Figure 7.

How the value of N_Tm and m can be determined. (a) For a given mean source community abundance, the frequency with which an organism is observed is related to N_Tm, the total number of individuals multiplied by the immigration parameter. At high N_Tm values, the local distribution is tightly clustered around the mean meta-community distribution (in this case 0.2). As the N_Tm values drop, the distribution widens and eventually the mode of the curve falls below the detection limit and the organism is no longer observed. There is therefore a relationship between the mean source community abundance N_Tm and the frequency with which an organism is observed. (b) The expected frequency–relative abundance relationships observed in a local community for differing N_Tm values. Thus, for a given dataset, the N_Tm values can be found by fitting a line to these data; where the value of N_T is known, m can be easily inferred.