1 Competing theories of community assembly are very difficult to test. Four main theories exist. The Stochastic theory sees species assembly as being random. The Humpty Dumpty/Alternative Stable States (ASS) theory suggests that a community may be unable to reassemble itself from its constituent species. The Deterministic theory suggests there will be convergence to one stable state. The Pre-adaptation theory is similar to the Deterministic theory but emphasizes that many species fit the stable state because of characters acquired elsewhere.
2 The reassembly of a flora into new communities in a different country, or its assimilation as a major component of such communities, offers a means to test these theories. The invasion of British plant species into New Zealand, and their reassembly into roadside communities there, is a good example of such a natural experiment.
3 Plant communities of NZ roadsides were compared to the communities of the British National Vegetation Classification (NVC). British roadside communities were also compared to the NVC as a control. New Zealand roadside communities provided a fit to the NVC communities of only 54.7% on average. After excluding species that are not present in NZ, and therefore could not possibly reassemble, the fit increased to 61.1%. British roadsides gave a 65.8% fit. The NZ figures are similar to the fit obtained with random data (58.7%), indicating that the NZ communities bear little relation to the ones formed by the same species in Britain.
4 Similarity between roadside communities in NZ and Britain was low, forming two almost distinct sets of communities.
5 Some of the predictions of the Stochastic, Humpty Dumpty / ASS and Deterministic models are borne out, but others are not. It is concluded that British species have reassembled into communities in NZ most of which are new, i.e. distinct from those that occur in the native range of the species in Britain. The evidence points to a process of community assembly by pre-adaptation.