Yaël: So, Don, did you know that food webs are affected by even the smallest changes      at the bottom of the chain?

D: Uh, yes. Sure, Yaël.

Y: Do you have any idea what I'm talking about?

D: Uhm . . . no.

Y: OK. Another term for food web is food chain--insects eat plants, birds eat insects,      bigger birds eat smaller birds, even bigger animals eat large birds, and so on. With      me so far?

D: Yep.

Y: Good. Now, scientists have understood for a long time that everything on a food web      is interconnected. But one study actually demonstrates that changing even one      element at the bottom of a food chain can have far-reaching consequences.

     For example, researchers in the Netherlands and in England studied aphids--tiny      insects that are eaten by lots of other insects. In a field experiment, the scientists      compared a group of aphids that lived on Brussels sprouts to a group that lived on      cabbage. Because sprouts aren't as nutritious for aphids as cabbage, the sprout-      eating aphids were smaller and there were fewer of them.

D: And I bet that the bigger cabbage-eating aphids attracted more and larger predators      like wasps and other insects.

Y: Exactly. And the sprout-eating aphids attracted fewer predators. So if aphids are      thriving, that means wasps and other insects are thriving, which in turn means that      there's a greater diversity of other animals filling out the other parts of the web. But      something as small as a change in a single plant can affect not only the food chain but      wider ecological issues like pest control.  

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