Balancing energy and nitrogen in the rumen is a key to both profitability and environmental sustainability. Four dairy cows were used in a Latin square experimental design to investigate the effect of severe nitrogen underfeeding (110 vs. 80% of requirements) and the type of carbohydrate consumed [neutral detergent fiber rich (FIB) vs. starch rich (STA)] on the rumen ecosystem. These dietary treatments modified both rumen fermentation and microbial populations. Compared with STA diets, consumption of FIB diets increased bacterial and fungal diversity in the rumen and also increased the concentrations of cellulolytic microorganisms, including protozoa (+38%), anaerobic fungi (+59%), and methanogens (+27%). This microbial adaptation to fiber utilization led to similar digestibility values for the 2 carbohydrate sources and was accompanied by a shift in the rumen fermentation patterns; when the FIB diets were consumed, the cows had greater ruminal pH, ammonia concentrations, and molar proportions of acetate and propionate compared with when they consumed the STA diets. Certain rumen microorganisms were sensitive to a shortage of nitrogen; rumen concentrations of ammonia were 49% lower when the low-protein (LP) diets were consumed as were total bacteria (-13%), anaerobic fungi (-28%), methanogens (-27%), protozoa (-19%), cellulolytic bacteria, and microbial diversity compared with when the high-protein (HP) diets were consumed. As a result, the digestibility of the LP diets was less than that of the HP diets. These findings demonstrated that the rumen microbial ecosystem is directly linked to the rumen fermentation pattern and, to some extent, to the efficiency of diet utilization by dairy cattle.