Listen to a conversation between a student and an ecology professor.

Student: Hi, Professor Charlesworth. Professor: Hi John, how can I help you?

Student: Well, I've been working on our report for the field survey we did- for the Forest Ecology unit. Professor: OK, good.

Student: And the assignment said to count a tree as mature if it's more than two meters high. Professor: Right?

Student: And you can't close the fingers and thumb of your hand around its trunk? Professor: That's right.

Student: Well, I was working with Tim Jackson. He surveyed a hundred trees and I surveyed a hundred trees. Professor: Okay.

Student: But his hands are a lot smaller than mine, so...

Professor: Ah...okay, yeah, of course, it's not an exact measurement, just a guideline so that we're all talking in roughly the same terms when we say "mature tree." It is important to define our terms, but in ecology defining things can sometimes be challenging.

Student: Yeah, uh, in class yesterday when you were defining ecosystem, I didn't quite get it when you started talking about some of the fish that live in streams- salmon, I think. Professor: Yes.

Student: OK. Well, based on what you said about them-I couldn't tell-is a streaming ecosystem or not?

Professor: Well, the point I was making there is that there's still some debate about it.

Student: Yeah, cause you said the salmon swim to the ocean and grow there.

Professor: Yes. And then they returned to the stream where they were born and die there. So that does bring nutrients from the ocean ecosystem into the stream. Salmon are also sometimes preyed on by land animals like bears, in which case the nutrients in the salmon might also be transported out of the stream and might fertilize the terrestrial ecosystem.

Student: Then how do you define an ecosystem?

Professor: Well, there are different ways to do that, depending on your research focus. Take the wetland lab assignment we'll be doing next week.

Student: Oh, is that next week? You know I don't think I need to get much wetter after the forest assignment and rain the whole time.

Professor: Sorry, but that's ecological research for you. If we let a little rain stop us, where would we be? Student: I guess.

Professor: Anyway, the boundaries of the wetland can be really hard to find. Land characteristics can change as rainfall varies over time. Some people even say that all communities of organisms can be considered wetlands, because they depend on water to survive. But I don't think that's a very useful definition. So for our purposes, we'll be looking at two features: the type of vegetation we find. Student: OK.

Professor: And the level below which the ground is saturated with water. We'll use this information to identify where the wetland ecosystem gives way to what we call the upland ecosystem.