definition

derek a. zelmer zelmeda4 at WFU.EDU
Wed Feb 15 12:39:23 EST 1995


Brian Keas stated the following requirement for an organism to be 
considered parasitic:

> 1. Thus, if an organism (or life-cycle stage)  causes a net negative 
> effect on host resources it COULD be a parasite.  {If it causes no net 
> negative effect, it CANNOT be a parasite (e.g. flagellates in a termite 
> gut use host resources but make cellulose available to the termite and 
> so there is a positive effect on host resources; not a parasite).}

Consider, if you will, the question asked of J. Holmes by N.A. Croll: 
"Are predators alway idiots?" in the context of parasites that alter 
intermediate host behavior in order to complete their life cycle by 
increasing the probability of the predator eating an infected 
intermediate. The question implyed that there must be selective pressure 
against eating infected intermediates if the resulting infection has a 
negative effect. Holmes response was that the tradeoff must be favorable 
to the host; i.e. the energy gained from the easier capture and ingestion 
of the prey must be greater than the energy consumed by the parasite that 
establishes within the host. In other words there is a net benefit to the 
host as a result of the infection. According to your definition these are 
not parasites. This intermediate host manipulation is characteristic of 
all acanthocephalans, and I would personally mourn the loss of this group 
from my area of study.

> 2. An organism (or life-cycle stage) that ABSOLUTLEY CAN ONLY use one host 
> at a time to obtain host resources COULD be a parasite.  {An organism that 
> could POSSIBLY obtain resources from two hosts simultaneously CANNOT be a 
> parasite (a cow could possibly obtain resources from two separate plants 
> (grasses) in the same mouthful; not a parsite).}

Some people would argue that this is simply a matter of scale. If 
parasites were larger, perhaps they could feed on two organisms at one 
time, or perhaps cows are just large parasites.

Just more fuel for the fire....Derek A. Zelmer




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