Size Related Bias in planktonic Foraminifera

Arnold et al. (1995) agree that there is an observed trend towards larger size present in the foraminifera which they study in their paper, but the question they pose is whether the assumed explanation for this trend, an organismal adaptation, is valid (Arnold et al., 1995). To test Cope’s rule, this study takes into account a size bias which occurs during collection of the foraminifera samples. Specimens are commonly collected in a sieve with 150-micrometer opening to allow for sand and silt to filter through (Arnold et al., 1995). This sieving procedure would make the likelihood of recovering smaller specimens very low. In order to counteract the effect of the sieving procedure, the authors imposed an upper size limit to the data in a symmetrical fashion to the lower limit (Figure 3)(Arnold et al., 1995). The authors then used the data contained between the two limits to test for the cause of Cope’s rule.

Figure 3. Lower size bias imposed by straining techniques and equal upper size bias imposed in the laboratory to counterbalance the effects. (after Arnold et al., 1995)