How can competition reduce species diversity with respect to named crop plant and a named weed?

All plants are competing for the same resources (water, sunlight, nutrients), so if any species has an advantage of getting one or more of the resources (as in deeper root system, more extensive root network, ability to store water for periods of drought, taller or broader leaves to shade the others, earlier germination time, more efficient in water or nutrient takeup, type of photosynthesis [C3, C4, CAM] compared with environmental conditions) over the others, it can out-compete them to the point they no longer are able to survive or produce reproductive structures.  Once a species dies, or can't reproduce itself any longer, the biodiversity will decrease.