Predation as a driver of gastropod distribution in north-eastern New Zealand kelp forests

Gastropods play an important ecological role in kelp forests; however, this role is dependent on where exactly in such communities these animals are located. We assessed the spatial and temporal patterns in the utilisation of available reef and macroalgal substrates by three gastropods in a north-eastern New Zealand kelp (Ecklonia radiata (C. Agardh) J. Agardh, 1848) forest, focusing on predation as a factor influencing these patterns. Although kelp provided a large habitable surface area for gastropods, much of this area was under-utilised. During the day, gastropods on E. radiata were largely confined to the primary laminae. At night, gastropods on the reef migrated into the algal canopy where they occupied regions of the kelp that were not occupied during the day. Over a 36-day period, 93% of the gastropods tethered to the reef were consumed by predators, whereas none tethered in the algal canopy was consumed. We consider that night-time vertical migration and dispersion through the algal canopy may be primarily a response to predation by diurnally feeding fish and nocturnal benthic predators such as lobsters. Our research highlights the importance of considering diel changes in epifaunal distribution and abundance when assessing their ecological role.

Establishment of the introduced kelp Undaria pinnatifida following dieback of the native macroalga Phyllospora comosa in Tasmania, Australia

The Asian kelp Undaria pinnatifida has recently invaded the coastlines of several countries across both hemispheres. Although the occurrence and subsequent spread of the alga has been well documented, the processes underpinning establishment and spread remain poorly understood. Recent work involving canopy manipulations has demonstrated that disturbance to the native algal canopy facilitates establishment of U. pinnatifida sporophytes at high densities, however, the kelp’s response to a natural disruption of the native algal canopy has not been assessed. In summer/autumn 2001, we examined the response of U. pinnatifida to the significant dieback of a common native canopy forming macroalga (Phyllospora comosa) on the east coast of Tasmania. The response of U. pinnatifida and native algae to the dieback was observed during the season for growth of U. pinnatifida sporophytes (spring 2001) and compared with adjacent areas where dieback did not occur. Undaria pinnatifida sporophytes established at high densities (6.75 ± 1.99 stipes m–2) in dieback areas, but remained rare or entirely absent in control areas where the native canopy was intact. The dieback also resulted in bleaching of encrusting algae and increased cover of understorey algae and sediment. The results support the findings of our recent artificial disturbance experiments, confirming the importance of disturbance events for the successful establishment of U. pinnatifida at high densities.