Final answer:
The sampling technique that represents an index of abundance is the number of bear droppings along a trail, as it's an indirect measure and not related to specific area units like the quadrat and mark and recapture methods, which provide estimates of population density.
Step-by-step explanation:
Population Sampling Techniques
The technique that represents an index of abundance rather than an estimate of density in the given options is B) number of bear droppings along a trail. This is because indexes of abundance, like the count of bear droppings, are indirect measures of population size that do not directly relate to specific area units for calculating density. On the other hand, techniques such as the use of a quadrat to count oak trees or the mark and recapture method used on mice provide more direct measures that can be used to estimate population density, which is the number of individuals per area unit.
For immobile or slow-moving organisms, a quadrat is placed on the ground in random locations, and the number of individuals within it is counted. This count is then used to calculate population size and density. With the mark and recapture method, animals are marked, released, and then a second sample is taken later to infer total population size, which can be divided by the area to find population density.