Dear Ecolog: I would appreciate feedback/insight clarifying the difference between two mechanistic hypotheses explaining the species-area relationship (SAR) among discrete habitats that vary in size:
1) random placement (e.g. Coleman 1981) 2) passive sampling (e.g. Conner and McCoy 1979) My understanding is that random placement is a mathematical artifact of sampling that can explain an observed SAR: count more individuals and you increase the probability that you will have sampled a greater number of species. Larger habitats are predicted to contain more individuals than smaller habitats, hence larger habitats are more species rich. To test this, one would examine the relationship between number of individuals sampled (within the entire habitat?) and richness. The key here being that random placement stops short of saying anything about the DENSITY of individuals (per unit area) on habitats of varying size. Random placement can be viewed as the most parsimonious explanation for SAR, essentially the null hypothesis. Passive sampling differs from random placement in that it predicts that larger habitats support greater immigration rates than their smaller counterparts (hence larger habitats 'sample' a greater numbers of colonists to a habitat). This results is greater densities of individuals per fixed sampling unit on larger habitats versus smaller. Just as in random placement, more individuals increase the probability of sampling more species, so larger habitats are more diverse. Here, however, its because larger habitats actually 'attract' more colonists per unit area than smaller habitats. That's my understanding at this point. I often see these two hypotheses treated as one and the same, but it appears to me and others (see Matias et al. 2010, Ecology) that there are important differences between the two. I am grateful to any clarification/insight/confirmation. Thanks in advance, Brittany Huntington
