Spellerberg, I. F. (1991). Monitoring ecological challenge. Cambridge University Press.
Lua i Stíl Chicago (17ú heag.)Spellerberg, Ian F. Monitoring Ecological Challenge. New York: Cambridge University Press, 1991.
Lua MLA (9ú heag.)Spellerberg, Ian F. Monitoring Ecological Challenge. Cambridge University Press, 1991.
Rabhadh: Seans nach mbeach na luanna seo go hiomlán cruinn i ngach uile chás.