This may be an evolutionary stable strategy (https://en.wikipedia.org/wiki/Evolutionarily_stable_strategy https://en.wikipedia.org/wiki/Evolutionarily_stable_st

This may be an evolutionary stable strategy (https://en.wikipedia.org/wiki/Evolutionarily_stable_strategy https://en.wikipedia.org/wiki/Evolutionarily_stable_strategy). If nitrgoen-fixating species already cover N% of the niche, other species may not be pressed too hard to retain their own nitrogen-fixing ability.