We recently came back from a trip to Florida and one of my first questions to the friends we were visiting was about the invasive snakes.  This article popped up on the Twitter feed.  I like to use current events of relevant biology topics as they provide good introductions and starting points from which to build discussions.  I find students are more curious about pythons in Florida than garlic mustard or zebra mussles in Iowa, so these types of articles can be good to initiate topic exploration.  Students can then bring their learning and research and tie it into locally relevant examples.
http://smithsonianscience.org/2012/04/invasive-pythons-in-florida-now-eating-bird-eggs-straight-from-the-nest/