Final answer:
The 'mark and recapture' method provides information on both the size and density of a population, typically used for mobile organisms like mammals, birds, or fish.
Step-by-step explanation:
The method that provides information to an ecologist about both the size and density of a population is mark and recapture. This technique is especially useful for mobile organisms such as mammals, birds, or fish. It involves capturing a number of individuals, marking them, and releasing them back into the environment. After some time, another sample of the population is captured, including both marked and unmarked individuals.
From these samples, scientists can estimate the total population size and density using statistical models. In contrast, quadrat sampling is typically used for immobile or slow-moving organisms like plants. A quadrat is a square structure placed at random locations to count individuals within its boundaries. Although quadrats can provide information on density, they are less often used to estimate population size for mobile animals.