after talking with a friend of mine, I understood the following (if I'm not mistaken):

-ANCOVA evaluates whether population means of a dependent variable (DV) are equal across levels of a categorical independent variable (IV) often called a treatment, while statistically controlling for the effects of other continuous variables that are not of primary interest, known as covariates (CV) or nuisance variables.

-the covariate would be log volume in that case (interaction, not confounding)

-the main effect would be the effect of decomposition stage (categorical variable) on the abundance of arthropods

-and the interaction term would be the effect of both independent variables on the dependent variable (not so sure?)

What I am still not sure is that I feel like I should have numerical values to quantify these effects, but I don't understand where to find them.