I am conducting a research project on the vegetation composition of an abandoned mine site to determine the environmental factors that influence species abundance (cover) and composition.

Study Design: I have 10 sites, within each site 10 randomly placed 1x1m quadrats were sampled for plant species percent cover (abundance) along with a soil sample that was tested for heavy metals concentration(Pb, As, Cu, Zn), pH, NPK, and OM. Factors such as slope, aspect were also recorded.

Analysis: From species abundance data I created the response variables: total cover, exotic cover, native cover, and species richness. I would like to investigate if my environmental variables are influencing the response variables I described above. I have tried using lm in R but my response variables are not normally distributed and therefore I believe violate the assumptions of linear modeling. I have also tried glm but since my cover abundance data contains many 0's and is not bounded at 100%, as there can be more than 100% cover in a quadrat, I don't think any of the error distribution families of glm can accommodate this.

So, I would appreciate any input on selecting an appropriate model for my response variables. If possible I would like to see the size of the effect or r2 rather than just a P value. Thank you very much for your time. If you would be interested in the data I could provide it.

  • $\begingroup$ Welcome to the site. Questions about code are off topic here, which is probably why your question has close votes. I think your question is more about statistics, so I am voting to leave it open, but you might want to make it clear you are interested in statistics, not code. $\endgroup$ – Peter Flom Jan 31 '19 at 10:40

You can try to transform your responses.

If it is continuous and right skeewed, try a cube root transformation since it deals with zeros. Otherwise, check if the species abundance in counts are hightly correlated with their cover %, if they are, you can try a count distribution family, such as poisson or negative binomial in case of overdispersion.

How is it possible to have a cover greater than 100%? (curiosity)

Another detail, are you inspecting for normality in the model residuals or at response variable itself?

You should inspect for normality at the model residuals, such as:

plot(model) #residuals versus fitted should show homocedasticity
hist(resid(model)) #should looks like a bell shape
shapiro.test(resid(model)) # not significative= normality

Since your quadrats are nested in site (non indenpendence-pseudoreplication), it is worth to consider a mixed modeling approach, where you should use sites as random factors.

Good luck!

  • $\begingroup$ Thanks for your suggestions! To answer your question about cover, in my study you can have different layers of cover so maybe a shrub covers 100% of the quadrat but has an herb growing beneath it, the cover will be >100%. $\endgroup$ – montanaplants Jan 31 '19 at 17:33
  • $\begingroup$ Also, species counts are not correlated with cover% so I can't use that. As for inspecting normality, I am looking at both, my response variables are left skewed and when using untransformed they don't make pretty residual graphs. I will take a look at mixed modeling approach! $\endgroup$ – montanaplants Jan 31 '19 at 17:42
  • $\begingroup$ fmwww.bc.edu/repec/bocode/t/transint.html take a look at this. $\endgroup$ – maiava Jan 31 '19 at 18:43
  • $\begingroup$ Since you are more interested in r2 rather than p values, may it is worth to use a random forest algorithm, which gives variables importance and do not make distribution assumptions $\endgroup$ – maiava Jan 31 '19 at 18:44

First, ordinary least squares regression does not make assumptions about the distribution of the variables, only about the errors - estimated by the residuals.

Second, assuming that your residuals are not normal, then, rather than transforming your variable, I suggest you use a method that makes fewer assumptions. One such is quantile regression - I have done a lot of this, but in SAS, not R, but there is an R package quantreg that may help. Quantile regression makes no assumptions about the residuals.


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