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 t
hat a community may be unable to reassemble itself from its constituent spe
cies. The Deterministic theory suggests there will be convergence to one st
able state. The Pre-adaptation theory is similar to the Deterministic theor
y but emphasizes that many species fit the stable state because of characte
rs 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 Zeal
and, and their reassembly into roadside communities there, is a good exampl
e 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 communi
ties were also compared to the NVC as a control. New Zealand roadside commu
nities provided a fit to the NVC communities of only 54.7% on average. Afte
r excluding species that are not present in NZ, and therefore could not pos
sibly 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 fo
rmed by the same species in Britain.
4 Similarity between roadside communities in NZ and Britain was low, formin
g two almost distinct sets of communities.
5 Some of the predictions of the Stochastic, Humpty Dumpty/ASS and Determin
istic models are borne out, but others are not. It is concluded that Britis
h 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 Bri
tain. The evidence points to a process of community assembly by pre-adaptat