Most organisms in any ecosystem are

the correct answer is anaphylaxis.

anaphylaxis is a condition caused by insect bites, foods, medications, manifested by tongue swelling, shortness of breath, vomiting, low blood pressure. it sometimes may occur without an obvious reason. it is treated with epinephrine injection.other tretments include antihistamines and steroids,

for a different answer set the answer would be because the oxygen in the water had depleted (apex)