Characterising alligator weed (Alternanthera philoxeroides; Amaranthaceae) invasion at a northern New Zealand lake

Exotic plant invasions are a key threat to New Zealand biodiversity. Alligator weed (Alternanthera philoxeroides; Amaranthaceae) is an invasive, herbaceous weed native to South America. Little is known about its dynamics in natural ecosystems in its introduced range, despite known agricultural impacts. We quantified alligator weed infestation at Lake Rotokawau, Northland, and investigated alligator weed’s relationship with other vegetation, both native and exotic, over a year (Nov. 2005 to Sep. 2006). We also examined the relationship between native vegetation and ‘other’ exotic vegetation at the site. Alligator weed, at its peak in spring, covered over 20% of the surveyed lake margin. Plant community composition of plots without alligator weed differed significantly from invaded plots even when alligator weed itself was removed from the analysis. Uninvaded plots were characterised by low beta-diversity and predominantly terrestrial plant species, with Phormium tenax contributing 41% of within-group similarity. In contrast, invaded plots had higher beta-diversity and were characterised by a variety of emergent sedges and herbs. Alligator weed cover was negatively related to cover of natives but not cover of ‘other’ exotics. Alligator weed cover was not related to species richness of natives or ‘other’ exotics. ‘Other’ exotic species were positively related to native cover and richness, likely due to shared responses to favourable environmental conditions.