Manuscript-Vole population dynamics: factors affecting amplitudes of fluctuation (original) (raw)
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Acta Theriologica, 2007
We studied factors affecting highest densities achieved (peak densities) and amplitudes of fluctuation during 39 annual population fluctuations of Microtus ochrogaster in alfalfa, bluegrass, and tallgrass habitats over a 25-year period. Thirtytwo of the 39 population fluctuations reached a peak in autumn or winter. Length of the increase period and initial population density appeared to have the greatest correlation with peak densities and amplitudes of fluctuation. Cessation of growth of population fluctuations reaching peak densities in autumn-winter resulted from a reduction in survival, perhaps enhanced by winter reduction in reproduction. Cessation of growth of population fluctuations with peak densities during spring-summer appeared to result mainly from reduction in survival. We conclude that increased mortality is the primary factor stopping population growth during population fluctuations of M. ochrogaster. We suggest summation of independent mortality effects from multiple predators may be responsible for high population fluctuations some years and not others.
Journal of Mammalogy, 2006
We studied factors associated with occurrence of high-amplitude population fluctuations of the prairie vole (Microtus ochrogaster) in alfalfa, bluegrass, and tallgrass habitats in east-central Illinois for 25 years. Increased survival was the most important factor associated with initiation of a population fluctuation during a given year. The proportion of reproductively active adult females was not associated with initiation of population fluctuations. The interval between fluctuations was not correlated with the previous peak density. We propose that population fluctuations in M. ochrogaster were initiated by the net effects of relaxation of predation pressure of multiple generalist predators, which occurred erratically across years.
Factors associated with initiation of population cycles of Microtus ochrogaster and M. pennsylvanicus were studied in alfalfa, bluegrass and tallgrass habitats for 25 years. For both species, increased survival appeared to be the most important factor associated with initiation of a population cycle during a given year. There was no difference in reproduction the previous winter or during spring (both species) and autumn (M. ochrogaster) of cycle and non-cycle years. Weather differences, including episodes of extreme conditions, were not associated with cycle and non-cycle years. There was no indication that cyclic phenomena were a result of habitat degradation owing to high densities during peak phases. We found no relationship between peak densities and rate of decline, length and extent of the decline, population density during the subsequent trough, or the interval until the next cycle. Population cycles appeared to be initiated by relaxation of predation pressure which occurred erratically across years.
We analyzed monthly survival rates, persistence of young, reproduction, recruitment of young, immigration rates, body mass, and sex ratio for fluctuating sympatric populations brought to you by CORE View metadata, citation and similar papers at core.ac.uk provided by Illinois Digital Environment for Access to Learning and Scholarship Repository Getz, et al. 2 of Microtus ochrogaster and M. pennsylvanicus in alfalfa, bluegrass and tallgrass over 25 years. M. ochrogaster underwent 13 population cycles in alfalfa, 12 in bluegrass and five in tallgrass. M. pennsylvanicus displayed five cycles in alfalfa, nine in bluegrass and was acyclic in tallgrass. Among the demographic variables considered, only increased survival and persistence of young were associated consistently with the increase phase of population cycles in both species. Survival rates, persistence of young and reproduction of M. ochrogaster were lower during the decline than during the increase phase in alfalfa and bluegrass, but not in tallgrass. There were no demographic differences between the decline and increase phases for M. pennsylvanicus in either alfalfa or bluegrass. Most differences in demographic variables within phases among habitats involved survival and explained among-habitat demographic differences of each species. Differences in demographic variables within phases between species also mainly involved survival; these differences were consistent with differences in demography of the two species in alfalfa. Overall, our data indicate that changes in survival were most important in determining patterns of fluctuation in our study populations.
Habitat-Specific Demography of Sympatric Vole Populations Over 25 Years
Journal of Mammalogy, 2005
We studied the influence of preferred food, forbs, and vegetative cover on survival and reproduction at different population densities of Microtus ochrogaster and M. pennsylvanicus in alfalfa, bluegrass, and tallgrass prairie habitats in east-central Illinois for 25 years. Population densities of M. ochrogaster were greatest in alfalfa, least in tallgrass, and intermediate in bluegrass, whereas those of M. pennsylvanicus were greatest in tallgrass, least in alfalfa, and intermediate in bluegrass. For both species, preferred food availability was greatest in alfalfa, intermediate in bluegrass, and least in tallgrass. Vegetative cover was relatively sparse in alfalfa, especially in winter, and dense throughout the year in bluegrass and tallgrass. Variation in survival emerged as the most important factor explaining population differences between the 2 species. Reproduction had little differential impact on abundance of either species in any of the 3 habitats. Survival of M. ochrogaster was higher in alfalfa than in bluegrass or tallgrass; survival of M. pennsylvanicus was higher in tallgrass than in alfalfa or bluegrass. Differential survival among habitats and between species was influenced primarily by amount of vegetative cover. We suggest that M. ochrogaster is less susceptible than M. pennsylvanicus to predation by raptors and large carnivores (predators that hunt from above vegetative cover), whereas M. pennsylvanicus is less susceptible than M. ochrogaster to snakes and small carnivores (predators that hunt under vegetative cover).
Experimental tests of predation and food hypotheses for population cycles of voles
Proceedings of the Royal Society B: Biological Sciences, 2000
Pronounced population cycles are characteristic of many herbivorous small mammals in northern latitudes. Although delayed density-dependent e¡ects of predation and food shortage are often proposed as factors driving population cycles, ¢rm evidence for causality is rare because su¤ciently replicated, large-scale ¢eld experiments are lacking. We conducted two experiments on Microtus voles in four large predator-proof enclosures and four unfenced control areas in western Finland. Predator exclusion induced rapid population growth and increased the peak abundance of voles over 20-fold until the enclosed populations crashed during the second winter due to food shortage. Thereafter, voles introduced to enclosures which had su¡ered heavy grazing increased to higher densities than voles in previously ungrazed control areas which were exposed to predators. We concluded that predation inhibits an increase in vole populations until predation pressure declines, thus maintaining the low phase of the cycle, but also that population cycles in voles are not primarily driven by plant^herbivore interactions.