Population Dynamics of the Perennial Herbs Plantago Major L. and P. Rugelii Decne

Demographic information about plant populations has accumulated over the last few years, and some generalizations are now possible from these data. Sarukhan & Harper (1973) used published information on the survivorship of several herbs and grasses from different geographic regions, and from their own studies on buttercups, to show that established plants of perennials, including Plantago lanceolatat (Sagar 1959), suffer approximately constant annual mortality, though the mortality rate varies between species. Mortality rates appear to be affected by environmental conditions in the year of germination and during subsequent years (Williams 1970; Antonovics 1972). This information has been used to prepare preliminary models of the regulation of plant populations (Harper & White 1971). The complete life cycle has rarely been assembled in a life table (Sharitz & McCormick 1973), and then may be useful for annual species only. Sarukhan & Harper (1973) recognized that mortality rates of perennials need not be constant. Although most survivorship curves for plant populations approximate to the Type II described by Deevey (1947), those for range grasses studied by Canfield (1957) were negatively skewed (Deevey type I) while those of grasses studied by Williams (1970) were slightly positively skewed (Deevey type III). The few studies available on the survivorship of populations of seedlings show that the mortality rate declines with age (Sharitz & McCormick 1973; Sarukhan & Harper 1973), eventually attaining a constant loss rate (Harper 1967; Hett & Loucks 1971; Hett 1971; Antonovics 1972; Thomas 1972). Mortality rates in populations of dormant seeds, like those for established plants, are usually fairly constant (Roberts 1962; Sarukhan 1974). Only two studies are known to us where differences in longevity, mortality, natality, and density were examined in closely related species under natural conditions. Sarukhan & Harper (1973) investigated the demography of Ranunculus repens, R. bulbosus and R. acris, and Thomas (1972) investigated the dynamics of Hieracium florentinum, H. floribundum, and H. pratense. Such taxonomically related species can live in the same area if the factors regulating the size of their populations, at any stage of the life cycle, are different (Harper et al. 1961). For example, Sagar (1959) showed that Plantago major, P. media and P. lanceolata could cohabit, in part, because seeds of each species had different requirements for germination.