R: Sum each column, average cover value and relative average cover value

I have a dataframe that contains percent cover data of many plant species in several plots:

Example

        sp1 sp2 sp3 sp4
PlotA   1   12  0   0
PlotB   0   3   5   0
PlotC   3   0   0   1
PlotD   0   6   0   0
PlotE   1   1   0   0

I would like to calculate a few things for each individual species/column:

How many plots does each species occur? (ex. sp1 = 3, sp2 = 4, sp3 = 1)

What is the average pct. cover per species per plot? (total cover/total plots, ex. sp1 = 1.0, sp2 = 4.4)

What is the relative avg. pct. cover per species per plot? (avg. pct. cover/sum of all avg. pct cover)

Will this be more easily achieved by making the species the rows and the plots the columns?