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.
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
- 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
- a number of alternative mechanisms, unrelated to the source of impact, were examined and
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
"... 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".
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.