Comparisons of population demography of Microtus ochrogaster and M. pennsylvanicus

C om parisons of population dem ography of Microtus ochrogaster and M. pennsylvanicus.A cta theriol., 24, 24: 319-349 [W ith 6 T ables, 8 Figs.& 1 A ppendix].D em ography of tw o population cycles of Microtus ochrogaster and o ne of M. pennsylvanicus w ere com pared in alfalfa, blu eg rass and ta ll grass p rairie h ab itats in ea st-c e n tra l Illinois, 1972-1976.P eak densities of M. ochrogaster w ere g reatest in alfa lfa (240 and 110/ha), low est in p ra irie (37 and 12) and in te rm ed ia te in bluegrass (125 and 60); p eak d e n sities of M. pennsylvanicus w ere: bluegrass, 49/ha; p rairie , 31; alfalfa, 11.Length of the breeding season, as established by food av a ilab ility , a p peared responsible for the am plitu d e of flu ctu atio n of M. ochrogaster populations, b u t did not drive the p opulation cycle.Food av a ila b ility did not influence am p litu d e of flu ctu atio n or the p o pulation cycle of M. pennsylvanicus.R eproduction and su rv iv al w ere g rea ter d u rin g th e in crease th a n durin g the decline phases in M. ochrogaster.T h ere w ere no phase differences in reproduction and su rv iv al of M. pennsylvanicus.T here w as a n egative correlatio n betw een reprodu ctio n and p o p u latio n density in M. ochrogaster, but not in M. pennsylvanicus.C hanges in re production ap p eared to be a m ajo r facto r responsible for in itiatin g a M. ochrogaster population decline; increased juvenile m o rta lity w as of secondary im portance.Increased juvenile m o rta lity ap p eared to be a m ajo r fac to r in the population decline of M. pennsylvanicus.