It's difficult to give definitive advice about what it means, but the 
simplest approach is to look at what it means: plot the model and see 
what it looks like. Basically, the interaction says that as the 
variables both increase, the relationship gets less negative. The 
details depend on the ranges of the variables, hence plotting the 
results is helpful.

I would write some code, but that would probably mainly show that I'm 
out of date with R. But you could use expand.grid() to create new data 
to predict, use predict() to predict it, and then plot it as several 
lines on one plot (e.g. plot against C.abundances, with one line for 
each value of C.diversity).

Bob

On 05/08/2013 02:56 PM, "Iris Kröger" wrote:
> Dear list members,
>
> I want to analyse the impact of a competitor community (i.e. "community abundances" on the one hand and "community species diversity" on the other hand) on mosquito larval populations of species A and B. Each variable on its own has a negative impact on mosquitoes - but when both variables are interacting, there is a positive impact... How can I interpret that? For mosquito A only C.diversity has a significant impact - but the interaction between C.abundances and C.diversity is significant? What does that mean?
>
> I used the model:
> lm (mosquito ~ C.abundances * C.diversity)
> output Mosquito A:
>   Estimate Std. Error t value Pr(>|t|)
> (Intercept) -0.2120 0.1159 -1.829 0.074855 .
> C.abundances -0.1277 0.1616 -0.790 0.434067
> C.diversity -0.5787 0.1385 -4.178 0.000155 ***
> C.abundances:C.diversity 0.4096 0.1712 2.393 0.021476 *
>
> Output Mosquito B:
>   Estimate Std. Error t value Pr(>|t|)
> (Intercept) -0.2900 0.1220 -2.377 0.02233 *
> C.abundances -0.3856 0.1701 -2.266 0.02891 *
> C.diversity -0.3470 0.1458 -2.381 0.02213 *
> C.abundances:C.diversity 0.5367 0.1801 2.980 0.00489 **
>
> Thanks a lot for your help!
> Iris
>
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-- 

Bob O'Hara

Biodiversity and Climate Research Centre
Senckenberganlage 25
D-60325 Frankfurt am Main,
Germany

Tel: +49 69 798 40226
Mobile: +49 1515 888 5440
WWW:   http://www.bik-f.de/root/index.php?page_id=219
Blog: http://blogs.nature.com/boboh
Journal of Negative Results - EEB: www.jnr-eeb.org


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