation.
> 
>1. Predator killings: 
> 
>  A. The predator to deer ratio has all of a sudden doubled. As a result, the
>     death rate due to predation will *increase*. While it probably wouldn't
>     double - the scarcity of deer would lower the predator's success rate - 
>     it would clearly increase. Since predation has no direct effect on the 
>     birth rate, the resultant increased death rate would lead to a further
>     *reduction* of the deer population, leading to an even further increase  
>     of the death rate. 
>

Why do you assume the predators success rate wouldn't be cut in half? This 
would result in weakening of the individual predators and further decline in
their ability to hunt.

>     Of course, at the same time the predators are in decline due to
>     starvation, setting up a differential equation which would require more
>     details to solve. At a rough level, though, it seems to me that 
>     either this positive feedback (decreased deer pop. -> increased death  
>     rate -> decreased pop) would drive the deer
>     population to 0, or the decline of the predator population would 
>     asymptotically catch that of the deer, achieving a new equilibrium
>     at lower population levels. 

I think you're are correct on this point. (new equilibrium at lower level)

In either case, no restoring mechanism
>     exists to return to the original population levels (except possibly
>     if the predator decline overshoots that of the deer, yielding an  
>     oscillating system which could conceivably (not likely) converge to
>     previous levels).
>     

The increase in food supply (if it occurs) should result in higher fertility
and winter survival rates.


>Summary:
>
>Food source as a controlling mechanism, either through starvation or through 
>fertility regulation, provides a *restoring* mechanism that drives the 
>population back to the equilibrium point. Predator control, on the other 
>hand, exacerbates the problem, leading to a divergence from the equilibrium 
>point. Viewed this way, predator control would seem to be a really shitty way 
>to maintain equilibrium - Nature usually does better than this. 
>
>Comments?
> 

Basically, I think that by breaking the population dynamics into individual
cause/effect relationships you are oversimplifying the system. Although it 
is easier to grasp and analyze, it is not a realistic representation. You 
are analyzing these as single variables, when what you really need is 
multivariate analysis.

Jeff Achenbach
Newsgroups: talk.politics.animals