Comparisons of population demography of Microtus ochrogaster and M. pennsylvanicus

Abstract

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. [iDept. Ecol., Ethol. and Evolution, Univ. Illinois, U rb a n a, Illinois 61801 (LLG, LV, DEA); Dept. Biol., Colby College, W aterville, M aine 04901 (FRC); Dept. Biol., C alifornia S tate Univ., F resno, C alifornia 93740 (JEH)].