Colonization of Non-Traditional Range in Dispersing Elk, <em>Cervus elaphus nelsoni</em>, Populations

Abstract

On traditional ranges, animals typically concentrate use in the most productive patches of habitat (Charnov 1976;Parker and Stuart 1976). However, intraspecific population pressure, often associated with population growth, can cause a species to increase its range of habitat use in less predictable ways (Rosenzweig 1991). For non-territorial species, the tendency to increase range of habitat use with increasing population density has been formalized as the Ideal Free Distribution (IFD) Theorem. The Ideal Free Distribution Theorem asserts that individuals choose habitats at densities associated with equal levels of fitness (Fretwell and Lucas 1970). Thus, IFD predicts that when populations occupy new ranges at low densities, individuals will occupy only optimal habitat. As density increases, so does interference with conspecifics, causing individual fitness to decline. Individuals leave optimal habitat when fitness drops below what they could achieve in a suboptimal habitat at a lower population density.Among ungulates, the behavior of Elk (Cervus elaphus nelsoni) in populations experiencing long-term population growth corresponds to predictions of IFD. Dispersive behavior associated with population growth and an increasing range of habitat use might contribute to the initial stages of colonization and the establishment of a permanent population in a previously unoccupied area (Safriel and Ritte 1983). The colonization of vacant habitat without decreased fitness of colonists is an alternative to increased intraspecific competition on traditional range. Although the IFD Theorem makes density-dependent dispersal theoretically plausible, such dispersal has been difficult to demonstrate in ungulates (Clutton-Brock et al. 1985;Boyce 1989), and determination of the mechanisms of colonization by individuals dispersing from established ungulate populations, or their relation to population growth, has been problematic.In three increasing populations in south-central Montana, USA, Elk demonstrated changes in habitat use and home-range characteristics, increasing spatial separation of adjacent populations, and fissioning of individual populations into multiple populations (Van Dyke and Klein 1996;Van Dyke et al. 1998), all of which can lead to an increasing range of habitat use. The last response, population fissioning, was initiated by the formation of a dispersing group that made seasonal movements to new range, often containing a distribution of vegetation communities different from the traditional range of the established population. In investigating the role of such groups in expanding range and habitat use of established populations, I sought to determine what changes dispersing Elk make in their use of home range and habitats compared to their populations of origin that might enable them to persist on new ranges that the established population did not use. Study AreaThree populations of Elk in south-central Montana (USA) were investigated, locally known as the Line Creek (LC), Picket Pin (PP), and Silver Run (SR) pop...