What is paleobotony and what significance does this field have to our understanding of plants?

Paleobotany is a branch of the paleontology and the paleobiology. Its main focus are the recovering and identification of the plant remains (the plant fossils) found into the geologic contexts.

The paleobotany is a very important branch as it is the one that gives us the answers about the emergence of the plants, their evolution and development over time. Also, it gives us answers how the plants from the past lived, what was the environment in which they existed, their needs, and how the plants influenced the environment around them.

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

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.