"In a particular case of secondary succession, three species of wild grass all invaded a field. By the second season, a single species dominated the field. A possible factor in this secondary succession was ."

A possible factor for the above is best described by the inhibition model. This is a model of succession that allows for early colonizers to modify their environment making it only conducive for them hence ensuring their survival. If this is done successfully, by the second season, of the the 3 species of  wild grass, only one will thrive in the field as a result of the phasing out of the other 2.

Explanation:

The inhibition model occurs as a result of the environment being modified by of the 3 species of wild grass by the 2nd season. This may have been as a result of it secreting biotoxins into the environment that destroys the other 2 wild grass, thus inhibiting their growth. This helps the organism thrive well during the 2nd season  

geologic time scale (for gradpoint) =)