In a good web, primary producers, like plants, generate food. Primary consumers, like herbivores (such as deer), consume these primary producers and in turn, are consumed by secondary consumers (such as lions, wolves, etc.). If, by some mechanism, all the primary consumers were removed from the food web (say by predation, death, lack of food, immigration, etc.), secondary consumers will have no food source to survive on. This situation will ultimately result in death of all the secondary consumers (or their movement out of the food web). To prevent this situation, we have to ensure the presence of primary consumers and that can only be possible by the presence of primary producers. Thus, we have to protect all the components of our food web, since as discussed above, degradation at one level will result in degradation at all subsequent levels.
Hope this helps.