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Following my answer to this question, a debate ensued on whether the loss in population of one species (namely red squirrels) due to its lesser resistance to a pathogen brought by a competing species (namely squirrel parapoxvirus and grey squirrel) should be labelled as a result of interspecies competition or not. Here I will try to state it as a question with in a more general setting. I'll give my own answer to it, and would welcome other views on the issue.

Assume the following: Species $A$ lives in a closed ecosytem and has reached carrying capacity $A=M_A$. At time $0$, a new species $B$ is introduced which is in competition with $A$, and additionally bears a pathogen to which it is resistant but which causes a high lethality rate in $A$.

It is observed that $A$ dimishes while $B$ thrives. Can we necessarily ascribe it to interspecific competition?

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  • $\begingroup$ If you are specifically thinking about longer-term, evolutionary effects (see tag) I think that you should make that more clear in the actual question. As it is written now, I don't really see why evolution is relevant. $\endgroup$ – fileunderwater Aug 1 '16 at 22:00
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No, you cannot for certain ascribe it to competition, without further information. The mediation by a pathogen is similar to the effects on prey species that are indirectly caused by shared predators (e.g. Prey B increase -> Predator X increase -> more predation on Prey A). Such effects, which can be tricky to separate from direct competition, are usually called apparent competition (see e.g. the classic paper Holt. 1977. Predation, apparent competition, and the structure of prey communities. Theor Popul Biol 12).

However, to some extent, the terminology used may depend on what you view as "the environmental background". All species have lots of adaptations to certain aspects of their living environment, and these naturally differ between species. Some might be more resistant to a certain pathogen (of to drought or other abiotic factors), but this might trade-off against other traits. I that respect, you might view species B as outcompeting species A in a certain environment (e.g. in the presence of the pathogen). However, since you specifically know about the pathogen (which is the focal point of the question), and that species B will probably function as a host/source population of the pathogen, I think it is more useful and informative to label the effect as apparent competition. To partition the direct and indirect effects you will however need moe information.

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There are two possible effects on the evolution of A:

(i) interspecies competition with B which effectively reduces the carrying capacity, which can be modelled by writing $M_A^{\textrm{i}}(B)=M_A \frac{A}{A-\alpha B}$ (Lotka-Volterra model), and

(ii) infectivity with an infection rate $k_I A B$ which can be treated as direct mortality if pathogen is 100% lethal for A (which is a good approximation for red squirrels it seems, here we've even supposed A once infected didn't live long enough to pass disease).

The question is: can we have a population reduction even if there is no interspecies competition, i.e. $\alpha=0$? The answer is yes, since as long as $B>0$, there'll be a decay of $A$ to a fraction of $M$ (with a simple logistic model, this would be $\frac{r}{r+k_I M B} M$ where $r$ is the rate of natural increase)

This matches with the interpretations of Tompkins et al transcribed in National Geographic

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  • $\begingroup$ You should give your suggested answer with the question. I edited your question accordingly, feel free to edit further. I also tried to make it a bit clearer as I did not understand some of your phrases. Please also check for misconceptions I introduced. I will try to suggest an answer to your question in the following days (little time these days ...). $\endgroup$ – AlexDeLarge Jul 29 '16 at 12:46
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    $\begingroup$ I'm not sure why I should put the answer in the question-- it's a possibility offered by SE to provide your own answer to your questions, and allow SE process to discuss and improve them independently of question. Thanks for your other edits anyway, I'm incorporating them. $\endgroup$ – Joce Jul 29 '16 at 12:50
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    $\begingroup$ @AlexDeLarge self-answering questions is not only allowed, it's encouraged. $\endgroup$ – MattDMo Jul 29 '16 at 14:07
  • $\begingroup$ @MattDMo I know. But I just felt it would fit the question better than as a self-standing answer and therefore suggested an alternative. I have no problems with it being the way it is, though. :) $\endgroup$ – AlexDeLarge Jul 29 '16 at 14:26

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