We assess the strength and duration of competitive interactions between the
closely related vole species Microtus montanus and M. longicaudus in the
Greater Yellowstone Ecosystem (GYE). We directly manipulate modern
populations of these species in the field and compare these data to species
abundance data from two late-Holocene fossil records. Both modern and
ancient data were replicated.  Removal experiments conducted along an
environmental gradient revealed a habitat-dependent competitive interaction
between species: M. montanus excludes M. longicaudus from resource-rich
meadows; M. longicaudus is limited to patchy, ecotonal habitats which M.
montanus does not colonize. The paleontological data demonstrate that the
habitat associations of the modern species are concordant with habitat
associations during the late-Holocene. Further, a spike in M. longicaudus
abundance in open meadow habitat during a Medieval Warm Period (650-1,000
ybp) decline in M. montanus abundance suggests that the species was
experiencing competitive release, akin to what is seen in the modern
experiments. The fossil and modern data combined suggest that the outcome
of the asymmetric competition between these species is influenced by local
environmental conditions over millennial time-scales. Furthermore, we show
that the same interspecific interactions which influence local abundance
and distribution in the modern and enable community-wide coexistence of
competitors operate over millennial timescales to enable long-term
evolutionary co-occurrence of competitors.