Hill, David Arnold; Matthew Fasham; Graham Tucker; Michael Shewry; Philip Shaw (2005). Handbook of biodiversity methods: survey, evaluation and monitoring. Cambridge, UK: Cambridge University Press. pp. 219–222. ISBN0-521-82368-4. OCLC61439712.
Hill, David Arnold; Matthew Fasham; Graham Tucker; Michael Shewry; Philip Shaw (2005). Handbook of biodiversity methods: survey, evaluation and monitoring. Cambridge, UK: Cambridge University Press. pp. 219–222. ISBN0-521-82368-4. OCLC61439712.