The Phi statistic is often used to represent Indicator Species. For example, the popular R package indicspecies relies heavily on reporting the Phi statistic from Chytrý et. al 2002

Indicator Species are those which signal some feature of the environment, some location, or some grouping. For example, salamanders species as a whole are indicators of stream water health, but the Red Hills Salamander (Phaeognathus hubrichti) may be an indicator of the Red Hills Region of Alabama, USA. Sometimes indicator species are used to represent the best species in some statistical grouping. 

The Phi statistic in Tichy and Chytrý 2006 scores individuals presence and absence in groups to deliver a value between -1 and 1. 

Example: Wetland vs Dry Forest, using Wetland as our "target site group".

- A value of 0 indicates a species does not show any presence any preference or avoidance to a particular group.

      - Species has no preference for wetland or dry forest. 

- A value of 1 indicates a species is present in all sites of the target, but absent in all non-target sites

      - Species is only found in the wetland, not the dry forest. This species is indicative of wetlands.

- A value of -1 indicates a species a species is absent in all target sites, but is present in all non-target sites. 

      - Species is not found in the wetland, only the dry forest

Further Reading: 
1. Chytrý, M., Tichý, L., Holt, J. and Botta-Dukát, Z. (2002), Determination of diagnostic species with statistical fidelity measures. Journal of Vegetation Science, 13: 79-90. https://doi.org/10.1111/j.1654-1103.2002.tb02025.x

Open Access Examples: