Distributional expansion of Carpophyllum flexuosum onto wave-exposed reefs in north-eastern New Zealand

Abstract

The native fucalean macroalga Carpophyllum flexuosum (Esper) Grev. has recently occupied many reefs on wave-exposed coasts in north-eastern New Zealand. At recently colonised sites, C. flexuosum occurs mainly on coralline-dominated areas with moderate densities of sea urchins (Evechinus chloroticus). At these sites, stipes of C. flexuosum are shorter, with more branching, reduced laminae, and increased stipe weights compared to those at sheltered sites. Survivorship of tagged individuals was less on wave-exposed than on sheltered reefs. On sheltered shores, small C. flexuosum suffered moderate mortality, but large individuals suffered little mortality from any source. All individuals transplanted from sheltered to wave-exposed shores died or became moribund as stipes, whereas those which survived transplanting from wave-exposed to sheltered shores assumed a morphology more characteristic of sheltered sites. Population modelling suggested that the existence of stands of C. flexuosum on wave-exposed shores is dependent on regular recruitment, though it is not clear whether populations are maintained by recruitment from within or without. The establishment of C. flexuosum on wave-exposed shores is correlated with decadal scale decreases in wave energy. We predict that the patchiness of recruitment of C. flexuosum in space and time (and the ubiquity and reproductive characteristics of the dominant laminarian Ecklonia radiata) and its susceptibility to wave action will prevent the fucalean from inducing long-term changes of assemblage organisation.