hypotheses. In benthic ecology, the same hypotheses recur: assemblages of animals
change progressively with depth, sediment type, organic richness, and character of the
overlying water. Perhaps that would be predicted without even looking at samples, but
it is fully confirmed by data from many sites, which is more satisfying.
(^) Radically simplified explanations of several commonly used ordination techniques
are provided in Box 14.1. Two books providing detailed explanations of various
methods with ecological examples are Legendre and Legendre (1998) and McCune
and Grace (2002). Like many other forms of writing (for example, instructions for
complex software), descriptions of ordination often assume that the reader knows
things that he or she does not. Because the authors know all the details and
definitions, they occasionally fail to consider that you might not know one or the
other. Also, ordination procedures are feasible largely because computers can carry
out the complex calculations, particularly so for nMDS (see Box 14.1). Once the
computer program is working, there is a tendency to just send the reader along to try
it, assuming that experience will somehow complete the transfer of understanding.
(^) Roper et al. (1989) compared the benthic fauna in the near vicinity of city sewer
outfalls debouching in the subtidal at about 15 m depth offshore from Gisborne and
Hastings on the eastern shore of North Island, New Zealand. They evaluated the
density of a very large number of benthic species at a series of stations close to and
variously far from the outfalls (Fig. 14.9 for Gisborne) along the same isobaths as the
outfalls. At Gisborne, the analyses included 42 species of polychaetes, 12 mollusks,
27 crustacea, four echinoderms, and four others – a total of 89 species generously
represented among 2735 specimens. The Gisborne [species x stations] matrix was
evaluated with PCA (Fig. 14.10) and showed distinct community modification near
the outfall (in agreement with a cluster analysis also applied). Roper et al. then
overlaid the positions of the stations in the PCA plot with the amounts of oil/grease in
the sediment at the stations, showing that the near-outfall stations (only two close in)
were indeed strongly polluted. The Hastings results were similar. Somewhat
surprisingly, areas affected by sewage outfalls are of modest size, a few kilometers.
They are larger for outfalls from very large cities, like the Hyperion outfall from Santa
Monica, California, USA, but still not as large as might be guessed from the tonnages
of organic matter coming down the pipes.
Fig. 14.9 Location map of Gisborne, New Zealand, sewage outfall with positions of
benthos sampling stations.
(^) (After Roper et al. 1989.)