What group of archaebacteria live on the surfaces of swamps, marshy lands, and intestines of cows?

Methanogens or methane producing bacteria.

Explanation:

This bacterium uses carbon and hydrogen as a carbon and energy source to metabolize and produce methane. Methanogens are halophiles, meaning they flourish in environments that have a high salt concentration. This bacterium play a vital role in the ecosystem by removing excess hydrogen from the air produced during anaerobic respiration.

answer; /// i believe the correct answer is (reactants);

any abiotic or biotic factor in an ecosystem that causes a population's size to slow or decrease is a limiting factor.