Practice question

Sørensen similarity for presence–absence data is 2c/(a+b), where c is the number of shared species and a and b are the richnesses of the two communities. Counting shared taxa in the source dataset makes communities A and C the pair with the largest coefficient. Alpha diversity describes composition within a community, whereas pairwise similarity compares composition between communities. Abundance-based indices require relative frequencies; incidence-based coefficients require shared and unshared taxa. High richness does not guarantee high evenness, and high diversity within two sites does not guarantee that they contain the same species. Reading an index therefore requires knowing its formula, scale, and sensitivity to dominant versus rare taxa. In this context, the keyed term, A and C, identifies the relevant mechanism or quantitative relationship and links the observed pattern to its underlying ecological cause. The distinction is testable by measuring changes in organisms, resources, or process rates through time rather than relying on the label alone.