Treating Food Webs as Undirected Networks

After the discussion of the ecology papers in class on Friday I got curious about whether anyone had attempted to address the tendency of many people studying food web networks to treat them as undirected despite their directed nature. I spent some time doing some additional reading, and I found that the trend was pretty consistent through the papers that I found. The two papers that offered a justification for it (Williams et al., 2002; Dunne et al., 2002) both stated that a directed graph could be treated as undirected, because "effects can propagate through the network in either direction.". It seems to me that whether this is a valid assumption depends heavily on what exactly the network is being used to study. If one was studying the effects on the food web from the removal of certain species then perhaps it would be an okay idea to work under, because the effects of prey and predator removal would have similar effects on organisms throughout the food web. On the other hand if one was looking at transmission of toxins through an ecosystem then one would have to use a directed network to model it, because they are only going to move in one direction. In almost every other aspect of network analysis I would think that the directed nature of the food web is too much a key part of how the web works that it should not be ignored. There are perhaps a few situations in which an undirected food web could be applicable, but it seems to me that none of the articles that I found provided sufficient justification for an undirected food web.