Question 18N.3.HL.TZ0.b.ii

The graph shows a theoretical model that divides species into five ecological groups (I to V) related to the degree of tolerance to an increasing pollution gradient.

[Reprinted from Marine Pollution Bulletin, 40, A. Borja, J. Franco and V. Perez, 'A marine biotic index to establish the ecological quality of soft-bottom benthos within European estuarine and coastal environments', pp. 1100–1114, Copyright (2000), with permission from Elsevier. https://www.sciencedirect.com/journal/marine-pollution-bulletin]

The biotic index is calculated using the formula shown.

$\text{BI}=\frac{\mathrm{\Sigma }\left({\text{n}}_{\text{i}}\times {\text{a}}_{\text{i}}\right)}{N}$