I am a teacher in university in environmental science, but unfortunately students don't have enough motivation until the 4th term, when they have to pass a public course, so I would like to recommend them an interesting book about biodiversity.
Try the United Nations Environmental Program's Millenium Ecosystem Assessment, particularly the synthesis report, "Ecosystems and Human Well-being: Biodiversity Synthesis." It's free to download, from MAweb.org. While this won't serve as a basic textbook, it is easy to read and has basic information as well as data about the current state of the world's biodiversity, and it could serve as a supplement. I'll try to think of some more, as this is an area of interest to me.
You might also want to have a look at Biodiversity and Ecosystem Functioning: Synthesis and Perspectives, edited by Michel Loreau, Shahid Naeem, and Pablo Inchausti. It may be too much for an intro class, but it consists of a collection of papers dealing with different aspects of the impact of biodiversity decline on ecosystem functions and different dimensions of biodiversity, itself. One of the authors, Shahid Naeem, wrote another book on Biodiversity and Human Wellbeing, which I haven't looked at, but which looks interesting.
I've been looking at the impact of biodiversity decline on outbreaks of pests and pathogens, as a sidebar to my current research interest. I could recommend some good articles on that subject, if you like.
I can recommend "Conservation biology for all". This publication is available for free under the link: http://www.conbio.org/images/content_publications/ConservationBiologyforAll_reducedsize.pdf