I don't have any experience in ultras, but here is my current view from the outside looking in. I welcome any ultra runners to educate me.
-ultra running is hard, it hurts, takes determination, etc.
same for marathons
-ultras must be specifically trained for to complete adequately.
same for every distance, depending on your definition of "adequately". Certainly not going to win a 200m race while training for 10k, or vice versa.
-speed at shorter distances does not guarantee success at ultra, but it helps. (e.g. 10k vs marathon)
same for almost every distance when compared to other distances.
-durability (from injury) is a larger part of the talent needed for success
Maybe I guess, if you have trouble upping your weekly volume? On the flip side of the higher chance of repetitive stress injury for long distances, straining a muscle like hamstring or groin is more likely to happen to sprinters than long distance guys.
-the ultra community is small and typically older, with a shallow young talent pool
same for marathons, the younger talent runs shorter distances until they "graduate" to the long ones. Recently the top marathon runners' ages have dipped down compared to the past. But WS was won by a 20yo. The 100mi AR was set by a 29yo. It's more a matter of younger runners generally taking time to build up from short distances, where the money is. And if you can make real money running 10K as a pro, you'd be stupid (or foolishly idealistic) to run 100mi for peanuts. That is far more the problem than it being the case that there's no talent for long distance.
-there is generally more emphasis on completion rather than competition.
among some... but no more emphasis than at the marathon, half-marathon, or hell even 5Km level races... 30,000+ people turn up to the Rock'n'roll half marathons and 98% of them only give a crap about completion. It's only in elite field competition for track that you see Competition over Completion as the only viewpoint, and there are ultras like that too. 24hr World Championships, 100k WC, Desert Solstice. There aren't as many elite-field ultra events because there aren't as many people trained to run at the level necessary, and you can't run at that level nearly as many times in a season as you can for say 5k. Three 24hr heats in a week or two? Ouch.
-if the talent that populates the shorter distances moved to ultra en masse, the current scale of slow vs good vs world class would be shattered.[/quote]
Same for 5k, half-marathons, marathons, etc... There's far more casual 5k runners than ultramarathoners, proportionally it's about equal. The last half-marathon I ran I came in top 10% of the field.... with a measly 1:45. Last Ultra I ran I came in at pretty much the exact same top 10%, with 70+mi in 12 hours. I generally place at about the same percentile in both short distances and ultras at the exact same fitness level.