Size-Based Demography of Vertebrates

Age has traditionally been the categorical variable used in demographic models of vertebrate populations (76, 87, 89, 132). This may be due in part to the historical development of demographic theory as a means of describing human population dynamics for social and political purposes (76). The term demography originally referred to the science of vital statistics of human populations, although the term has lost its human (or even animal) orientation in the ecological literature (63). However, humans are not good models for other animals because, even among other vertebrates, they are among the most long-lived, show little variation in litter size, and are extremely agespecific in rates of both fecundity and survival (76, 87). Most vertebrates have shorter life spans, display more variation in litter size, begin reproduction as soon as they are physiologically able, and do not experience the long postreproductive lives found in humans (5, 6, 26, 43, 61, 77). For nonhuman vertebrates, body size has been a suggested alternative to age as a demographic variable (35, 60, 77, 128-130). The use of body size (generally measured as body mass) in demography has been explored for several taxa of vertebrates (60, 61, 77, 128-131), and there is increasing theoretical support for the use of body size as the relevant parameter in vertebrate life-history studies (4, 18, 112, 134, 144, 145, 168). Werner & Gilliam (166) recently reviewed the ecological relevance of body-size changes during ontogeny and introduced a continuous-time model of sizebased demography. However, no review exists of the methods of size-based