Horses and Weeds: To Blame or Not to Blame

I saw this interesting but ultimately frustrating piece in the LA Times a few days ago. For years horses have been blamed as part of the problem for the spread of non-native plants and noxious weeds into National Parks and Wilderness Areas in this part of the world. The reasoning goes that horses eat these plants and deposit the seeds in their manure where they later sprout in new locations. Apparently the NPS has required horsepackers operating in their parks to use certified weed-free feeds based on this.

Recently the NPS hired a university researcher to do a complete literature search on the subject of horse-spread weeds. The results of this showed that no empirical studies of the subject had been conducted. Everyone assumed that horses were an agent but no experiments had ever been done to actually prove it.

So the NPS paid these guys to develop a protocol for sampling manure, putting it in pots, watering it and seeing what sprouted. Sounds like a good idea to me. The preliminary results of the experiments reported here show a few non-native plants and no noxious weeds have sprouted and the article uses this for its headline: "Who Spreads the Weeds? Don't Bet on the Horse." The reporter asserts that this means the notion that horses spread weeds, "...looks like a myth." Later you see that this lack of weeds in the samples is attributed to - use of weed-free feeds.

So what good are the studies if you don't control for what the horses are eating? I'm sure Senator Proxmire is at least twitching in his grave. It's also scary that the reporter doesn't seem to realize his copy doesn't support his headline. This is an issue that really does need to be studied, but we're sure wasting time and money running in circles with this approach.