Abstract
The Hewatt transect is a permanently marked area perpendicular to shore running through
the protected rocky intertidal zone at Hopkins Marine Station. Species abundance data
collected in 1933-37 and 1993-95 showed significant differences for many species during
the intervening years. In order to determine how representative the Hewatt transect is of
the protected rocky intertidal zone at Hopkins, a 90 x 15 m area surrounding the Hewatt
transect was compared to three adjacent areas of the same size. Three organisms were
sampled in two or three 15-m transects within each of the four areas. Ten quadrats were
counted in each transect. Additionally, the substrate rugosity for each area was measured.
For Anthopleura sola, a solitary anemone, there were significant differences among areas
(P=.02), but not among transects-within-areas (P=. 16). For Serpulorbis squamigerus, a
tube dwelling gastropod, there were no significant differences at either level (P=. 15 and
15). For the alga Endocladia muricata, there were no significant differences among
areas (P=.23), but highly significant differences among transects-within-areas (P.001)
Substrate rugosity showed no significance between areas (P=. 72). These data indicate
that while there are some differences between the Hewatt area and adjacent areas, there is
no evidence that the Hewatt area is non-representative of the HMS intertidal. While the
intertidal zone is certainly not homogeneous, it is possible to conclude that the changes
seen in the 60 year interval were due to significant species abundance change and not
stochastic variation. Additionally, the environmental processes that caused these changes
are likely to have influenced the distribution of organisms across the intertidal zone.
Introduction
In the 1930's, Hewatt (1933) conducted an ecological survey of a permanent
transect he established in the protected rocky intertidal at Hopkins Marine Station. This
study provided extensive information on species distribution and abundance. Sagarin and
Gilman (1993) relocated the transect and resampled 19 of Hewatt's original quadrats.
Their data showed significant changes for many species. Notably, many species which
increased in abundance had geographic distributions with northern boundaries at Cape
Mendocino ("southern" species), and the species that decreased in abundance had
geographic distributions with southern boundaries at Point Conception ("northern'
species). Concurrent increases in sea surface temperatures implicated global climate
change as a possible cause of these changes (Barry et al. 1995). This result has important
implications for the potential environmental impacts of global warming in intertidal
communities.
While many anecdotal observations of changes in species abundance have been
made, quantitative studies such as this are necessary to provide robust evidence of
environmental change. Unfortunately, with limited amounts of reference data such as
Hewatt’s, these studies are rare. Furthermore, they are confounded by natural variation.
In order to make inferences concerning potentially crucial changes occurring in large
areas, it is necessary to understand the spatial and temporal scales at which nature varies.
The rocky intertidal is a highly variable environment. A great deal of work
conducted in Australia (Underwood and Chapman1996, 1998; Underwood 1998) has
focused on this problem, because anthropogenic stress on the southern coastlines of
Australia is increasing and may be affecting intertidal communities. The purpose of this
research was to better understand spatial patterns prior to attributing causation for
temporal changes. Natural spatial variation makes anthropogenic impact difficult to
quantify. A number of statistical techniques have been developed to quantify spatial
patterns so that changes can be detected (Underwood 1998). A primary conclusion of
these studies is natural variation is high and spatial patterns are difficult to quantify.
The potential anthropogenic impacts upon the rocky coast of California are also
huge, and spatial variation is equally high. Fu (1995) studied organism abundances at
various spatial scales and found that different species were patchy at different scales.
Considering such high and inconsistent levels of variation, it is appropriate to question
how representative the Hewatt transect may be of the intertidal zone at Hopkins Marine
Station prior to making inferences about environmental change. This study was designed
to address this question by sampling two intertidal invertebrates and one intertidal alga.
The animals I chose are both southern species (Barry et al. 1995) that have significantly
increased in abundance along the Hewatt transect. This makes them particularly
interesting in the context of climate change hypotheses. In order to conclude that
southern species are able to "invade" the intertidal zone because of rising sea surface
temperatures, it is necessary to show that increased abundances noted along the Hewatt
transect are representative of areas elsewhere in the intertidal zone.
Two of the study organisms are sessile invertebrates. The anemone Anthopleura
sola is abundant and widespread in the mid-intertidal zone. A sessile predator during high
tide, it retracts its tentacles and its shell-covered body wall becomes visible when tides are
low. Serpulorbis squamigerus is a low-intertidal tube building snail. It is a suspension
feeder, spinning webs of mucus to catch particles as they drift by (Morris et al. 1980). In
previous years, it was present as a solitary, primarily subtidal species, but recently it has
become extremely abundant in the intertidal zone, forming mats of coiled tubes. The third
species I sampled was the alga Endocladia. While Hewatt (1933) did not survey algae,
some understanding of the variation in algal cover is useful because algae is often the
predominate occupant of space in the intertidal, and plays an important role in determining
the local animal community. Simons and Leydig (1996) found a 1 ft. downward shift in the
Endocladia zone compared to data from the early 1960's (Glynn 1965), as well as
associated changes in its assemblage of organisms. This suggests Endocladia may be an
indicator species for intertidal change.
Materials and Methods:
This study was conducted in the sheltered rocky intertidal area west of Bird Rock
at Hopkins Marine Station in Pacific Grove, CA, USA (36° 37' N, 121° 54' W). This
study site was determined by the location of the Hewatt transect. The transect runs 100
yards perpendicular to shore and is marked by brass bolts. I randomly sampled within an
approximately 90 x 15 meter area that contained the Hewatt transect and compared it to
three adjacent areas of comparable size and tidal height. The four areas were designated
West, Hewatt, East1, and East2 (Fig. 1, Fig. 2). Areas closer to Bird Rock were excluded
as they incorporated primarily high intertidal areas. The four study areas are crossed by
two permanent channels that run parallel to shore. The zone is also protected along its
seaward margin by high rocks, preventing high wave exposure in the interior.
Study design and Sampling
A preliminary study was conducted to determine the optimal quadrat size for A.
sola and S. squamigerus. Both animals were counted in haphazardly placed quadrats of
25-m and 1-m sizes. Precision, a measure of repeatability, was calculated by the
following equation:
P-SE/X
where SE is the Standard Error of the sample and X is the sample mean.
For S. squamigerus, 25-m quadrats gave the greatest precision, while for A. sola
1-m’ quadrats were more precise. The alga Endocladia was surveyed in 25-m" quadrats
without any comparison to other quadrat sizes.
This study used a nested Analysis of Variance (ANOVA) design (Fig. 3). For each
species, two or three 15-m transects were sampled in each of the four areas. Ten quadrats
were randomly located and counted per transect. For Endocladia and Anthopleura, three
transects per area were haphazardly placed through the mid-intertidal zone (approximately
2 - 4 feet above MLLW). Whenever possible, these transects were kept roughly parallel
to the Hewatt transect. For Serpulorbis, two transects were haphazardly placed within the
low zone (approximately -1 to +1.5 feet above MLLW). There was insufficient space for
three non-overlapping low zone transects within each area.
Animals were counted if they fell within the randomly located quadrat. Tube snails
were counted as number of apertures within each quadrat rather than tubes. Individuals
along the edges of a quadrat were counted if their aperture fell inside the quadrat. For A.
sola, a relatively large species, animals were counted as halves on the rare occasion that
they fell exactly on the edge of the quadrat. For Endocladia, percent cover was estimated
using a 25m' quadrat subdivided into 25 5 x 5 cm squares.
For each quadrat sampled, the estimated average tidal height was measured using a
surveyor's transit and stadia rod. Measurements were cross-referenced with a brass bolt
that marks the beginning of the Hewatt transect at 3.82 feet above MLLW.
Physical Measurements
In addition to ecological sampling, physical topography measurements were taken
in each area. A straight line distance of 1.88 meters was measured with plumb lines hung
from a leveled PVC pipe. The surface distance between those points was determined
using fine link chain that conformed to irregularities and cracks in the rock. This length,
divided by 1.88, was used as an index of the substrate rugosity. Five replicate lengths
were haphazardly placed per area at random compass bearings.
Data Analysis
Cochran's tests for heterogeneity of variances were performed on the within-
transect variances for Serpulorbis, Anthopleura, and Endocladia. For each organism, the
test was significant (PS.05), indicating their variances were too heterogeneous to meet the
assumptions of ANOVA. To decrease the heterogeneity of variances, Serpulorbis data
were log transformed (log (x+1)). Anthopleura data were square root transformed
(V(x+.5)). Endocladia data were arcsin transformed (sin (x)). After transformation,
variances were no longer heterogeneous.
The data were analyzed using a two-factor nested ANOVA. The tests were
conducted at q= 10 rather than the more conventional .05 for the following reasons. Two
types of statistical error are possible: Type I occurs when a true null hypothesis (Ho) is
rejected; its probability is a. Type II, with probability B, is the opposite mistake of
retaining an incorrect null hypothesis. Type I errors are more serious in biological
experiments, because the acceptance of an incorrect alternative hypothesis will probably
not lead to further experiments. By contrast, Type II, or the acceptance of an incorrect
null hypothesis, probably will lead to further experiments and the error will be detected.
In this study, a larger a was considered appropriate due to the potential result of
Type I and Type II errors. With Ho of "no differences among areas," Type I error would
conclude the Hewatt transect is not representative of the intertidal at Hopkins Marine
Station when it actually is, and no inferences about the generality of environmental
processes would be made. Conversely, Type II would suggest the Hewatt transect is
representative of the intertidal when it is not, which in this case is an equally serious error.
The probability of Type II error, ß, cannot be specified without a specific (quantitative)
alternative hypothesis (Hi), but it is clear from Fig. 4 that their probabilities are
complementary: an increase in a would decrease ß, and vice versa. For this reason, a
more conservative approach that would balance the likelihood of either error was to set
a=. 10 rather than the conventional .05.
Differences among areas for each species were compared using Tukey's post-hoc
tests.
Rugosity data were analyzed using a single factor ANOVA.
Results
Anthopleura densities ranged from 0-23 per m’ (Fig. 5). There were no significant
differences between transects-within-areas (P= 16, Table 1), but there were significant
differences between areas (P=.02, Table 1). Post-hoc comparisons showed that the
Hewatt area did not differ from either of the other three, the only significant differences
were between West and East1 and West and East2 (Table 2). Furthermore, 77% of the
summed variance components was associated with residual, or among-quadrat, variation.
while only 4% was associated with transects-within-areas, and 19% was associated with
areas (Fig. 9).
Percent cover of Endocladia ranged from 0-93% (Fig. 6). There were highly
significant differences among transects-within-areas, especially in West and Hewatt (Fig.
6, Table 1). However, there were no significant differences between areas (P- 23, Table
1). Post-hoc comparisons among areas revealed no significant differences between any of
the areas (Table 2). Fisty percent of the summed variance components was associated
with among-quadrat variation, 39% was associated with transects-within-areas, and 11%
was associated with areas (Fig. 9).
Serpulorbis densities ranged from 0-856 individuals per m’ (Fig. 7). There were
no significant differences among areas (P= 15, Table 1), or among transects-within-areas
(P=.15, Table 1, 2). Sixty-eight percent of the summed variance components was
associated with among-quadrat variation, 1 1% was associated with transects-within-areas,
and 23% was associated with areas (Fig. 9).
For rugosity indices, conformed distances ranged from 1.96 to 3.60 meters, with
corresponding ratios of 1.04 and 1.91. Means per area ranged from 1.38 through 1.56
(Fig. 10). There were no significant differences between areas (P=.72, Table 1)
Discussion
The study attempted to answer the question of how representative, not how
identical, the Hewatt transect is of the rocky intertidal at HMS. Considering the high
spatial variation in this environment, some differences will always be present. However, in
order to be representative, comparisons of the Hewatt transect and other areas should
show no more differences than the other areas do when compared among themselves. A
consistent pattern of differences between the Hewatt transect and adjacent areas would
indicate the transect is not representative at all.
Post-hoc Tukey's tests were used because they have the ability to analyze all
possible pair-wise combinations of areas. These tests showed no significant differences
between any areas for Serpulorbis and Endocladia. For Anthopleura, two out of six
possible comparisons were significant and neither of them involved Hewatt. The
significant difference (P=.020) noted in the overall ANOVA occurred because West was
significantly different from both East l and East2.
The significant differences between West and East l/East2 for Anthopleura are
most likely related to microhabitat variation. West is located just prior to the seaward rim
of the intertidal, and probably receives considerably higher wave exposure. Anecdotally, it
is also host to another species of anemone, Anthopleura xanthogrammica. A.
xanthogrammica is typical of wave exposed areas (Ricketts et al. 1985). The two East
areas, by contrast, receive far less wave exposure than West. The substratum in these
areas tends to have many more shallow crevices, where A. sola is most abundant. It is
interesting to note that the statistical analyses for differences in substrate rugosity did not
differ among areas (Fig. 10, Table 1)
The low abundance of Endocladia found in Hewatt (Fig. 6) is due to sampling
artifact: nearly all quadrats fell on inappropriate tidal heights. Fig. 7 shows a skewed
distribution between quadrat count and average tidal height for each of the four areas.
Most of the zero values occur in Hewatt below 2.5' above MLLW. Endocladia is
abundant between fairly specific upper and lower boundaries, and the lower boundary is
approximately 2.5' above MLLW (Simons and Leydig 1996). If quadrats with zero
percent cover extended along the x-axis of Fig. 7, I would conclude that the dearth of
Endocladia was unique to Hewatt. However, as the zero values are only found below the
lower boundary for the alga, and since many Hewatt quadrats are found at or below this
height, it is more likely that portions of Hewatt are simply too low for Endocladia.
While these tests show that the Hewatt transect is representative of the intertidal
area at Hopkins Marine Station, they raise the general question of spatial heterogeneity in
the intertidal. Variation occurs among areas and among transects-within-areas, but based
on the components of total variation, the greatest variation occurs between quadrats.
This is due to differences in microhabitats that average out at larger spatial scales.
While there is a great deal of small scale variability in the intertidal zone at
Hopkins Marine Station, there is no consistent pattern suggesting the Hewatt transect is
much different from surrounding areas. Considering these results, it is possible to
extrapolate that the environmental processes responsible for intertidal change since 1933
10
have acted on the whole intertidal at Hopkins Marine Station. While these processes are
far from cle
iportant to note that temporal variation across sixty years was far
r, it is
more significant than spatial variation in this study. This provides compelling evidence
that although stochastic variation is always a problem, carefully studied transects can act
as indicators for large-scale change.
Literature Cited
Barry, J.P., Baxter, C.H., Sagarin, R.D., Gilman, S.E.(1995). Climate related, long-term
faunal changes in a California rocky intertidal community. Science 267:672-675
Hewatt, H.G., (1934). Ecological studies on selected marine intertidal communities of
Monterey Bay. Doctoral thesis, Biological Sciences, Stanford University
Fu, I. (1995) Optimal sampling design and spatial scales of variation in the abundance of
three intertidal invertebrates. Unpublished student paper, Hopkins Marine Station,
Stanford University.
Glynn, P. (1965). Ecological studies on the Endocladia muricata - Balanus glandula
association in the intertidal zone in Monterey Bay, California. Stanford, California.
Morris, R, Abbott, D., Haderlie, E., (1980). Intertidal invertebrates of California.
Stanford, California, Stanford University Press.
Ricketts, E.F., Calvin, J.C., Phillips, D.W. (1985). Between Pacific Tides 5“" ed. Stanford
University Press, Stanford, California, USA.
Sagarin and Gilman (1993). Temperature correlated long-term faunal changes in the
rocky intertidal. Unpublished student paper, Hopkins Marine Station, Stanford
University
Sagarin, R., Gilman, S., Baxter, C., Barry, J. (in press) Climate-Related Change in an
Intertidal Community over Short and Long Time Scales. Ecological Monographs.
Simons, A., Leydig, E. (1996). Long term changes in the Endocladia-Balanus community
at Hopkins Marine Station, Pacific Grove, California. Unpublished student paper,
Hopkins Marine Station, Stanford University.
Sokal, R., Rohlf, F.J. (1995). Biometry: the principles and practice of statistics in
biological research. New York: Freeman.
Underwood, A.J. (1998). A method for analysing spatial scales of variation. Oecologia
117(4):570-578
Underwood, A.J., Chapman, M.G. (1996). Scales of spatial patterns of distribution of
intertidal invertebrates. Oceologia 107:212-224
Underwood, A.J., Chapman, M.G. (1998). Spatial analyses of intertidal assemblages on
sheltered rocky shores. Australian Journal of Ecology 23:138-157
Table 1. Analysis of Variance (ANOVA)
Anthopleura sola
Source
Areas
Transects(Areas)
Within
108
Serpulorbis squamigerous
Source
Areas
Transects(Areas)
Within
Endocladia muricata
Source
Areas
Transects(Areas)
Within
108
Rugosity Index
Source
Areas
16
Within
MS
8.280
1.394
0.913
MS
12.728
4.097
2.364
MS
0.918
0.52
0.059
MS
0.036
0.082
5.939
0.020
1.527 0.156
3.107 0.151
1.733 0.152
0.231
1.766
8.787 0.000
0.460 0.720
Table 2. Results of Tukey's tests. Differences between Area means (row minus
column); P-values in parentheses.
Anthopleura sola
Hewatt
East2
0.107 (0.984)
East2
Hewatt
-0.592 (0.285)
-0.699 (0.179)
-1.041 (0.042)
West
-1.121 0.026)
-0.422 (0.542)
Serpulorbis squamigerus
East!
East2
Hewatt
-1.545 (0.216
East2
-1.629 (0.190)
Hewatt
-0.084 (0.999)
-1.608 (0.196)
-0.063 (1.000)
West
0.021 (1.000)
Endocladia muricata
East
East2
Hewatt
East2
0.081 (0.971)
-0.320 (0.376)
Hewatt
-0.400 (0.217)
West
-0.025 (0.999)
-0.106 (0.939)
0.295 (0.439)
Figure Legends
Fig. 1. Aerial map of Hopkins Marine Station showing the location of the Hewatt
Transect. Dotted lines indicate the approximate locations of the four areas under study:
West, Hewatt, East1, and East2.
Fig. 2. Photograph of the study site taken at low tide.
Fig. 3. Schematic representation of the two factor nested design with areas, transects-
within-areas, and quadrat replicates.
Fig. 4. The relationship between the F-distribution for Ho, the F-distribution for Hj, a,
and B. When Ho is true, the probability of incorrectly rejecting it is a, and when Hi is true
the probability of incorrectly rejecting it is ß. Note that without a specified alternative
hypothesis, ß is unknown. However, the relationship between a and ß indicates that an
increase in a will decrease B.
Fig. 5. The distribution of Anthopleura. Each bar represents the mean of one transect.
Error bars are Standard Errors.
Fig. 6. The distribution of Endocladia. Each bar represents the mean of one transect.
Error bars are Standard Errors.
Fig. 7. The distribution of Serpulorbis. Each bar represents the mean of one transect.
Error bars are Standard Errors
Fig. 8. Correlation between Endocladia percent cover per quadrat and average tidal
height per quadrat. Lower than two feet above MLLW is considered low zone, which is
an inappropriate area in which to sample Endocladia.
Fig. 9. Components of variance contributing to total variation seen in the three organisms
sampled. Serpulorbis is represented by black, Anthopleura by light gray, and Endocladia
by dark gray. Variance is divided into areas, transects-within-areas, and residual, or
among-quadrat, variance.
Fig. 10. Average substrate rugosity for each area. The height of each bar is the sample
mean. Error bars are Standard Errors.
FIGURE 1

Monterey
Bay
25 km


West Beach
t Hewatt area
2 Wect arca
.larea
CLa4 2 ara

Hewatt's:
K
Transeét:
goig




Loeb Laboratory □

N


Bird Rock
J
Agassiz


2
Seal Rocks

Fig. 3
Nested Analysis
Intertidal
West
Hewatt
Eastl
Transect 1 Transect 2 Transect 3
Quadrat 1 .... Quadrat 10
East2
FIG. 4

::

P=.05
14
12 -
10
0 +
Anthopleura Distribution


West Area Easti Area
East2 Area Hewatt Area
Fig. 5
60
50
20
10
0 +

West Area
Endocladia Distribution

Easti Area East2 Area

Hewatt Area
Fig. 6
20
Serpulorbis Distribution
AE
1


West Area Easti Area East2 Area Hewatt Area
Fig. 7
100
60
40
20
0
Endocladia Height Distribution
88.
v


O
wdrae
Tidal Height (Ft)
West Area
East1 Area
East2 Area
V Hewatt Area
Fig. 8
90 -
80
70
60
50
5 40
30 -
20 -
Variance Components
— Serpulorbis
-
E Anthopleura
Endocladia
HE

Area
Transect(Area)
Source of Variation
Within
Fig. 9
Fig. 10
West Area
Average Rugosit)
Easti Area
East2 Area
Hewatt Area