Case studies

California

Australia

Apart from the studies of the effects of kelp harvesting discussed in Chapter V, there has been almost no scientific work in European kelp forests that has had the objective of monitoring or surveying change within kelp biotopes. In other parts of the world, and notably in California, South Africa and Australia, however, scientists have been interested in or funding has been available for examining the effects of some environmental impacts on the inhabitants of kelp forests. Two methodological examples deserve mention here.

California

Schroeter et al. (1993) conducted a case-study of the impact of human activities on the invertebrates of kelp forests in Southern California, which attempted to separate human perturbations from the considerable natural temporal variability displayed by most populations, using the Before-After/Control-Impact (BACI) sampling design, in which Impact and Control sites are sampled contemporaneously and repeatedly in periods Before and After the human perturbation of interest.

In systems where plants and animals are long-lived and recruit only sporadically, the rates of change in population density are often so low that sampling more than a few times per year will introduce serial correlations in the data. As a result, for studies of only a few years’ duration, few samples will be taken. The resultant small sample size means that the tests of the assumptions underlying the statistical analyses (e.g. independence and additivity), will have low power. This injects uncertainty into the conclusions. Small sample size also means that the power to detect any but very large effects will be low.

In this study, sampling periods of 2-3 years both before and after the impact were not long enough to detect a halving or doubling of population at the impact site. Nevertheless, the authors concluded that there were significant environmental impacts because:

  • the effect size was generally very large (almost 75%);
  • there was a consistent pattern among species;
  • there were two Impact sites, and effects were larger at the site nearest the discharge;
  • the observed effects accorded with physical changes that could be linked with the source of impact;
  • a number of alternative mechanisms, unrelated to the source of impact, were examined and rejected.

Relative to control populations, there were statistically significant reductions in density of snails, sea urchins, and sea stars, all of which occurred primarily on rocky substrata, although populations of two filter-feeding species, a gorgonian coral and a sponge, showed relative increases in density. The authors concluded that:

"... monitoring studies of relatively long-lived organisms will often have low power to detect ecologically significant changes in density".

Although their study of kelp forest organisms extended over nearly 6 years, the resulting statistical tests generally had power of <30% to detect a doubling or halving in density at a significance level of 0.05.

"In such a community it would be a mistake to conclude that there were no significant ecological effects based on conventional hypothesis tests. Unless there is a willingness to accept the fact that changes in natural populations of the order of 50% will often go undetected, the standards and types of evidence used to demonstrate environmental impacts must be changed".

Australia

On the basis of a study of the fluctuations in the distributions and abundance of species in sublittoral kelp forests in New South Wales, Kennelly & Underwood (1992) concluded that the structure and dynamics of such systems

"... vary, and do not conform to predictions of simple models about processes in habitats dominated by one or a few large organisms. When kelp forests are described using quantitative data at several spatially replicated scales, few general conclusions can be made about the structure of these assemblages".

  • The implications of the above results for monitoring the effects of human and natural perturbations on the European kelp ecosystems are considerable.

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