Here are my thoughts relative to your situation. First, I would buy a radio that will last a long time. Whether or not you are interested in radio systems that are already digital or not, I would purchase a radio that will handle it. Another factor to consider is if the CPU or central processing unit can be upgraded. The PRO-96 cannot be upgraded as it turns out and when the radio came out it was said that it could be upgraded. The PSR-500 (handheld) and PSR-600 (mobile) radios can be upgraded. The reason this is important is that a new band is being opened up right now, and it is the upper 700 MHz band. The final outcome of how that band will be allocated is not known and GRE, the manufacturer of the PSR radios, is already working on a CPU update that will handle some recent changes in the 700 MHz band allocation. These upgrades can be accomplished by the radio owner using a computer and the Internet, precluding the need to send the radio back to the manufacturer for upgrades.
You might not think you are interested in the new 700 MHz band as it will be used mainly in urban areas, however, you just don't know what is going to happen in the future. I bought a PRO-96 three years ago because I just could not wait to have a digital capable radio with me when I visited metro areas. I held off on getting the PRO-2096, the mobile version, even though it had ten times the memory capacity as the mobile scanner I was using. I waited until the PSR-600 came out as I knew that the purchase of the 2096 was going to cause greater expenditures in the future because it could not be upgraded. The same principle applies to trying to get a radio that is cheaper now. It might be cheaper right now, but because it does not have the latest features and capabilities it will have to be replaced sooner, and in the long run going with the cheapest now will cost more money.
You indicated that you wanted to listen in on National Park Service communications in order to get an indication of where wildlife is being sighted. My career experience and listening to land management agencies while traveling might provide some insight on this. In a National Park it is the law enforcement or "protection ranger" that handles most of the human/wildlife interactions. They will often call in or be called to these interactions and this generates radio traffic. Sometimes wildlife management personnel will be roving and will respond to these types of incidents as they will often want to record data on the animals involved such as size, appearance, or have some animals tagged. Wildlife biologists and other resource management people use the radio far less. They don't usually have a requirement to call in and out of service and don't generally talk with each other on the radio. Resource management personnel often have an aversion to using radios at all. Sometimes this is due to personality but is also because they don't want the information about the movements of animals becoming common knowledge. Yellowstone has experienced a fair amount of poaching for animal parts that are sold internationally, such as elk antlers and bear gall bladders. Most of the time sightings of wildlife are not reported on the radio unless there is a human management need caused by the sighting, such as traffic problems, people feeding wildlife, safety problems caused by people, or similar. It would be very surprised if any park personnel were to announce a particularly good sighting of wildlife if a human problem does not exist because to draw attention to the situation may cause a human problem they didn't otherwise have to handle.
Quite often communications regarding wildlife are very vague and hard to understand. Sometimes locations and species of animal are numbered or lettered and you might only hear something like "I have 53 160's in sight at EF" or something similar. Sometimes they call in locations obtained by GPS but use the "Universal Tranverse Mercator" (UTM) instead of longitude and latitude, and for some people not familiar with UTM it can be difficult to understand how to calculate the location.
There are exceptions to this lack of wildlife related radio traffic, one being the bear management personnel in Yosemite National Park who work all night and call sightings in to each other so they can "haze" black bears. Hazing is a process of using various methods to teach the bears that human encounters are not pleasant. Those methods being loud noises, rubber bullets, unpleasant odors, etc. If such methods are used in Yellowstone on either black or grizzly bears you might hear wildlife management personnel on the radio. Generally this type of activity is done on tactical or simplex frequencies. For a long time 168.350 and 163.100 were used as tacticals in almost every park. With narrow banding making far more frequencies available many parks are now obtaining tactical frequencies unique to that park. These frequencies are getting difficult to figure out as frequency guides, many of which were posted on the Internet, or available to many hobbyists with agency contacts, have become "for official use only." I'm retired from the Forest Service and those guides are no longer available to me, even when I promise to keep the information on a "my eyes only" basis.
Hopefully you have some outdoor experience, and some with wildlife. The wildlife in Yellowstone can be dangerous with the obvious animal being the grizzly, but the bison are also dangerous, given the behavior of well meaning, but inexperienced park users. Yosemite National Park has never had a human fatality from a black bear, but mule deer have killed four people since the Park's establishment in 1890. In the western U.S., some of the worst injuries and some fatalities from wildlife have involved elk. If I had a nickel for every sighting of dangerous human behavior around wildlife I ever made, where the person said "I was just trying to get a good picture," I would probably have a barrel full of nickels, or at least a good size bucket.
You should also be aware that Yellowstone is an "exclusive" jurisdiction, that is state and local authorities have no jurisdiction inside the park boundaries, unless the federal government has given them authority, such as under the Clean Water Law or Clean Air Act. Don't bother trying to listen to the Wyoming Game and Fish Department as they have no authority inside the park, nor do any of the surrounding state wildlife agencies (Idaho and Montana).
I show the following frequencies in use at Yellowstone: 166.375 repeater output/169.975 input for Mt. Holmes for the northern portion of the park and 165.5875 output/164.800 input for Mt. Sheridan for the southern portion of the park. With no specified location I show 167.150 output/163.125 input for an "Emergency" repeater(s). It is pretty old information so some changes could have been made, especially after the narrow banding requirements were established in 2005.
I wish I could get back up there this year. I was on a "self-guided, all expense paid tour" of Yellowstone during August - October in 1988. I was a crew boss assigned to the North Fork fire, which burned about 500,000 acres in the southwestern portion of the park. I would like to retrace some of my travels and see how the area has changed since the fires.
Hope this helps.
