Combine the given two facts to write a concluding fact. Note that there should be some parts of the first and second facts that are not mentioned in this conclusion fact. Your combined fact should be the result of a chain between the two facts. Chains form when two facts connect together to produce a concluding fact. An example of a chain is: "pesticides cause pollution" (fact1) + "pollution can harm animals" (fact2) → "pesticides can harm animals" (conclusion fact). Parts of the concluding fact overlap with the first and the second fact. To construct such a  concluding fact, a useful way is to borrow the subject from one fact and the ending conclusions from another fact.

Fact 1: landfills have a negative impact on the environment. 
Fact 2: An ecosystem is an environment and all the members of that environment.
Landfills have a negative impact on the ecosystems.

Fact 1: a valley is formed by a river flowing. 
Fact 2: Rivers erode the rocks they flow over.
Water erodes rocks as they flow over, creating a valley.

Fact 1: Ribosomes are small organelles where proteins are made. 
Fact 2: Bacteria contain no organelles.

Bacteria do not have ribosomes for protein production.