Some may consider my suggestion quaint (and I also wager many of those same people haven't even read it).  I strongly suggest The Origin.  So many fundamental ecological questions are addressed (even introduced) in it that many people don't give Darwin credit for.  Once you are comfortable with the Victorian prose, it becomes a really intriguing read.  Also, I was surprised to find out how many of my colleagues in biology had not yet read it.

When I read The Origin, it was in a seminar setting with eight - ten faculty members and graduate students; every week we would read a chapter and discuss it, then later in the week we would read a modern primary journal article that built off the ideas discussed in said chapter.  If you haven't read the book, and you need some convincing, read chapter 3, "Struggle for Existence", it is an ecological tour de force condensed into 19 pages.

Added bonus: it is free! http://www.gutenberg.org/files/1228/1228-h/1228-h.htm

I strongly suggest the first edition - Darwin watered it down quite a bit in subsequent editions.

I would definitely assign "The third window" by Robert Ulanowicz. I read it three times in a row....

Recently, I found Resolving Ecosystem Complexity by Os Schmitz (http://press.princeton.edu/titles/9380.html) really heuristic.  It is by no means a classic (too new etc...), but it is a really great explanation of how reductionist research can help explain complex systems across space and time.  It has been very inspiring to some of my students.

Wow, that's a great idea. A good way to start them talking ang thinknig. But it also depends on how much students today are even willing to read...

How about some dystopian literature like Orxy and Crake by Margaret Atwood? She's a great nature lover and an environmentalist at heart and it shows. Or even worse - The Road by Cormac McCarthy. Depressing, but good and unfortunately, environmentally relevant...

Or something by Richard Dawkins? He's easy enough to read and to understand and can be very thought-provoking.

Last Chance to See by Douglas Adams and Mark Carwardine would also definitely work, it's funny and moving at the same time.

I think I'll try some of these out on my students next semester!

Desert Solitaire by Edward Abbey

Some may consider my suggestion quaint (and I also wager many of those same people haven't even read it).  I strongly suggest The Origin.  So many fundamental ecological questions are addressed (even introduced) in it that many people don't give Darwin credit for.  Once you are comfortable with the Victorian prose, it becomes a really intriguing read.  Also, I was surprised to find out how many of my colleagues in biology had not yet read it.

When I read The Origin, it was in a seminar setting with eight - ten faculty members and graduate students; every week we would read a chapter and discuss it, then later in the week we would read a modern primary journal article that built off the ideas discussed in said chapter.  If you haven't read the book, and you need some convincing, read chapter 3, "Struggle for Existence", it is an ecological tour de force condensed into 19 pages.

Added bonus: it is free! http://www.gutenberg.org/files/1228/1228-h/1228-h.htm

I strongly suggest the first edition - Darwin watered it down quite a bit in subsequent editions.

I'm a fan of Silent Spring, but maybe that should be read before grad school. 

I have to agree with Thomas. The Origin of Species is chock full of insightful ecology, and a constant reminder of the timeless advice, "If you think you have an original idea, first go back and check Darwin".

+1 for The Origin of Species. Definitely a must-read. I'd also recommend some of Richard Dawkins's books such as The Selfish Gene and The Greatest Show on Earth.

I got really inspired by reading "The Song of the Dodo: Island Biogeography in an Age of Extinction" by David Quammen when I was a student. Quammen follows the ideas and journeys of important naturalists but also looks at isolation and extinction today.

We also read "The Origin of Species" in a seminar and enjoyed it a lot.

I think these are all excellent ideas.  I realize the potential with The Origin but I guess I sort of thought that was a given for all ecology students to read at some point.  But as one person pointed out, it's likely many have not or never will read it.  I was sort of thinking along the lines of influential texts that students might not necessarily think to read if it wasn't assigned to them. Perhaps classics like Systematics and the Origin of Species (Mayr) or Adaptation and Natural Selection (Williams) or maybe some more recent contemporary contributions like a few noted above.  Thanks for all the suggestions and it would be great to hear more ideas!