I agree with the last post - it would be interesting to understand how people affect the bird habitat and population in the park - joggers, bikers, birders, photographers, dump-trucks alike. I've seen people digging through bushes, walking into nesting grounds and protected areas trying to get 'the perfect shot' or 'see the birds up close' - each causing their own negative impact.
I'm sure the beaver has an impact - and it sounds like people are taking notice (re: wire-mesh around trees), but what other factors are at work here?