Use the empirical relationship between size and population density observed in the studies by Damuth (1981) (see fig. 9.19) and Peters and Wassenberg (1983) (see fig. 9.20) to answer the following: For a given body size, which generally has the higher population density, birds or mammals? On average, which lives at lower population densities, terrestrial or aquatic invertebrates? Does an herbivorous mammal twice the size of another have on average one-half the population density of the smaller species? Less than half? More than half?