FW662 Lecture 9 – Immigration and Emigration 1Lecture 9. Role of immigration and emigration in populations.Reading: Sinclair, A. R. E. 1992. Do large mammals disperse like small mammals? Pages 229-242 in Stenseth, N. C., and W. Z. Lidicker, Jr. Eds. Animal dispersal smallmammals as a model. Chapman and Hall, New York, New York, USA. 365 pp.Optional:Greenwood, P. J. 1983. Chapter 7. Mating systems and the evolutionary consequencesof dispersal. Pages 116-131 in I. R. Swingland and P. J. Greenwood, eds. Theecology of animal movement. Clarendon Press, Oxford, England.Stenseth, N. C. 1983. Chapter 5. Causes and consequences of dispersal in smallmammals. Pages 63-101 in I. R. Swingland and P. J. Greenwood, eds. Theecology of animal movement. Clarendon Press, Oxford, England.Dispersal -- “waif of population ecology” (Ricklefs 1990:373).Important, but difficult to measure. Dispersal is defined as the one-waypermanent movement away from an established home range or natal area. In contrast, migration is the two-way movement between 2 areas. Philopatry is the fidelity or tenacity to an area or home range. Immigrationcan be thought of as dispersing animals that are leaving the area ofinterest, whereas emigration is the arrival of dispersing animals onto thearea of interest.Two types of dispersal (Lidicker 1975)Saturation dispersal, population crowded with respect to resources and aggressiveindividuals force others to leave.Presaturation dispersal, individuals with an innate predisposition to wander andleave. Dispersers leave their current place of residency before the patch'scarrying capacity is reached.FW662 Lecture 9 – Immigration and Emigration 20 50 100 150 200 250 300 N(t)0 20 40 60 80 100 Time (t)Saturation DispersalPre-Saturation DispersalProximate causes of dispersal: competition for mates, avoidance of inbreeding (Bollingeret al. 1993), and competition for resources. Boolinger et al. (1993) found thatmeadow voles (Microtus pennsylvanicus) released into experimental grasslandplots with siblings were more likely to disperse from these plots than were volesreleased into similar plots with nonsiblings. Furthermore, voles that dispersedfrom sibling groups did so sooner than dispersing voles from nonsibling groups.Dispersal, how the mechanism evolved (see Emlen 1984, Chapter 13)Evolve a genotype that leaves an area where population is close to carryingcapacity.This genotype would be deleterious at low densities, but beneficial at highdensities (assuming areas away where fitness is increased)Role in population regulationKeeps population from getting too high in one areaEvidence for population regulation provided by "fence effect" (Krebs1992)Buffers population over several subpopulationsMaintains genetic structureMaximum dispersal takes place at MSY when there are the most juveniles available todisperse, at least according to common perceptions. However, this perception hasnot held up with experimental work. Andreassen and Ims (2001), in anexperimental study with 12 enclosed, patchy populations, found that dispersal inroot voles (Microtus oeconomus) was strongly density-dependent, and most so forsubadult animals. However, high-density patches had low emigration rates. Rootvoles immigrated onto patches with a smaller number of individuals, especially ofFW662 Lecture 9 – Immigration and Emigration 3their own sex and reproductive state, than were present in the patch they left. Most shifts between patches took place from patches with relatively low densityto patches with even lower density. Small patches had higher spatiotemporalvariability in density, and demographic composition than large patches, and thisprobably caused most of the demographic turnover in small patches. In particular,emigration was the main demographic parameter behind declining numbers andpatch extinction in small patches with few individuals. The kind of density-dependent emigration-immigration dynamics found by Andreassen and Ims(2001) does not match the common perception that dispersal works primarily toreduce extinction probabilities through rescue effects. In particular, the impact ofemigration as a factor that may increase the extinction probability of small,isolated patches with few individuals is an important aspect of metapopulations.Dispersers should be either young animals which have not yet attained maximalreproductive value (and which have not yet established a breeding site), or mucholder individuals whose prospects for future reproductive output is low (but stillgreater than zero) (Morris 1982). Examples of older individuals leaving are malelions from pride, or 10+ year old African buffalo males leave the breeding herds(Sinclair 1992).Applications of dispersal theory in applied ecology (Hansson 1992). Generallyimmigration is more pertinent than emigration.Pest control -- removal of pest and time to re-colonizationPest outbreaks (jack rabbits, house mice)Settling in human habitations (annual fall mouse trapping episodes)Disease transmission, both within the species and to other species, e.g., livestock,humans.Re-colonization of areas with extinct populations (Yellowstone wolf -- is itnecessary to transplant individuals)Dispersal distances -- how to measure? (Porter and Dooley 1993).Most studies get a biased view of dispersal distances because of distance-weightedsampling procedures. Radios provide the best method for estimating dispersaldistances, but even new technology can be flawed if the animals move far enough. Dispersal distances are important in metapopulation models because distance(along with frequency) determines time to re-colonization of a vacant patch andgene flow frequencies. Good example of a study with radios to measure dispersalis Larsen and Boutin (1994) on 250 red squirrels (Tamiasciurus hudsonicus) inAlberta, Canada. Foray distance was not related to age or size of the offspring. Offspring that settled relatively farther away from their natal territory were morelikely to obtain larger territories, with traditional hoarding and overwintering sites(middens). These offspring also had higher overwinter survival, suggesting thatthe costs of making forays off the natal territory may be balanced by theadvantages of locating a superior territory.FW662 Lecture 9 – Immigration and Emigration 4Modeling dispersal and population dynamics -- Hastings (1993). Dispersal stabilizeschaotic discrete logistic models. “A study of one of the simplest