[R-sig-eco] interpretation of interaction between explanatory variables

2013-05-08 Thread Iris Kröger 
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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[R-sig-eco] question on gls model interpretation

2013-01-29 Thread Iris Kröger 
Dear R-Sig-Ecology,

I'm analysing an experiment about mosquito larvae beeing treated with 3 
conditions (no competitors added = algae, few competitors added =new, many 
competitors added =old). After 2 weeks I contaminated all conditions with 3 
insecticide concentrations (no treatment = control, low concentration =low, 
medium concentration = medium, high concentration =high). The effect of 
treatments was observed over a time period of 35 days. 
Now I would like to analyse, if there is a synergistic effect of condition and 
insecticide treatment. I used a gls model (see below).
(Important: the mosquito larval abundances differed significantly between 
conditions when insecticide contamination was applied. I'm not sure if this is 
a problem for the further analyses)

Obviously there is a synergistic effect (condition * contamination, p0.001), 
but I don't know how to interpret the output. Which treatments did the model 
compare?Does it mean, that the effect of medium and high contamination 
increased significantly at old-condition compared with the effect at 
algae-condition? (This I can hardly believe. Mosquito larval abundances were 
close to zero even without contamination at condition-oldcontamination-control 
, hence an additional effect of insecticide treatment should be hardly 
detectable. I would rather expect an effect at condition-new in combination 
with contamination...)

I used:
mod1-gls(log10(mosquito+1) ~condition * contamination , data=MK ,
 correlation = corAR1(form=~day |bucket),
 weights=varIdent(form= ~ 1|condition),
 method=REML)
summary(mod1)
anova(mod1)

The output was:

 anova(mod5)
Denom. DF: 258 
 numDF F-value p-value
(Intercept) 1 221.04772 .0001
condition 2 243.79461 .0001
contamination 3 24.71058 .0001
condition:contamination 6 23.92481 .0001

Coefficients:
 Value Std.Error t-value p-value
(Intercept) 1.9778939 0.1391071 14.218493 0.
conditionnew -0.771 0.1610017 -4.823963 0.
conditionold -1.8958920 0.1461519 -12.972068 0.
contaminationhigh -1.2825250 0.1840214 -6.969433 0.
contaminationlow -0.2044417 0.1840214 -1.110967 0.2676
contaminationmedium -0.7833268 0.1840214 -4.256715 0.
conditionnew:contaminationhigh 0.1011773 0.2129852 0.475044 0.6352
conditionold:contaminationhigh 1.2005231 0.1933408 6.209364 0.
conditionnew:contaminationlow -0.1917487 0.2129852 -0.900291 0.3688
conditionold:contaminationlow 0.1496570 0.1933408 0.774058 0.4396
conditionnew:contaminationmedium -0.1454105 0.2129852 -0.682726 0.4954
conditionold:contaminationmedium 0.7940461 0.1933408 4.106978 0.0001

Can anyone help me?
Many thanks,
Iris

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