A demographic and experimental evaluation of plant and herbivore interactions in subtidal algal stands

Abstract

Some of the ecological factors influencing the local distribution and abundance of fucoid and laminarian algae in northern New Zealand were examined. Transect and quadrat sampling in a localised area showed a dominance of fucoid algae in shallow subtidal areas and laminarian dominance in deeper water. Biomass distribution reflected numerical abundance. Areas of intermediate depth were often barren of large brown algae and were characterised by an abundance of turfing red algae, sea urchins and herbivorous gastropods. This pattern of algal and herbivore distribution was typical of coastal sites of northern New Zealand. Offshore islands may have deep fucoid forests, unlike coastal sites. At all localities, however, both algal numbers and biomass tended to decrease with depth, regardless of whether or not sea urchins were present. Individually tagged plants of Sargassum sinclairii, Landsburgia quercifolia, Carpophyllum maschalocarpum and C. angustifolium were censused regularly in the field for 2 to 3 years. Sargassum growth, reproduction and survivorship were monitored between sites and at different areas within a site. There were differences in peak size, mean growth rate and reproductive output for algal plants at different sites and depths. These differences were not related to age. There were also significant differences in sizes reached and reproductive output for plants on different boulders at the same site. For all species examined, plants in high density, monospecific stands grew faster, became larger and bore more receptacles than plants of the same age at low density. Settlement experiments with Landsburgia zygotes showed poor survivorship for plants in both shallow and deep areas. Sargassum germlings had better survivorship in the shallow area than in the deep area, and higher survivorship overall than Landsburgia. For both species, plants grew faster in the shallow area after the leaves began to form. Canopy removal experiments showed that mature canopies of fucoid and laminarian algae inhibited recruitment of most species. Recruitment was much greater on substrata cleared to bare rock than on natural substrata with a cover of encrusting red algae. For each species, most recruits appeared within two metres of adult plants. Field experiments indicated that sea urchins removed juvenile algal plants in the proportion at which each species was encountered. Feeding preferences for large plants of each species were demonstrated experimentally, but these preferences bore little relation to the order in which algal species were removed by urchins in natural situations. Instead, algal removal was related to plant morphology and vulnerability to being pulled from the substratum by urchins. Herbivorous gastropods can also affect algal recruitment and may account for some of the local patchiness of algal distribution. The grazing effects of three species of gastropods were not the same; the turbinid, Cookia aulcata, had the greatest influence on algal recruitment. Because of differences in the timing of reproductive activity and recruitment of the various species, the timing of space availability can be an important factor in determining which algal species captures space. The laminarian, Ecklonia radiata, was the only species which was capable of invading stands of other species and could be considered the competitive dominant in potential, if not always in fact. However, in most cases a clear competitive dominant was not evident. Instead, free space was usually re-colonised by the algal species with a nearby canopy. The evidence from this study is that the subtidal algal system is driven, in the immediate sense, by the life history characteristics of the various species. Physical requirements such as light and nutrients provide broad borders for algal presence. Grazers are an important source of free space and help provide the broad scale mosaic of algal patches within a depth stratum. The relative roles of each of these influences are not predicted by the more traditional approaches to algal studies.