Leimer AK, Boykin K, Anderson C.A., Steele C.  2019.  Applicability of functional groups as indicators of resilience and redundancy in the San Pedro Watershed, Arizona. AIMS Environmental Science. 6(3):127-146.

Whitford WG.  1996.  The importance of the biodiversity of soil biota in arid ecosystems. Biodiversity and Conservation. 5:185-195.