Spell out zero and one if they are used in a general sense:
One of the species was found.
… on one occasion.
His level of understanding was zero.
Use numerals if the zero or one is connected with a unit or specifies a value:
1 mL 1 year a mean of 0 y = 0
or occurs in a series of related numbers:
separated into 1, 3, 5 or 7
1 fruit, 9 vegetables and 20 cheeses
On a property with 900 cattle, 1 died and 6 were sick.
