Prey Populations

After posting my sightings at Plum Island the other day I started to think about the the abundance of birds of prey in that area. I went back and reviewed the recent sightings on the island for 1/8 - 1/9 and there were reports for the following:

Snowy Owl (2)
Short Eared Owl (1)
Northern Harrier (5)
Red Tailed Hawk (1)
Rough Legged Hawk (1)

I singled these birds out because a large portion of their diet consists of rodents (moles, voles, mice, etc). Then I started to think about how many rodents there must be on the refuge to support these predators (plus the coyotes and foxes that must be there as well). That's 10 birds weighing 1 to 5 pounds a piece. They must consume a lot of rodents every week. The refuge is obviously large enough to support them (4600 acres) but just how large is the rodent population? And, I've wondered if its possible to estimate the minimum population of prey by the number of predators.

To do this, you've got to assume that even with all of the predation, the rodent population is steady. I know from some talks that I've attended on insect population, only a small fraction of the eggs need to survive to maintain a population. I did a little research and found an Introduction to Population Ecology site which has a table that details mortality factors for spruce budworms. Only 1.25% of the eggs need to survive to keep the species alive. I have no idea if this magnitude of losses would apply to rodents but I would think that it could be over 80%.

If anyone comes across information on this subject, I'd be interested in reading it.