Download (20MB) - Repository@Napier

Pattern Analysis - Grid Surveys
Survey design - Statisticians have usually condemned systematic sampling in favour
of random sampling (Krebs, 1989). However, McArdle and Blackwell (1989) stated
that when investigating spatial patterns a systematic sampling design (e.g., grid
sampling) will usually provide the smallest sampling error of the mean (Milne, 1959).
The possibility of a periodic variation in the variable being studied leading to a biased
estimate of the mean is negligible (Milne, 1959; Krebs, 1989). Another advantage of
grid sampling is the ease with which spatial patterns can be detected and displayed
(McArdle and Blackwell, 1989). Consequently, this type of sampling is
recommended in pattern analysis studies (Greig-Smith, 1952; Angel and Angel, 1967;
Pielou, 1969; Hill, 1973; Elliot, 1977; Jumars et al., 1977).
Pattern analysis was carried out by performing 3 separate surveys, each at a different
scale (lag). The sample designs for each of these grid surveys were very similar to
each other (Figure 2.2), except the lags were different in each case. The lags used for
the 3 surveys were lm, 8m and 40m and, from here-on will be referred to as the 1 m,
8m and 40m surveys respectively. These lags were chosen so that spatial patterns
could be investigated at a relatively small scale (1m survey), at an intermediate scale
(8m survey) and at a relatively large scale (40m survey) within the 400x250m study
area. The lm and 8m surveys were composed of plots forming an 8x8 array while
those of the 40m survey formed a 9x7 array so that the design could be positioned
within the study area. The positions of the lm and 8m surveys were determined by
random co-ordinates while the 40m survey covered the majority of the 250mx400m
survey area.
At each plot faunal abundances were assessed in two ways. A single box-core sample
(625cm2) was taken in an identical manner as described for the pilot survey: this
method sampled the majority of the faunal species present. The results from the
transect survey revealed that the lugworm Arenicola marina and the sandmason
Lanice conchilega could not be sampled in this way since the majority of these
individuals live deeper in the sediments than 15cm. Ragnarsson (1996) suggested that
there was almost a 1:1 relationship between A. marina individuals and their casts on
the Ythan Estuary, Aberdeenshire, and by direct observations (following Carey, 1982)
22