About 2200 lumens, of 550nm light would be the cut-off point after which absorbance goes below levels that would allow plant life. Now, if you go a little below this(and are assuming in all cases that the dimming mechanism doesn't affect the wavelength of sun's light) then only certain oxygen generating algae will survive. But I think the aliens will find that desirable, cos now all land crops are dead, (unless supported by artificial lights in grow roomsin which case they're thriving), but the humans are still surviving on oxygen from algae, and food from the oceans.
Gradually, natural selection will promote mutations that allow for lower intensity light to be able to support new plants, and life might keep going on normally, albeit after a couple millennia of trajectory changing.
(Un)fortunately, there's not much research on this, as it's not a very practical question for institutions to use their funding on. Most photosynthesis-light intensity based studies are on finding the maximum intensity after which more increases don't yield proportional gains in yield.