Feature

Breaking Neptune's Necklace

Your casual beach walk across seaweed can have surprisingly drastic consequences for intertidal ecology.

David Schiel

Research on rocky intertidal shores in temperate areas of the world has generated much of the ecological theory relating to factors that affect the structure of populations of organisms. In many ways, rocky shores are natural experimental laboratories, where most plants and animals (such as barnacles, mussels, algae) are attached to the substratum, where many predators (such as whelks and starfish) can be abundant and fairly confined in their movements, and where grazers (such as limpets) are abundant.

The physical environment of rocky shores varies considerably across small areas because the daily rise and fall of the tides provides a gradient of exposure to desiccation and heat stress. Also, because coastlines vary in their shape and orientation, even sites that are close together can experience quite different wave forces.

In contrast to terrestrial systems, in which the abundance and distribution of the major interacting components of natural communities -- predators, grazers, vegetation -- usually vary over several kilometres, these components can be dramatically different over even a few metres on marine shores. Unfortunately, the localised nature of marine populations also makes them particularly vulnerable to disturbances, both by natural forces such as storms and by human activities such as fossicking and trampling over plants and animals.

We are testing ecological hypotheses relating to the effects of disturbance to shores, while developing an understanding of the factors affecting intertidal populations and the spatial and time frames in which they act. Much of this work is being done on intertidal rock platforms along the east coast of the South Island, at Cape Campbell, Kaikoura, and Moeraki.

Perennial brown algae, particularly Hormosira banksii (known commonly as Neptune's necklace because it looks like a string of beads), dominate the middle and lower shores, and beneath it turfs and crusts of tough, calcified red algae. Interspersed along reefs are bare patches dominated by large numbers of grazing limpets, and patches of seagrass.

The effects of disturbance, and the timing and process of recovery of algae after disturbances were tested by removing the Neptune's necklace at different levels of the shore in different seasons. Because Neptune's necklace populations are reproductively active virtually year-round, it was expected that recovery would be relatively rapid. Much to our surprise, however, it took over three years to recover from these small disturbances.

The reasons are complex. When Neptune's necklace is removed, the underlying red turf is exposed to the sun and burns away, leaving bare rock. The rock is mostly mudstone, which continually erodes and is generally too unstable for Neptune's necklace spores to attach to successfully. The firmly attached and relatively stable red algal crusts are usually necessary for successful recruitment of Hormosira. However, the crusts are extremely slow to recover from disturbance, and most of those in the upper algal zone have still not recovered 4 years after the experiments started.

Furthermore, Hormosira must recruit in extremely high densities to produce viable adult populations, a situation that occurs no more than once a year. Finally, Hormosira plants grow very slowly (about 30 mm per year) so recovery after disturbance is a slow, chancy process driven by the life histories of the algae interacting with their immediate environment.

These results, and others obtained from experimental manipulation of algae, grazers and predators on the shore, stand in contrast to much of the marine ecological literature, particularly that of North America. The "textbook" explanation of marine population structure usually involves either a dominant "keystone species" (such as a starfish) removing dominant space-occupiers (such as mussels), which lets other species recruit; or else, when a keystone species is not present, the frequency and intensity of disturbance allows differing successions of species into the population structure. Neither of these explanations accounts for what occurred in this study, where only two perennial algae (calcified red algae and Neptune's necklace) ever came into disturbed areas.

Managing Tourists

In addition to testing ecological principles, this work is also providing information necessary for informed management of intertidal platforms in New Zealand. For example, the effects of pedestrian traffic on platforms was tested by small-scale trampling experiments -- as few as 10 people walking along a bed of Neptune's necklace produced effects mimicking those of the disturbance experiments described above. With the equivalent of 50 walkers, the effects were much more dramatic, leaving a legacy of bare space and slow recovery.

It is becoming increasingly common for bus loads of tourists to walk onto platforms, especially at Kaikoura, and it is not difficult to see where they have been because of the bare corridors in algal beds. Similar effects have been documented in work on seagrass patches, which are also easily damaged by large numbers of walkers and occasional motorbike riders.

Dr David R Schiel is in the Department of Zoology at the University of Canterbury.