1 Community structure was examined by analysing the spatial distribution of
species' biomass-abundance, as well as species' occurrences, in two semi-a
rid grassland sites in New Zealand. This is the first attempt to search for
a range of assembly rules (variance in species occurrence, guild proportio
nality and intrinsic guilds) using quantitative data.
2 One site, with a greater proportion of native species, had a larger speci
es pool, greater species richness and greater biomass. At this site, varian
ce in richness was generally greater than expected at random, but variance
in biomass tended to be less than expected at random, probably indicating c
ompetition. Microsites which had lower biomass and fewer species tended to
be dominated by exotic species; it is suggested that these are more recentl
y disturbed microsites.
3 At a second site, dominated by exotic species, there was no significant e
vidence for community structure, i.e. no non-random patterns in richness or
biomass, i.e. no assembly rules.
4 Guild proportionality was not seen in a priori guilds: dicots, monocots,
cryptogams, native species and exotic species.
5 Searches for intrinsic guilds produced no evidence of guild structure, in
the Pianka sense of groups of species that use similar resources, compete
strongly with each other, and thus are especially subject to mutual competi
6 It is concluded that interspecific competition limits the biomass of each
species in these grasslands, but there is no evidence from the community s
tructure that it causes local competitive exclusion. There was no indicatio
n at either site that the communities were structured by guilds. It is conc
luded that, at least at the scale examined and at the time examined, Pianka
-type guilds do not exist in this vegetation.