How can you determine an ecosystem's biodiversity? a. determine what each species eats, and identify its place in a food web. b. count the number of species, and measure their relative abundance. c. weigh all the species, and measure their total biomass. d. identify the keystone species, and find all the species that depend on it.

The correct answer is-

B. Count the number of species, and measure their relative abundance.

Biodiversity of ecosystem can be described as variability among different life forms  ( such as plants, animals, and microbes) that are present within an ecosystem.  

An ecosystem biodiversity can be measured by counting the total number of living organisms, which are present in that ecosystem by using several indices.

For instance Simpson diversity index, which includes measuring species number present in an ecosystem and their relative abundance, termed as as species richness.

Thus, option B). count the number of species, and measure their relative abundance is the right answer.