"Why do field researchers and laboratory researchers use tools that are similar and tools that are different?

Field research and laboratory research are different from each other on various accounts. In a lab, the study is not on such a broader level as that on a field.

Some of the tools in a lab and a field study will be same because such tools cannot be changed. For example, for amplification of genes, PCR will be used whether a field research is being made or a lab research.

On the other hand, some of the tools will be different. For example, to study field plants we'll use quadrants for counting but in a lab, as a lesser number of plants are understudy so we won't need a quadrant.

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.