There doesn't seem to have been much research on the role of sousliks as a keystone species in Europe, but they do seem to be very similar to prairie dogs in terms of their environmental impacts:
- Their old burrows are used as hibernacula by green toads, smooth snakes and other herptiles; the Isabelline wheatear also nests almost exclusively in souslik burrows
- They are also a very important prey species - they have specialised predators such as Eastern imperial eagles, saker falcons and both marbled and steppe polecats that only really thrive in the presence of large souslik colonies, but they are also eaten by a number of less-specialised predators including booted, lesser spotted and short-toed eagles, goshawks, common and long-legged buzzards, golden jackals and red foxes.
As for the second question - there certainly were large steppe areas of Europe where sousliks lived. Wisent are a bit more controversial - there has been argument in recent years whether they are naturally a forest species or a grassland species that only occurs in forests because of past hunting. The latter idea seems to hold water better. So it is quite likely that there were herds of wisent which helped keep certain habitats open.
Rather no contact between these species: wisents live in forest and sousliks in open steppe. And yes: sousliks serve as "european praire dogs". In Poland nearly extinct (two species) and now are reintroducing in south-west and south-east Poland (among others breeding programs in Poznań and Opole zoos).
