The statistical mechanics of community assembly

Theoretically, communities at or near their equilibrium species number resist 34 entry of new species. Such ‘biotic resistance’ recently has been questioned because of successful 35 entry of alien species into diverse natural communities. Data on 10,409 naturalizations of 5350 36 plant species over 16 sites dispersed globally show exponential distributions for both species over 37 sites and sites over number of species shared. These exponentials signal a statistical mechanics 38 of species distribution, assuming two conditions. First, species and sites are equivalent, either 39 identical (‘neutral’), or so complex that the chance a species is in the right place at the right time 40 is vanishingly small (‘idiosyncratic’); the range of species and sites in our data disallows a neutral 41 explanation. Secondly, the total number of naturalisations is fixed in any era by a ‘regulator.’ 42 Previous correlation of species naturalization rates with net primary productivity over time 43 suggests that regulator is related to productivity. We conclude that biotic resistance is a moving 44 ceiling, with resistance controlled by productivity. The general observation that the majority of 45 species occur naturally at only a few sites but only a few at many now has a quantitative 46 [exponential] character, offering the study of species’ distributions a previously unavailable rigor. 47