Geologists use and fossils to infer what the environment of a place was like when a layer of it was forming.

rocks

Explanation:

A geologist is a person who studies the rocks and the earth's surface to find different phenomenon. The study of rocks and he fossils found there helps the geologist to understand and  conclude the type of climate and the environment it formed the fossils of a place on earth. It provides large amount of information about the climatic conditions of the place when the layer of it was forming.

the right option is; decreased genetic variation  

decreased genetic variation describes the effect of this environmental change.

genetic variation is the naturally occurring genetic diversities or differences between organisms of the same species in a population. genetic variation allows the survival and adaptability of a species when they encounter changing environmental conditions. when an endangered species reduces, it loses genetic variation which may not be restored even if the species rebound and will only be recovered through the gradual collection of mutations over many years.