The article What’s behind the biological classification system in use today? goes beyond memorization and the mnemonic devices used to remember the hierarchy (King Philip Came Over From Germany Smiling was my bio teacher’s favorite). The author reviews the rationale for classification and describes the cladistics mechanism used in the classification today. Although there are SciLinks entries with the keyword classification at all three levels (K–4, 5–8, 9–12) there is more background information on the  phylogenetic approach to classification at the 5–8 and 9–12 levels. Start with The History of Life: Looking at the Patterns and Using Trees for Classification.
The activity in How Symbiosis Creates Diversity can help students to understand the behaviors of symbiosis, mutualism, commensalism, and parasitism. The SciLinks for symbiosis seem to focus more on parasitism, but Ecological Communities: Networks of Interacting Species is a nice overview.
With a “wintry mix” of precipitation outside my window, I wish that I could join in on a Journey to the Reef. To complement a unit such as this, Scilinks has many resources on Coral Reefs. I especially like the Corals site from NOAA.

Clues to the Past uses the Bald Eagle as an example of a species made a comeback from the brink of extinction in the lower 48 states not long ago. Altough the SciLinks collection Mass Extinctions focuses mainly on dinosaurs, it might be interesting to compare and contrast the reasons for extinction as a follow-up discussion.
Feathering Your Nest shows how younger students used the process of observation, measurement, and communication to study the relationship between bird nests and their habitats. The pictures of bird nests suggested by the authors are interesting, especially since as the article notes, collecting bird nests without a permit may be illegal in your state.
I was blown away by the article Clash of the Titans, and not just by the photograph on the title page. The author shows how two very powerful instructional approaches – the 5E learning model and WebQuests can be integrated, rather than used as separate entities. The lesson described by the author also incorporated “driving” questions, graphic organizers, formative assessments, rubrics. It would be interesting to see additional lessons created in this way. Every time I read this article, I see more applications.
Check out the Connections for each issue. Even if the article does not quite fit with your lesson agenda, this resource has ideas for handouts, background information sheets, data sheets, rubrics, etc.