ABSTRACT
Nineteen square yards of W.G. Hewatt's (1934) ecological transect in the intertidal
zone at Hopkins Marine Station were replicated in order to determine changes in
populations of intertidal invertebrates. Forty-two invertebrate species showed an
appreciable change in number between the two studies. It was hypothesized that some of
the changes might represent shifts in species range due to a temperature increase which was
observed in historical sea surface temperature data for the intertidal zone at Hopkins. The
species were divided by their listed ranges into northern, southern, and cosmopolitan
categories to determine any correlation between changes in a species population and its
listed geographic range. Changes in the physical habitat, introduction of the predators
Haematopus bachmani and Enhydra lutris to the area, increases in sea gull populations,
and increases in Macrocystis were also considered as factors driving the changes in some
of the species. When these factors were accounted for, increases in 8 of 10 southern and
decreases in 4 of 5 northern species were observed, suggesting a temperature dependent
species shift.
INTRODUCTION
In 1930, Professor G. E. MacGinitie established four brass benchmarks in
the intertidal zone off of Cabrillo (China) Point on the southern end of Monterey Bay.
These four markers became the eastern edge of a 108 yard long by 1 yard wide ecological
transect surveyed by W. G. Hewatt from October 1931 to June 1933 (see Hewatt 1934
and 1937). Hewatt recorded the abundance of 90 species in 81 square yards of the
transect. This work is still the most broad scope study of invertebrate populations in the
intertidal zone at Hopkins Marine Station.
In the 60 years since Hewatt's transect work, numerous changes have occurred in
the region surrounding his study area. These changes include: physical processes such as
erosion; population increases of predators, such as sea otters, oystercatchers, and seals; the
collapse of the canneries and changes in exploited fish populations; human population
growth and changes in industrial activities in the surrounding towns; and a possible large
scale climate change. Several studies have documented some of the faunal changes in the
region of the study area(see Lowry and Pearse 1973, Kovnat 1982, Hahn 1985).
This study began as an attempt to identify changes in the population of one
such faunal species, the vermetid gastropod, Serpulorbis squamigerus. Since its presence
was noted within the area of Hewatt's study, the transect was considered as a way to
document these changes. Once work began on the transect, however, we realized that
changes is Serpulorbis could best be viewed as one small part of the more general question
of changes in all species along the transect.
MATERIALS AND METHODS
Site Description
The geology of the study site was described by Hewatt (1934) as quartz-diorite,
characterized by "...many large, granite boulders and by rocky islands which project above
the water level..." Hewatt also provided descriptions of the water and air temperatures,
light conditions, precipitation, and tidal conditions of the site. With the exceptions of what
is discussed specifically in this paper, these are assumed to have not changed significantly.
Study Location
Using the descriptions and diagrams found in Hewatt, 1934, we located two of the
four brass pegs that mark the line on which Hewatt conducted his ecological transect. It
should be noted that although Hewatt mentions that the pegs were established with the help
of the United States Coast and Geodetic Survey, they are not official benchmarks, and
should not be confused with the official USC&GS benchmark which is located on shore
and to the west of the transect area. This confusion is likely the reason L.M. Hodgson
(1979) incorrectly marked the location of Hewatt's transect in her map of the Hopkins
refuge.
The first peg is set in a relatively flat slab of granite about 25 yards out in the
intertidal zone at a tidal level of 3.82 feet above the mean lower low water mark (MLLW)
and approximately one foot to the west of the eastern edge of the transect line (Hewatt,
1934). The second peg is located on the west facing side of a low, algal covered slab, 49
yards from the first peg at a bearing of N.38° (adjusted for declination). It is right on the
easter edge of the transect. Hewatt notes that there are two other pegs on the transect line;
one further up on shore from our first peg, and another further out from our second peg, in
the mussel beds, but we were unable to locate these. In order to make it easier for future
observers to find the line, we have included a map of the area with the transect line
correctly marked (figure 1) as well as several photographs of the transect line (figure 2).
We replicated 19 square yards of the transect during the course of this study. The
squares we studied matched Hewatt's squares 27-38 and squares 62-68. Square 27 marks
the lower limit of the Balanus/Chthamalus association. Square 38 marks the shoreward
edge of the large permanent channel which runs through the intertidal zone. Square 62
picks up on the other side of the channel and the transect continues along several yards of
flat, algae covered granite, until square 68, which marks the edge of a small, permanent
channel.
Survey Methods
So that we could replicate Hewatt's work, we ran a line between the two points,
adjusting for the fact that the first peg is not located exactly on the edge of the transect. We
marked rocks in several points on the line with red nail polish in order to more easily locate
the transect.
In order to most accurately compare our results to Hewatt's, we used the same
square yard unit. We found the location of the squares Hewatt counted by measuring from
either of the two pegs along the line we had marked. In addition, we were able to ensure
that we were measuring the same square by matching our quadrat to the sketches of the
rock structure of each square that Hewatt included with his thesis. To delineate the area we
were to count, we laid down a square yard quadrat, constructed of PVC pipe (figure 3),
with one edge of the square on the line. As this stiff square could not lay evenly on the
rock surfaces, we marked out the corners of the square on the rock by dropping a plumb
line from each of the corners of the PVC square. We then connected the four points we
had marked on the rock with lengths of flat-link brass chain, which could conform to the
varied terrain of the rock surface. In order to simplify counting, we divided the projected
quadrat into nine sections using chains (figure 4), and counted one section at a time.
We counted and recorded all animals found within the boundaries marked out by
the chains, with the exception of some bryozoans, sponges, tunicates, and amphipods,
which were considered too difficult to count. Large anemones were occasionally counted
as a half, or quarter, individual if their bodies were crossed by a chain boundary. Although
we lifted algal cover to count species on the substrate below it, we rarely removed the algae
altogether, and if we did, we counted all the species attached to the algae before removing
it. We turned over only one small boulder in order to count the species under it. This was
the only boulder we encountered that could be overturned.
Counting was done at low tide when most of the square being counted was above
the water level. This required that the counting be done at all hours of the day, which may
have skewed the results somewhat, as some species are normally only observed at night.
However, except for highly secretive or highly mobile animals, we assumed that any
variation in species behavior between night and day low-tides, such as differing
distributions on the rock surface, would not translate into a difference in the total number of
individuals observed per square.
RESULTS
A total of 8146 individual organisms belonging to 69 species were
observed. The abundance of two other species were noted but numbers of individuals were
not counted. The data for all 19 squares is presented in table 1, along with Hewatt's data
for comparison.
DISCUSSION
For the nineteen squares surveyed, we observed 22 species not noted by Hewatt in
his list of 170 species he found in the intertidal zone. Twelve species were observed by
Hewatt that would have been recognizable to us, but we did not find them. Nine species
observed by Hewatt may have been present during our study but we did not identify them
for various reasons. Of the species observable by both us and Hewatt, 31 increased their
numbers, 33 decreased, and six could not be compared because they had not been recorded
in quantifiable terms by one of the two studies.
We excluded 40 species which we felt were not abundant enough to consider from
further study. These were species which were absent in one study and present in numbers
of less than 10 individuals in the other study, or species which had numbers totaling 5
individuals or less in both studies combined. One species, Tegula pulligo, was excluded
because of concerns about correct identification. The remaining species are listed, with
totals, in table 2. Using a paired t-test, it was established that 10 of the increases and 7 of
the decreases were significant at the 5% percent level. They are denoted in table 2 by an
asterisk.
It should be clear to anyone comparing numbers of individuals in table 2 and
observing their distributions in table 1, that several species exhibit changes of a definite
biological significance between the two studies and yet are not ascribed as being statistically
significant. This is primarily due to the small sample size of the transect Because the unit
of measurement (a square yard) is so large and because many of the species have very
narrow habitat ranges, a significant number of species were present in only four or fewer
squares. With a sample size smaller than five, it is often difficult to establish reasonable
limits of significance. Because of this problem, we choose not to limit our analysis to only
the statistically significant changes, but also included any differences that appeared to be of
a considerable size and defensible on other criteria.
Climatological Changes
Surface water temperature is recorded daily at the Hopkins Marine Station and
salinity was likewise recorded until 1975 (Hopkins 1964, Scripps 1965-1991). Mean
annual salinity (figure 5) is almost constant at 33.5 0/00 for the 55 years of data available.
Figure 6 presents mean seasonal surface water temperatures for the ten years preceding this
study and the ten years preceding Hewatt's study. It shows a warming of about S degree
centigrade for the entire graph. A t-test calculated on the two data series presented in the
graph was statistically significant (t--13.65, pe.01), clearly showing that the area around
Hopkins is warming up. This is also supported by figure 7 which shows the mean annual
temperature for each year. A linear regression calculated on the data presented in figure 7
shows an increase of .Oll degree per year. Changes in mean annual maximum and
minimum temperatures have also been calculated to be increasing at rates of .017 and .O11
degree per year respectively (Denny, pers comm.). The effects of this warming trend will
be discussed later in the paper.
Air temperature and precipitation data are not collected at Hopkins. Following
Hewatt's example, we used data collected by the U. S. Weather Service at the Del Monte
Station in Monterey. Comparisons of seasonal means for both parameters are presented in
figures 8 and 9, air temperature data was not available for the period before 1931. These
figures, although not showing any significant changes, do suggest a shift to slightly
warmer and drier winters in the recent past.
Based on these changes we propose that many of the changés in species
composition are related to a shift in the geographic ranges of species associated with the
warming of the area around the Monterey Peninsula. To test this, we divided the 42
species listed in table 2 into three general geographic ranges, based primarily on the ranges
listed in Morris, et al (1980) (see figures 10-12). The 6 northern species have ranges
which are recorded to extend no further south than Pt. Conception, or are considered
extremely rare south of this point. The 10 southern species have ranges which are recorded
to extend no further north than Crescent City, California, or are considered extremely rare
north of this point. The 26 remaining species, whose ranges extend beyond both of these
markers, were put into the "cosmopolitan" group. Although considered cosmopolitan by
Morris, et al (1980) we considered the solitary form of Anthopleura elegantissima and A.
xanthogrammica southern and northern, respectively, based on L. Francis (1979). If a
warming of Monterey Bay is affecting the species present, then we would expect to see a
decrease in the northem species group, an increase in the southern species group, and no
change in the cosmopolitan species.
Looking only at species range, we found that 8 of 10 southern species increased in
population from Hewatt's results (figure 10), 4 of 6 northern species decreased in
population (figure 11), and I1 cosmopolitan species increased in population, while 15
cosmopolitan species decreased (figure 12). However, it was apparent that other factors
besides the temperature changes were affecting populations of certain species. These
factors can be generally classified as; changes in populations of associated species, changes
in predation patterns, and changes in physical habitat. We will discuss these factors before
returning to the question of species range.
Physical Changes
In his description of the site, Hewatt (1934) notes the presence of many large
boulders along the transect. Sixty years later, it appears that the majority of these stones
are no longer present. As mentioned, we found only one boulder on the transect that could
be overturned. This seems a likely explanation for the decrease in numbers of Petrolisthes
cinctipes and Amphipholis pugetana, both of which are creatures that reside under rocks
(Morris, et al, 1980).
Oystercatchers
In his study of the effects of black oystercatchers, Haematopus bachmani, at
Cabrillo point, Thomas Hahn (1985) found that just one pair of the birds can have a
significant effect on several limpet populations. Hahn found that the oystercatchers had the
greatest negative effect on Lottia digitalis, L limatula, and L pelta by feeding on these
species. Oystercatchers rarely feed on McClintockia scabra, because that species homing
behavior allows it to fit its shell closely to the granite substrate, making it difficult to
remove. We found decreases in both L limatula and L. pelta and an increase we M.
scabra which fits the selective predation pattern of the oystercatchers. Hahn did not
mention Tectura scutum in his paper, and it is likely that there were simply too few of this
species present at the time of his study. Whether the large decrease in T. scutum we
observed occurred before or after the pair of oystercatchers arrived at Cabrillo point is
unclear.
Sea Otters
Decreases in four of the cosmopolitan species, Strongylocentrotus purpuratus,
Mytilus californianus, Pugettia producta, and Pisaster ochraceus, could be the result of
changes in the population of a common predator, Enhydra lutris, the sea otter. Sea otters
were hunted to near extinction during the nineteenth century, but returned to the area
around the Hopkins Marine Station in the early 1960's (Lowry and Pearse, 1973).
It is difficult to establish exactly what constitutes the sea otters diet. Kovnat
(1982) studied intertidal otter prey specifically in the area around Hopkins. Thirty-five
percent of the prey items he observed were gastropods, 26% were "other," 18.8% were
"unidentified," and 14% were bivalves. This demonstrates precisely the problem in trying
to establish a local prey list for the sea otter, almost forty-five percent of the observed prey
in Kovnat's list are of unknown species.
Many diverse reports exist in the literature (see for example Riedman and Estes
1987, Kovnat 1982, Wild and Ames 1974). The preferred prey is perhaps best described
as sea urchins, abalones and rock crabs, but it is highly variable and depends on the
availability of prey. As otters expand their range and move into new areas they quickly
deplete the populations of these preferred prey and their diet becomes more varied (Wild
and Ames, 1974). Reports of otter prey include species of Tegula, Balanus, Pisaster,
Mytilus, Pugettia, Octopus, and various clams (Riedman and Estes, 1987).
Evidence does exist (Kovnat, 1982) that sea otters are responsible for the dramatic
decline in the numbers of Mytilus spp. around the marine station. By comparing size
distribution of mussels between Hopkins and an area known to be outside the otters range
(Pescadero State Beach), Kovnat showed that the differences in Mytilus populations
between the two sites was due to otter predation around the station. Also verbal history
reports extensive beds of large Mytilus disappearing within the first few months of the
otters' northward spread to Hopkins Marine Station in l9xx. (Baxter, pers. comm.)
That the sea otters are responsible for the decline in the subtidal populations of S.
purpuratus around the marine station was established by Lowry and Pearse (1973). They
point to a decline in the subtidal population from a density of 6 per square meter before the
return of the otters to 21 per square meter ten years after. They also point out that this
change in the sea urchin population may have effects on the subtidal kelp forest, which will
be discussed later.
The decrease in numbers of Pisaster, and another sea star Leptasterias hexactis,
may also be due to an increase in sea gull predation at the marine station. The region
surrounding the transect is a resting and feeding area for large numbers of gulls and other
shorebirds. The increase in human use of the shore area, combined with the establishment
of Hopkins as a protected area, may have led to an enhancement in the local population of
these predators since Hewatt's study. Sea gulls were observed preying in sea stars during
the course of this investigation.
Kelp changes
A secondary effect of the return of the sea otter is a change in the subtidal kelp
beds. As described above sea otters prey preferentially on Stronglyocentrotus spp., and
are responsible for a decline in subtidal populations of these species around the marine
station. Often in areas without otter populations, high numbers of sea urchins overgraze
the kelp forests, creating "urchin barrens." When otters move into the area, the urchin
numbers are reduced and kelp species return to the area (Estes and Harrold 1987, Lowry
and Pearse 1973). Since evidence exists for the presence of a Macrocystis kelp bed
offshore in 1935 (Andrews, 1945), an urchin barren could not have existed; but a decrease
of the magnitude described above must still have resulted in an expansions of the
Macrocystis beds when the otters returned.
10
If the return of the otters has caused an expansion of the Macrocystis kelp forest
around Hopkins, it would explain the increases in populations of Tegula funebralis and T.
brunnea. both of these species are preferential consumers of Macrocystis; Tbrunnea as a
subtidal grazer and T. funebralis on the drift kelp that is washed inshore (Morris, et al,
1980).
The increase in Crepidula adunca can be related to the absolute increase in Tegula
spp. since Crepidula reside almost exclusively on the shells of living Tegula (Morris, et al,
1980). But a further examination of the Crepidula population reveals that relative to the
number of available Tegula, Crepidula has decreased. Table 3 shows that the ratio of
Crepidula to Tegula has dropped from approximately 1 to 4 in 1934, to 1 to 7 in 1993.
This is what we would expect based on the classification of Crepidula as a northern species
(Morris, et al, 1980), if there was a temperature dependent species shift. It is even more
pronounced if only the higher quadrats are compared. In quadrats 27 through 33, where
there are no significant numbers of T. brunnea, our study shows one Crepidula for every
32 Tegula, whereas Hewatt found a ratio of one to eleven. The absence of T. brunnea
indicates that these quadrats are more exposed and so should show a greater response to
climactic changes.
Other Snails
The final group of changes occurred in the populations of small snails that reside on
the low, algal covered rocks on either side of the channel. The snails Mitrella carinata and
Amphissa versicolor showed a large drop in numbers; while Bittium eschrichtii,
Homaloploma luridium, and Lacuna marmorata, all showed significant increases.
Unfortunately almost nothing is known about these five snails (see Morris, et al, 1980). It
is conceivable that either temperature or ecological factors have caused a shift in the types
of algae present on the rocks on which these snails reside, but there is no information on
the algal conditions of the transect in past years, and even if there were, there is no
11
information on how these snails interact with various algal species. The reasons behind the
changes in these five snail populations must, at least for now, remain a mystery.
Shifts in Species Range
When the species primarily affected by these other factors were removed from
consideration, we found that increases in southern species remained at 8 of 10, but
decreases in northern species changed to 4 of 4, and cosmopolitan species showed an even
8 increases and 8 decreases, which is what we expected considering that temperature
variations would not as readily affect cosmopolitan species. These results are presented in
table 4.
For example, we have observed an increase in two predatory gastropods, Acanthina
punctulata and Ocenebra circumtexta, both of which are southern species. Neither of these
species were cited by Hewatt on his transect and they have now replaced the sea stars as the
major invertebrate predators of the upper intertidal zone. The increase in the population of
these species is certainly an ecologically significant change.
We have also observed some interesting and perplexing changes in barnacle
populations at China point. Hewatt found no Chthamalus in the intertidal zone during the
course of his study. We observed large numbers of Chthamalus both in the transect line
and in other parts of the intertidal zone at Hopkins. It is possible that Hewatt did not
differentiate between Balanus and Chthamalus. However, since both species were
described at the time, and Hewatt has shown himself to be a competent biologist, we feel
that this is highly unlikely. There are two species of Chthamalus which could be present at
our site, C. fissus and C. dalli. The two species can not be distinguished without a
microscope, but C. fissus is a southern species, with a northern limit of San Francisco.
The barnacle Tetraclita rubescens, another southern species with a northern limit of San
Francisco, has also shown a population increase. In the squares we studied, we found 882
Tetraclita, whereas Hewatt found none.
12
There is also evidence for changes in the populations of intertidal anemones. We
observed a significant increase in the number of Anthopleura elegantissima. In the study
area we found over 100 A. elegantissima, all of which were the solitary form. No clonal
forms were found in he transect, although they are present at other points in the intertidal
around Hopkins. Hewatt found only 10, and it is unclear whether these were solitary
forms or clonal forms. While clonal A. elegantissima is a cosmopolitan species, work
done by Lisbeth Francis (1979) shows that solitary elegantissima is a southern species.
We observed a decrease in Anthopleura xanthogrammica, which is shown in Francis' work
to be a northern species. We also observed an increase in Corynactis californicus, a
southern species which is primarily a subtidal form.
Perhaps the most dramatic evidence for change in intertidal species range has been
observed in the sessile snail, Serpulorbis squamigerus. In his list of 170 species observed
in the intertidal zone, Hewatt does not mention Serpulorbis. Yet, in just 19 square yards
we found nearly 500 of the snails. Serpulorbis is now one of the most abundant creatures
throughout this region. The northern limit of its range is listed as Monterey Bay by several
authors (see for example Morris, et al, 1980 and Hadfield 1966), but they report that
Serpulorbis lives only as single individuals this far north. However, we have seen large
aggregations of Serpulorbis in several locations in the southern part of the bay. In addition
to Hopkins Marine Station, the most notable concentration is the rocky intertidal of the
harbor behind the Coast Guard breakwater in Monterey. Mark Hadfield, whose 1966
thesis done at Hopkins, is the most comprehensive study of Serpulorbis to date, had to
obtain the Serpulorbis for his study from southern California. He would clearly not have
that inconvenience were his study done today.
The shifts in species range that these data suggest may have a relation to the location
of our study site. Hopkins Marine Station is located in the southern bight of Monterey Bay
which tends to be slightly warmer than the waters which surround it. It is often home to
species that are usually found further south and not observed to the north. It is also,
because of the presence of the marine station, subject to more intense observation that the
surrounding areas(Chuck Baxter, pers. comm.). In this sense, the intertidal zone here can
be used as an early warning system in terms of shifts in species range and their potential
consequences. In light of all of the resources currently being directed into the study of
large scale climatological change such global warming, the results of this simple study
suggest that intertidal organisms may be an easily observable method of monitoring such
changes. Comprehensive studies should have begun decades ago, but better late than
never.
Subtidal Evidence
Although the subtidal community was beyond the scope of this project, several
examples if similar species shifts in the subtidal came to our attention during the course of
our research. The Macrocystis kelp beds, for example, were described in 1935 as
consisting of"... almost pure strands of Macrocystis integrifolia..." (Andrews, 1945).
Observations by Pearse and Lowry (1974) indicate that the dominant species is now
Macrocystis pyrifera. According to Miller and Geibel (1973) M. integrifolia is a northem
species whose souther limit is Monterey Bay and M. pyrifera is a southern species that
only extends as far north as Half Moon Bay.
Another subtidal example is the southern species Kelletia kelletii, whose range is
listed as Point Conception to Baja California (Morris, et al, 1980). Herllinger (1981)
reports observing a total of five adult Kellitia at several different points in the kelp forests
along the shore of Monterey County. Since then, the population has apparently increased
and there is evidence that they are successfully reproducing this far north (C. Baxter, pers
comm.). These two examples suggest that a survey of subtidal populations around
Hopkins would produce results similar to those we observed in the intertidal.
CONCLUSIONS
The most important conclusion of this study is that there is great lack of
understanding about large-scale ecological changes that are occurring within the intertidal
zone at Hopkins Marine Station. This is not entirely surprising, considering that the last
broad focused ecological study of the area was done over 60 years ago by Hewatt. Our
study only begins to discover the many changes that have occurred in this ecosystem since
that time. Likewise, it only begins to suggest a few of the many questions that will need to
be answered if we are to understand and appreciate the effects of large scale climatic
changes. We hope that our work can be used as a baseline for future studies of that type,
and we further hope that these studies are carried out far more frequently than once every
60 years.
Acknowledgments
We would like to thank Jenny Hodge, Ravi Chandrasekaren and Kim Hoke for
getting up at 3 am and climbing around on the cold wet rocks when one of us couldn't
make it. Jim Watanabe for his help on analyzing our raw data. Alan Baldridge and Susan
Harris for putting up with our mess in the library. Molly "Mola-mola" Cummings for all
her enthusiasm and good spirit. The seals for their patience when we disturbed their naps
(they needed the exercise, anyway). And all the tiny critters who sacrificed their lives for
this project when we inadvertently squashed them while working in the intertidal.
And of course our deepest gratitude to our advisor Chuck Baxter, without whom
this project would never have happened.
References Cited
Estes J.A. and Harrold, C. 1987. Sea otters, sea urchins, and kelp beds: some questions
of scale, pp. 116-150, in G.R. VanBlaricom and J.A. Estes, eds., The community
ecology of sea otters. Berlin Heidelberg New York: Springer-Verlag. 247 pp.
Francis, L. 1979. Contrast between solitary and clonal lifestyles in the sea anemone
Anthopleura elegantissima. Amer. Zool. 19: 669-681.
Hadfield, M.G. 1966. The reproductive biology of the California vermetid gastropods
Serpulorbis squamigerus (Carpenter, 1857) and Petaloconchus montereyensis
Dall, 1919. Doctoral thesis, Biological Sciences, Stanford University, Stanford,
Calif. 174 pp.
Hahn, T.P. 1985. Effects of predation by black oystercatchers (Haematopus bachmani
audobon) on intertidal limpets (Gastropoda, Patellacea) Masters thesis, Biological
Sciences, Stanford University, Stanford, Calif. 70pp.
Herllinger, T.J. 1981. Range extension of Kelletia kelletii. Veliger 24(1): 78.
Hewatt, W.G. 1934. Ecological studies on selected marine intertidal communities of
Monterey bay. Dissertation submitted to the Department of Zoology, Stanford
University
—--
1937. Ecological studies on selected marine intertidal communities of Monterey
Bay, California. Amer Midl Natur 18(2): 161-206
Hopkins Marine Station. Temperature and Salinity Pacific Grove, 1919-1964.
(Unpublished data on file at Hopkins Marine Station Library).
Hodgson, L.M. 1979. Ecology of a low intertidal red alga, Gastroclonium coulteri
(Harvey) Kylin. Dissertation submitted to the Department of Biological Sciences,
Stanford University
Johnson, M.E., and H.J. Snook. 1927. Seashore animals of the Pacific coast. New York:
The Macmillan Company. 659 pp.
Kovnat, G.D. 1982. Intertidal foraging by the southern sea otter, Enhydra lutris. M.S.
Thesis. Stanford University.
Lowry, L.F. and Pearse, J.S. 1973. Abalones and Sea Urchins in an Area Inhabited by
Sea Otters. Mar Biol 23: 213-219.
Miller, D.J. and Geibel J.J. 1973. Summary of blue rockfish and lingcod life histories
reef ecology study and giant kelp, Macrocystis pyrifera, experiments in Monterey
Bay, California. Calif Dep Fish Game Fish Bull 158:1-137
Morris, R.H., D.P. Abbot, and E.C. Hadderlie. 1980. Intertidal invertebrates of
California. Stanford, CA: Stanford University Press. 690 pp.
Pearse J. S. and Lowry L. F. (eds). 1974. An annotated species list of the benthic algae
and invertebrates in the kelp forest community at Point Cabrillo Pacific Grove
California. Coastal Marine Lab, Univ of Calif at Santa Cruz. Tech Rep 1: 1-73.
Ricketts, E., and J. Calvin. 1939. Between Pacific tides. Stanford, CA: Stanford
University Press. 320 pp.
Riedman, M.L. and J.A. Estes. 1987. A review of the history, distribution, and foraging
ecology of sea otters, pp. 4-21, in G.R. VanBlaricom and J.A. Estes, eds., The
community ecology of sea otters. Berlin Heidelberg New York: Springer-Verlag.
247 pp.
Scripps Institute of Oceanography. Surface water temperatures at shore stations: United
States west coast and Baja California. Univ of Calif data rep: 1956 - 1991.
FIGURE LEGENDS
Figure 1. A map of the area around the Hopkins Marine Station. The transect is indicated
by the solid line.
Figure 2.
Photograph of the transect are
a. Photograph of the intertidal behind the Loeb Laboratory. The yellow line
marks the easter boundary of the transect.
b. Photograph of Hewatt's second benchmark, at the base of square 16.
c. Photograph of Hewatt's third benchmark, at the base of square 66.
Figure 3.
Photograph of the PVC square used in collecting data.
Figure 4.
Photograph of the same square laid out on the rock surface by means of PVC
chains. Additional chains were added to break the square up into smaller
squares to make counting easier.
Figure 5.
Mean annual surface water salinity, 1920-1975. Data is taken from daily
readings made at Hopkins Marine Station. No readings were taken after 1975.
Figure 6.
Mean seasonal surface water temperature. The solid line is the mean
temperature calculated for the years 1983-1993. The dashed is the same mean
for the years 1921-1931. Data was calculated from ten day means of daily
readings taken at the Hopkins Marine Station.
Figure 7.
Mean annual surface water temperature, 1920-1991. Data is taken from daily
readings made at Hopkins Marine Station. The shaded line represents a linear
regression of the data showing an annual increase of .012 degree per year.
Figure 8.
Mean monthly air temperature. The solid line is the calculated mean for the
years 1983-1993, the dashed line is the mean for the years 1931-1947. Data is
taken from mean monthly air temperature calculated by the U.S. Weather
Service for the Del Monte Weather Station.
Figure 9.
Mean monthly precipitation. The solid line is the calculated mean for the years
1983-1993, the dashed line is calculated for 1911-1930 and the dotted line
represents the years 1931-1952. Data is taken from monthly precipitation totals
published by the U.S. Weather Service for the Del Monte Weather Station.
Figure 10. Bar chart showing the changes in total numbers of species present for ten
southern species. Asterisks indicate changes significant at the 5% level using a
paired t-test. Solid bars are numbers observed in this study, hollow bars are
numbers observed by Hewatt (1934)
Figure 11. Bar chart showing the changes in total numbers of species present for six
northern species. Asterisks indicate changes significant at the 5% level using a
paired t-test. Solid bars are numbers observed in this study, hollow bars are
numbers observed by Hewatt (1934).
Figure 12. Bar chart showing the changes in total numbers of species present for twenty-
six cosmopolitan species. Asterisks indicate changes significant at the 5% level
using a paired t-test. Solid bars are numbers observed in this study, hollow
bars are numbers observed by Hewatt (1934). The four species on the right are
plotted on the second axis, with the darker bars representing our study.
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1920
1922
1924
1926
1928
1930
1932
1934
1936
1938
1940
1942
1944
1946
§ 1948
1950
1952
1954
1956
1958
1960
1962
1964
1966
1968
1970
1972
1974
1976
8
i
Concentration
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Degrees Centigrade

8
1920
1923
1926
1929
1932
1935
1938
1941
1944
1947
1950
1953
1956
1959
1962
1965
1969
1972
1974
1978
1981
1984
1987
1990
1993

Degrees Centlgrade
5
3











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Degrees Fahrenheit
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18
8
9
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Precipitation in Inches
8
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-..
900 -
800
700
600
500
400
300
200
100

Southern Species
H
Specles
600 —
500
400
300
200
100

Northern Species

Specles

5
Acmea mitra
Amphipholis
pugetana
Arabella iricolor
Bittium eschrichtii'
Emplectonema
gracile
Halosydna insignis'
Homalapoma
luridia“
Kellia laperousi
Lacuna marmorata"
Littorina scutulata
Lottia limatula“
L. pelta
L. paradigitalis
5 MoClintockia scabra
Mopalia muscosa
Mytilus
californianus"
Notoacmea
paleacea
Pachycheles rudis
Pachygrapsus
crassipes
Pisaster ochraceus
Pugettia producta
Strongylocentrotus
purpuratus'
Clavelina
huntsmanii
Mitrella carinata"
Tegula brunea"
T. funebralis'
Total Number Counted
8 8


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Total Number Counted
12