Thank you for your reply. I think you misunderstood my question. What I wanted to ask is when there are multiple environmental factors measured (in my case 22 variables), how should I do the selection of the independent variables/enviromental factors/covariates to fit the regression model?
In the beginning, I fit the model with all the covariates which I think they may influence the microbiome. After I saw that many of them do not explain much of the variance, I tried to fit with one variable first then add one by one and kept those at least explain 2% of data variance.
Do you think I am doing it right?