Assembly rules operating along a primary riverbed-grassland successional sequence

1 Assembly rules are broadly defined as any filter imposed on a regional species pool that acts to determine the local community structure and composition. Environmental filtering is thought to result in the formation of groups of species with similar traits that tend to co-occur more often than expected by chance alone, known as Beta guilds. At a smaller scale, within a single Beta guild, species may be partitioned into Alpha guilds - groups of species that have similar resource use and hence should tend not to co-occur at small scales due the principle of limiting similarity. 2 This research investigates the effects of successional age and the presence of an invasive exotic species on Alpha and Beta guild structuring within plant communities along two successional river terrace sequences in the Waimakariri braided river system in New Zealand. 3 Fifteen sites were sampled, six with and nine without the Russel lupin (Lupinus polyphyllus), an invasive exotic species. At each site, species presence/absence was recorded in 100 circular quadrats (5 cm in diameter) at 30-cm intervals along a 30-m transect. Guild proportionality (Alpha guild structuring) was tested for using two a priori guild classifications each containing three guilds, and cluster analysis was used to test for environmental structuring between sites. 4 Significant assembly rules based on Alpha guild structuring were found, particularly for the monocot and dicot guild. Guild proportionality increased with increasing ecological age, which indicated an increase in the relative importance of competitive structuring at later stages of succession. This provides empirical support for Weiher and Keddy's theoretical model of community assembly. 5 Lupins were associated with altered Alpha and Beta guild structuring at early mid successional sites. Lupin-containing sites had higher silt content than sites without lupins, and this could have altered the strength and scale of competitive structuring within the communities present. 6 This research adds to the increasing evidence for the existence of assembly rules based on limiting similarity within plant communities, and demonstrates the need to incorporate gradients of environmental and competitive adversity when investigating the rules that govern community assembly.