A Comparison of Species Composition and Diversity

Between Urchin Rocks and Northwest Island, WA

A class Project by the

Summer 2002 Walla Walla College Marine Station

Marine Invertebrates Class:

Anna Dyer

Brandon White

Brian Catelli

Christina Smith

Heidee Leno

Janisse Maxwell

Kelly Williams

Melissa McFadden

Nathaniel Charbonneau

Robbie Wheeling

and Ryan Lunsford.

Coordinated and Final Writeup by David L. Cowles, Ph.D.

Walla Walla College

Associate Professor of Biology

Abstract

1 m2 quadrats were examined along transects which spanned the intertidal zone at Urchin Rocks, a site which is frequently visited by people, and at Northwest Island, a nearby site with similar characteristics but which is seldom visited. Both sites were composed of gradually sloping gray bedrock with a northerly exposure. Overall animal species richness and diversity did not differ significantly between the sites. Many species, however, most notably the macroalgae, large barnacles, and motile and colorful species such as large seastars and urchins, Calliostoma and Ceratostoma snails, and Tonicella lineata chitons were scarce or absent in the lower intertidal of Urchin Rocks. The expected increase in algal and animal diversity in the lower intertidal was muted or absent there as well but was clearly seen at Northwest Island. On the other hand, cryptic species such as Leptasterias and some snails seem to be doing well at Urchin Rocks. Another group which is doing well includes species such as barnacles and some chitons which are armored and can cling tightly to the rocks, though the lack of large barnacles suggests that they may be impacted at later life stages. The lower intertidal of Urchin Rocks had a striking deficit of macroalgae combined with an extensive diatom mat in the same areas. These trends could be due to local competition but may well be due to species displacement due to repeated trampling in the lower intertidal zone.

Table of Contents

Introduction	1

Materials and Methods	2

Results	4

Discussion	7

Literature Cited	10

Table 1: Physical characteristics of the two transects	11

Table 2: Physical and descriptive transect data for Urchin Rocks transect	12

Table 3: Physical and descriptive transect data for Northwest Island transect	13

Table 4: Species catalogued in the transects	14

Table 5: Comparison of algal abundance and diversity among high and low intertidal

levels at Urchin Rocks and Northwest Island	16

Table 6: Comparison of animal abundance and diversity among high and low intertidal

levels at Urchin Rocks and Northwest Island	17

Table 7: Number of individual animals of selected species found in a concentrated search

over approximately 400 m2 at Urchin Rocks and Northwest Island	18

Figure Captions:	19

Figure 1: Locations of transect sites	21

Figure 2: Slope characteristics of the two sites	22

Figure 3: Photographic comparison of the community sampled at the two sites

at characteristic tidal heights	23

Figure 4: Total percent cover for brown algae and green algae by tidal height	25

Figure 5: Total percent cover for red algae and coralline algae by tidal height	26

Figure 6: Total percent cover for macroalgae and diatoms by tidal height	27

Figure 7: Percent cover and species richness for total algae by tidal height	28

Figure 8: Animal species richness and diversity by the rarefaction method by tidal height	29

Figure 9: Simpson's diversity and equitability index for all animals by tidal height	30

Figure 10: Shannon's diversity and equitability index for all animals by tidal height	31

Figure 11: Simpson's diversity and equitability index for all animals except

barnacles by tidal height	32

Figure 12: Shannon's diversity and equitability index for all animals except

barnacles by tidal height	33

Figure 13: Total cnidarian percent cover and count per square meter by tidal height	34

Figure 14: Total motile crustaceans and total motile mollusks by tidal height	35

Figure 15: Total barnacle percent cover and total barnacles by tidal height	36

Figure 16: Total animal percent cover and total animals by tidal height	37

Figure 17: Barnacle relative size by tidal height	38

Figure 18: Total mussel count per square meter by tidal height	39

Figure 19: Total percent cover by tidal height	40

Appendix 1: Photos of each quadrat at Urchin Rocks	41

Appendix 2: Photos of each quadrat at Northwest Island	46

Appendix 3A: Algal percent cover for Urchin Rocks	52

Appendix 3B: Algal percent cover for Northwest Island	53

Appendix 4A: Animal Count data (percent cover and number/m2) for Urchin Rocks	54

Appendix 4B: Size of area counted for each animal to generate the data for Urchin Rocks	57

Appendix 5A: Animal Count data (percent cover and number/m2) for Northwest Island	58

Appendix 5B: Size of area counted for each animal to generate the data for Northwest Island	61

Introduction

As the population of the Pacific Northwest grows, state parks and other recreation areas are receiving more and more impact from the thousands of visitors that enjoy them each year. A natural goal for the state parks is to provide the most natural experience for the large numbers of visitors while at the same time preserving the natural environment for generations to come. In order to do this it is important to know the amount and types of impacts that visitors are having on the natural habitats of the state parks.

Deception Pass State Park is one of the most popular seaside state parks in the state of Washington. Many thousands of visitors pass through the park each year, and many of them visit the intertidal zone during their visit. One of the most popular intertidal areas in the park is at Urchin Rocks in the Rosario Beach area. Urchin Rocks has a substantial amount of rocky intertidal habitat within very easy walking distance of a large picnic area and a paved parking lot. The substrate is mostly bedrock, with few rough boulders to make walking difficult so it is an easy site to visit. The area also includes several hundred yards of gravelly beach which further attracts large numbers of vacationers. Even at nearly high tide there are a few tide pools with hermit crabs and sculpins accessible, so the area is frequently visited at all tide levels and throughout the week, especially in the summer months.

Over the years, both qualitative and quantitative observations have suggested that the heavy foot traffic in the Urchin Rocks area has had an impact on the types and numbers of species present. Few large seastars, for example, can be found. Visual observations suggest that the rocks, especially in the most easily traveled areas, are more barren of life than are those in similar areas less accessible to foot traffic.

In the summer of 2002 the Marine Invertebrates class from the Walla Walla College Marine Station, which is directly adjacent to the Rosario Beach/Urchin Rocks area, performed a detailed ecological survey of the Urchin Rocks area, and compared it to a similar area on Northwest Island, a few hundred yards to the northwest from Urchin Rocks but inaccessible to foot traffic (Figure 1). The purpose was to characterize the species living on Urchin Rocks and compare that community to the very similar but untrampled reference community on Northwest Island.

Materials and Methods

Sites were chosen by visual inspection at Urchin Rocks and Northwest Island. Both sites were chosen with a northern exposure, on gently sloping, slightly undulating bedrock with very few loose boulders (Figure 2). The Urchin Rocks transect was deliberately chosen in the area most heavily trampled by visitors. The Northwest Island site was of similar slope and exposure but seldom visited by foot traffic. Belt transects were laid down starting in the supralittoral zone and proceeding just a few degrees to west of north down to the lowest level reachable on a minus tide during the days of the survey (July 24 and 25, 2002). These levels were minus 1.9 feet on July 24 and minus 1.6 feet on July 25. The transect was marked by laying down a leaded line which ran straight toward the water from the high point. In both transects the leaded line was secured to a sharp rocky ledge on a high point of rock at the upper end. A surveyor's measuring tape marked in meters was laid along the line to record horizontal meters along the transect down to the water's edge. Meter-square quadrats were then placed along the leaded line at 1-meter intervals all the way from the top to the bottom of the transect, and the species present within each quadrat were quantified. In every case the quadrat was placed with its western edge along the leaded line and its seaward end at the .6 meter mark for each meter (Note: The .6 meter mark was not intentional–it was selected at first because that was where the water's edge fell on the first transect, then continued for consistency). Students worked in specialty groups identifying the algae and invertebrates present within each quadrat and in the general area. One group measured the tidal height of each quadrat (measured at the .6 meter mark) by using a string level and vertical pole, referenced to the water's edge at low tide (determined by the Port Townsend tide tables). Besides recording the actual intertidal height, all quadrats were assigned a tidal height category of "upper intertidal" if they were at +3 feet or above, or "lower intertidal" if they were below +3 feet. The transect was done near the end of an intensive college-level course on marine invertebrates, so the students had acquired a fair level of proficiency in identification. The students who identified the algae were also taking a similar course in marine phycology.

Some animals, such as barnacles, were too dense to be counted in an entire square meter. In other cases the flooding tide forced species counts in some quadrats to be over less than a square meter. Furthermore, some important species such as large echinoderms and anemones were present in too small numbers to be adequately represented in 1 m2 quadrats. These species were quantified over a larger area. The species groups counted by the different specialty groups and the techniques they used are listed below:

Algae was measured only in terms of percent cover. To make these measurements, 25 equally spaced points (0.25 m apart) were identified on the mesh within each 1 m2 quadrat. The algal species most obviously visible under each of the 25 points was quantified for each quadrat, and these numbers were converted to percent cover by multiplying by 4. Algal percent cover often does not add up to 100% because of bare rock, barnacles, etc. Also, if several species of algae were overlapping one another, only the species on top was counted for each point.

Sponges were searched for but were not found in the relatively smooth topography of the quadrats. Their presence was noted at both sites near the transects, however. Cnidarians (anemones) were counted in full 1 m2 quadrats, plus percent cover was calculated for anemones in the same way as for algae. This was adequate for common anemones such as Anthopleura elegantissima but not for less common anemones such as Urticina crassicornis. Therefore, for Urticina, a representative band of shoreline parallel to the water at the 0 tide line, and approximately 100 meters long and 4 meters wide was searched intensively for anemones. All Urticina anemones found within this 400 m2 plot were recorded.

Barnacles were measured in a quadrat divided into 100 1 dm2 squares. Enough square decimeters were counted to represent at least 100 barnacles starting with the southeast decimeter, then the result was multiplied by an appropriate term to represent the entire square meter. In addition, percent cover was calculated for barnacles by identifying the species present directly under each of the 121 equally-spaced (1 dm apart) points within the quadrat. Motile crustaceans such as crabs and isopods were counted in the entire square meter.

Mollusks were counted in an area varying from ½ meter square to a full square meter, depending on the number of individuals present and how fast the tide was advancing. If less than a full square meter was counted, an area representing at least 100 individuals was attempted to be counted and then multiplied by an appropriate factor to represent the entire square meter. Careful inspection was made in crevices and under algae, but animals in cracks not clearly visible from the surface were not counted.

Echinoderms were not present in high enough numbers to be adequately counted in the belt transect, especially at Urchin Rocks. Therefore an intensive search was made for echinoderms in the same 400 m2 plot as was used for Urticina anemones. All echinoderms found within this plot were recorded.

Several physical and general observations were made as well. In addition to measurement of the tide level, a verbal description was made of the appearance of each quadrat, plus a digital photograph of the quadrat was taken. Several photographs were also taken of the layout of the entire transect as well in order to facilitate placement of future transects which may be made in the same area.

All data were compiled into master spreadsheets for each site. Animal species richness, and diversity and equitability indices for each quadrat at the two sites were calculated using Simpson's diversity index Ds and evenness index and Shannon's diversity index H' and evenness index J' (Brower and Zar, 1984). These indices have been criticized as misleading since they confound several parameters; in particular, they are easily biased by sample size (Smith & Grassle 1977, James and Rathbun 1981, Ludwig and Reynolds 1988). We therefore also calculated diversity by the rarefaction method (Sanders (1968), Hurlbert (1971), which allows standardization to the same sample size. For rarefaction analysis all quadrats were standardized to a sample size of 9, since this was the sample size for the least abundant quadrat. All diversity indices were recalculated after omitting barnacles since the barnacles are often so overwhelmingly numerically dominant that they eclipse any differences in diversity due to other species. The data and diversity indices were then imported into SPSS 10.1 for analysis. The two sites were compared graphically, by two-way ANOVA, and by one-way ANOVA with the tukey-b post-test in SPSS. Counts of the species inventoried in the 400 m2 survey were compared between sites by Chi-square.

Results

Both transects were on similarly sloping northern exposures of similar types of gray bedrock and were only a few hundred yards apart (Table 1).

Basic descriptions of the life seen in each quadrat for Urchin Rocks can be found in Table 2, and in Table 3 for Northwest Island. The Urchin Rocks transect appeared to have more bare rock, less macroalgal cover, and more diatoms in the lower intertidal than did Northwest Island. Several representative photos comparing the two transects at similar tidal heights are in Figure 3, and a complete photographic record of each quadrat is in Appendices 1 and 2.

Quadrat measurement data are in Appendices 3-5. Table 4 lists all the species catalogued as present in the transects. The table shows that a number of species are absent or less abundant at Urchin Rocks than at Northwest Island. For example, 10 macroalgal species, 1 anemone, and 4 mollusk species were found on the transect at Northwest Island but not at Urchin Rocks, while only 2 algal species, including a diatom mat, were found on the transect at Urchin Rocks but not at Northwest Island.

Comparisons of the relative abundances of different groups and species at different tidal heights between Urchin Rocks and Northwest Island reveals many other interesting relationships. Brown algae (mostly Fucus), for example, was at low abundance in the upper intertidal of both sites (Figure 4A). At Northwest Island brown algae % cover (Fucus and other species) steadily increased in the lower intertidal, while at Urchin Rocks a very different pattern was seen. In the lower intertidal at Urchin Rocks Fucus did not increase in abundance and other brown algal species such as Leathesia difformis and Hedophyllum sessile were rare. The same was true of red algae (Figure 5A). Many species of red algae readily found in the lower intertidal at Northwest Island were not found at all on the Urchin Rocks transect (Table 4). This scarcity of red and brown macroalgae in the lower intertidal (See Figure 6A) seemed to be due largely to the presence of a thick slimy diatom mat (Figure 6B), with perhaps a secondary cause due to an abundant but narrow band of Ulva near the -1 foot level (Figure 4B). Only at tidal levels below zero tide, where the diatom mat was not abundant, was macroalgae abundant at Urchin Rocks, and this was due mainly to Ulva.

Surprisingly, with all algal species considered together (including diatoms), the total algal % cover at Urchin Rocks was similar to that at Northwest Island, with the possible exception of the zone from +1 to +4 feet, where trampling may be severe (Figure 7A). Northwest Island quadrats tended to have higher algal species richness in all regions of the intertidal, however (Figure 7B).

Two-way ANOVA confirmed these relationships among algal percent cover at the two sites (Table 5). All categories of algae were significantly more abundant in the lower intertidal than in the upper, as expected. Brown algae, red algae, and total macroalgae were in significantly greater abundance at Northwest Island than at Urchin Rocks, while diatoms were more abundant at Urchin Rocks (since substantial cover of diatoms was not even noted at Northwest Island). However, the significant interactions among site and height for a number of these measures shows that algal abundance with intertidal height varied differently at the two sites. At Northwest Island all algal groups increased in the lower intertidal, as expected. All these increases except for green algae (P = .056) were significant by one-way ANOVA. At Urchin Rocks, on the other hand, brown, red, and total macroalgae decreased in abundance in the lower intertidal while diatoms increased. None of these decreases were significant but the increase in diatoms in the lower intertidal at Urchin Rocks was highly significant by one-way ANOVA. The increase in green algae was also significant (P = .048). No significant differences were seen between the two sites in total algae % cover (including both macroalgae and diatoms) or in algal species richness (though Northwest Island averaged greater richness at all tidal levels).

Similar relationships can be seen between the two sites in comparisons of animal abundance and diversity (Table 6). At Northwest Island animal species richness was significantly higher in the lower intertidal than in the upper (Figure 8A). This was not true at Urchin Rocks, where both values were intermediate. No differences in overall diversity were found between the sites. All the diversity indices indicated higher diversity in the lower intertidal, though most were not significantly so (Figures 8B, 9-12). Tellingly, the rarefaction index of diversity, which is likely the most reliable index, shows the strongest increase in diversity in the lower intertidal (Table 6, Figure 8B), and a significant effect. The two-way ANOVA interaction term indicates a difference in this trend between the two sites (Table 6). When the sites are considered separately by one-way ANOVA, all the diversity indices which omit barnacles increase highly significantly in the lower intertidal at Northwest Island, while at Urchin Rocks none of the changes in diversity indices with tidal height come anywhere close to significance (Table 6). Similarly, the increase in species richness in the lower intertidal at Northwest Island is highly significant, while at Urchin Rocks there is no significant increase.

Several comparisons showed trends which differ from these general observations. Anemones (Figure 13, in this case consisting entirely of Anthopleura elegantissima) and motile mollusks (Figure 14B) were significantly more abundant at the Urchin Rocks transect than at Northwest Island. Barnacles (Figure 15), total animal counts (which consists mainly of barnacles, Figure 16), and motile mollusks (Figure 14B) showed higher counts in the upper intertidal than lower at both sites. The higher motile mollusk counts seemed to be associated with large numbers of small snails and chitons in the upper intertidal. The higher barnacle counts seemed to be associated with the fact that Urchin Rocks had large numbers of small barnacles while both lower intertidal areas and the upper intertidal at Northwest Island had a smaller number of much larger barnacles (Figure 17). Note in Figure 17 that at Urchin Rocks most barnacles even in the lower intertidal were small but one patch of large barnacles was encountered in the transect. This can be seen by comparing the relative sizes of barnacles by height and site in Table 6. A relative barnacle size index was calculated from the data for each quadrat by dividing the quadrat's % barnacle cover by the barnacle count for that quadrat. Though the differences indicated by this rough comparison are not significant, barnacles at both high and low levels of Northwest Island average larger than those at Urchin Rocks. The larger average size of barnacles at the lower level of Urchin Rocks is due mainly to the presence of one small patch of very large barnacles at the +1 foot height. If this patch is not included, Northwest Island barnacles are significantly larger than those at Urchin Rocks, especially those in the lower intertidal.

Mussels (Mytilus trossulus) were distributed differently at the two sites as well (Figure 18), though mussels were not abundant at either site and their distribution was patchy enough that no statistical differences were seen (Table 6). At Northwest Island mussels were more abundant and were higher in the intertidal. At Northwest Island mussels were most abundant in the middle intertidal, while at Urchin Rocks the mussels found were several feet lower in the intertidal, slightly above the 0 tide line.

Total percent cover for animals and algae combined, which for animals consists primarily of barnacles and includes both macroalgae and diatoms, differed little between the sites (Table 6, Figure 19). At both sites total percent cover increased steadily down through the intertidal until it remained constant at nearly 100% cover or decreased slightly at minus tide levels.

Table 7 presents data for species which were present in too low numbers to be adequately sampled in the transects and so were counted in the separate 400 m2 survey. Most species surveyed were found at both sites, though in differing abundance. Urchins and Urticina anemones were found at significantly greater abundance at Northwest Island, while Anthopleura elegantissima anemones and Leptasterias hexactis seastars were found more abundantly at Urchin Rocks. Sea cucumbers were found in approximately equal frequency at the low levels of both sites. Few large seastars such as Pisaster were found in the area of the survey, though Pisaster is frequently seen at Northwest Island and a few individuals can be found on Urchin Rocks, especially in the outer, more accessible areas.

Discussion

These transects highlight some good news and some bad news for heavily visited areas such as Urchin Rocks. First, it is clear that marine life is able to flourish at such a site despite heavy trampling. Indeed, all levels of the intertidal zone at Urchin Rocks contained abundant life, and both algal and animal percent cover and species richness are not significantly different from that at Northwest Island. Similarly, the standard Simpson's and Shannon's diversity and equitability indices were not able to show much difference in diversity between the two sites. The intertidal community at Urchin Rocks, however, is both qualitatively and quantitatively different from the protected one at Northwest Island in a number of ways that likely reflect the impact of the frequent trampling and collecting that take place there. Highly visible, motile, and easily collected species such as Urticina anemones, urchins, large seastars, large motile limpets such as Acmaea mitra and Diodora aspera, and large or colorful snails and chitons such as Calliostoma sp, Ceratostoma foliatum, and Tonicella lineata seem to be absent or at reduced abundance there. The algal community is very different, especially in the lower intertidal where the usual abundant population of large brown and red macroalgae is reduced or missing, their place largely taken by a thick brown diatom mat. This impoverishment is clearly seen in the animal community as well. Animal species richness increased sharply in the lower intertidal at Northwest Island, while it increased only slightly at Urchin Rocks (Table 6, Figure 8). The low species richness in the upper intertidal at Northwest Island is likely due to the fact that the upper transect at that site had a slightly higher proportion of substrate very high in the intertidal and virtually devoid of life. Animal diversity, as measured by the rarefaction method, increased steadily through the lower intertidal at Northwest Island while at Urchin Rocks animal diversity was depressed at least until minus tide levels were reached (Figure 8).

Several algal and animal species, on the other hand, seemed to defy this trend. Barnacles, for example, were abundant at both sites, especially at the Urchin Rocks site. This trend is a telling one, though, in several ways. First, though barnacles were significantly more abundant at Urchin Rocks, the percent of the substrate covered by barnacles was actually slightly lower there (though the difference was not great enough to be significant) (Table 6, Figure 15). Visual observations suggested that there was actually a larger percentage of bare rock at Urchin Rocks, even though the barnacles were more abundant (This was not confirmed by measurements of total percent cover, Figure 19). The barnacles found at Urchin Rocks were smaller than at Northwest Island (Table 6, Figure 17), representing an earlier stage of barnacle succession and growth. A larger proportion of bare substrate combined with smaller individuals would suggest that Urchin Rocks has experienced recent disturbance(s) of a type and magnitude that has cleared many of the large, mature barnacles from the rocks.

A second species found abundantly at Urchin Rocks was the anemone Anthopleura elegantissima. This may be a real trend or may be an artifact of the specific sites chosen for the transect. Anthopleura elegantissima does occur in abundance at Northwest Island, often in large clonal groups. Others are scattered across the substrate, often in slight depressions. Examination of the distribution of this anemone at the two sites (Figure 13) suggests that perhaps most of the individuals encountered were scattered, with one substantial clonal group occurring in the transect at Urchin Rocks. The Anthopleura observations made in the 400 m2 survey (Table 7) need to be qualified with the fact that while Anthopleura elegantissima can be found in abundance in the lower intertidal, it is often most characteristically found at higher tidal levels than the near 0 tide level that was sampled in this survey.

The abundance of the seastar Leptasterias hexactis at Urchin Rocks came as a surprise since it was not particularly evident at first. This species is a small, cryptic species which was found primarily at the lower intertidal levels in crevices, overhangs, or under boulders. It is possible that this species is actually being selected for at Urchin Rocks, or its relative abundance there may be due to a greater abundance of crevices, to the lack of macroalgae making it easier to find, or to some other factor such as possible competition or predation from other seastars such as Pisaster.

While they were not encountered in great abundance in this study since the transects avoided large tidepools, the hermit crab Pagurus hirsutiusculus is another very common species at Urchin Rocks. Virtually every available shell in the tidepools contains one of these crabs.

One of the most striking differences between the community at Urchin Rocks and that at Northwest Island was the fact that an extensive diatom mat was observed at Urchin Rocks in the lower intertidal zone but not at Northwest Island. This mat occurred in the zone which would otherwise be filled with abundant macroalgae which are mostly missing at Urchin Rocks. The diatoms may be occupying a niche emptied of macroalgae by some other factor such as trampling or they may have directly displaced them.

While not enough counts of mussels were made to justify firm conclusions, the observed distributions suggest some interesting possibilities. Large mussel beds are uncommon at either site for unknown reasons. Mussels were more abundant at the protected Northwest Island site, and were found higher in the intertidal (Figure 18). Perhaps repeated disturbance by humans at Urchin Rocks, especially in the upper and mid intertidal zones, limited the ability of mussels to survive there. The small population peak observed at +1 to 2 feet at Urchin Rocks, on the other hand, may be correlated with the fact that Pisaster ochraceous, one of their primary predators, is uncommon at that site. Pisaster is known to forage primarily in the lower intertidal. Future projects should test for these possibilities.

Several qualifications and sources of error need to be considered along with these data. Data for any one species or group should not be considered as definitive since this is only one study over an arbitrarily selected transect. Instead, the general trends of multiple species and groups should be considered together, and similar transects should be made to test the indicated trends. These transects were deliberately chosen to represent the most heavily visited area of Urchin Rocks, and the trends observed would not be as distinct elsewhere. Many more large barnacles were observed, for example, on steep rocks near the Urchin Rocks transect where walking is difficult than in the flatter areas the transect actually covered. More algae may have been present there as well, though that could not be determined from our data and needs further investigation. The 400 m2 survey, on the other hand, covered a variety of substrates from steep to flat, from easily accessible to more difficult to reach, and from bedrock to boulders so trends observed there may reflect changes occurring over all of Urchin Rocks.

Though the students who conducted the transects had spent considerable time learning the proper identification of intertidal species, several known missed identifications occurred. Though Littorina snails were abundant at the upper intertidal of both sites, for example, the records of their abundances were lost or confused and so are not reported here. The barnacle Balanus nubilus occurred it the lowest intertidal regions, especially at Northwest Island, and it is likely that some of the large individuals of this species in the lowest quadrats were misidentified as B. glandula. Additionally, several similar predatory whelks were lumped under the category of Nucella emarginata and in the "unknown small/long" snail category.

The rarefaction diversity index cited in this study suffered from one factor which decreased its predictive power. The index can only be used when comparing predicted species numbers for counts involving equal numbers of individuals. Additionally, it cannot extrapolate beyond the number of individuals actually counted. Therefore we had to standardize all quadrats to the number of individuals found in the least populated quadrat. The least populated quadrat was found at Urchin Rocks, and had only 9 individuals. This is an unusually low number for calculating diversity by the rarefaction method, and undoubtedly underestimated diversity for many of the quadrats. Some of the quadrats at Northwest Island, in particular, had more than 9 animal species within them and many more than 9 individuals but standardizing to 9 individuals eliminated a number of these species from consideration for diversity. The rarefaction diversity index, then, should be used only for comparing different quadrats and tidal levels, and should not be considered an absolute measure of the actual diversity present in many of the quadrats.

These shortcomings, though they detract slightly from the analytical power of the study, are unlikely to have had any great material affect on the general conclusions of the study.

The general picture at Urchin Rocks, then, is that of a diverse intertidal community which nevertheless is clearly distinct from the undisturbed community at Northwest Island. Many species, most notably the macroalgae, large barnacles, and motile and colorful species such as large seastars and urchins, Calliostoma and Ceratostoma snails, and Tonicella lineata chitons were scarce or absent in the lower intertidal. The expected increase in algal and animal diversity in the lower intertidal was muted or absent. On the other hand, cryptic species such as Leptasterias and some snails seem to be doing well there. Another group which is doing well includes species such as barnacles and some chitons which are armored and can cling tightly to the rocks, though the lack of large barnacles suggests that they may be impacted at later life stages. Small anemones such as Anthopleura elegantissima may also be unusually abundant, though this conclusion needs more investigation. The striking deficit of macroalgae combined with the extensive diatom mat could be due to local competition but may well be due to species displacement due to repeated trampling in the lower intertidal zone.

Literature Cited

Brower, James E. and Jerrold H. Zar, 1984. Field and Laboratory Methods for General Ecology. Wm. C. Brown, Dubuque, Iowa.

Hurlbert, S. H., 1971. The nonconcept of species diversity: a critique and alternative parameters. Ecology 52: 1459-1461

James, F. C. and S. Rathbun, 1981. Rarefaction, relative abundance, and diversity of avian communities. The Auk 98: 785-800

Ludwig, J. A. and J. F. Reynolds, 1988. Statistical Ecology. J. Wiley and Sons, New York.

Sanders, H. L., 1968. Marine benthic diversity, a comparative study. American Naturalist 102: 243-282

Smith, W. and J. F. Grassle, 1977. Sampling properties of a family of diversity measures. Biometrics 33: 283-292

Table 1: Physical characteristics of the two transects
Substrate	GPS location of southern (shoreward) end	High point (feet above 0 tide mark)	Low point (feet from 0 tide mark)	Length (meters)	Slope
Urchin Rocks

(heavily impacted by foot traffic)	Gray bedrock	48o 25.024'N

122o 39.879'W	8.5	-1.6	23.6	16%
Northwest Island (Unimpacted control)	Gray bedrock	48o 25.172'N

122o 40.220'W	9.1	-1.9	18.6	14%

Table 2: Physical and descriptive transect data for Urchin Rocks transect, July 25, 2002. Transect is on the northern side of the rocks in the most heavily trampled area, trending northward from a rounded rocky knoll for 20m to -1.6 feet tide level. Substrate: bedrock with a few small tidepools. Upper end of transect (WAAS- enabled GPS coordinates 48o 25.024' N, 122o 439.879' W) is on a small rock rise attached to a notch in the rock near a small tidepool. Meter numbering starts from the upper end of the transect.
Meter	Tidal height (mllw)	Photo	Description of substrate and main cover
m	ft
1	2.58	8.5	UR01.jpg	Bare rock
2	2.45	8.1	UR02.jpg	Bare rock with barnacles
3	2.03	6.7	UR03.jpg	Rock, Fucus, barnacles, Anthopleura
4	1.81	5.9	UR04.jpg	Small crevices, barnacles, Fucus
5	1.63	5.4	UR05.jpg	Slope/bank, barnacles, Anthopleura
6	0.99	3.3	UR06.jpg	Rock, barnacles, Fucus, tidepool with encrusting corallines
7	0.89	2.9	UR07.jpg	Crevices, Fucus, barnacles, Anthopleura
8	1.00	3.3	UR08.jpg	Anthopleura, barnacles, Fucus, reds
9	0.60	2.0	UR09.jpg	Ridge/channel, Fucus, barnacles, Anthopleura
10	0.29	0.9	UR10.jpg	Small tidepool, Anthopleura, barnacles
11	0.37	1.2	UR11.jpg	Tidepool, rocks, barnacles, limpets
12	0.43	1.4	UR12.jpg	Ridge, Fucus, barnacles, Anthopleura
13	0.21	0.7	UR13.jpg	Ulva, barnacles, limpets
14	0.08	0.3	UR14.jpg	Barnacle-covered knobs, water in fissures, diatoms
15	0.13	0.4	UR15.jpg	Diatom-covered barnacles
16	-0.19	-0.6	UR16.jpg	Diatoms, barnacles, Ulva, Hedophyllum
17	-0.27	-0.9	UR17.jpg	Tidepool, broken rock, diatoms, algal debris
18	-0.13	-0.4	UR18.jpg	Rock ridge, Hedophyllum, limpets, Katharina, Mastocarpus tarspot
19	-0.42	-1.4	UR19.jpg	Ulva, diatoms, reds
20	-0.49	-1.6	UR20.jpg	Ulva, reds

Table 3: Physical and descriptive transect data for Northwest Island transect, July 24, 2002. Transect is on the northern side of the island, from base of cliff trending north for 24 m to -1.9 ft. Substrate: bedrock with a few small tidepools. Upper end of transect (WAAS-enabled GPS coordinates 48o 25.172' N, 122o 40.220' W) is on a small rock rise attached to a notch in the rock near a small tidepool. Meter numbering starts from the upper end of the transect.

Meter	Tidal height

Photo

Description of substrate and main cover
cm	ft
1	2.78	9.1	NWI1.jpg	Bare rock
2	2.59	8.5	NWI2.jpg	Bare rock
3	2.25	7.4	NWI3.jpg	Rock with algae in crevices
4	1.58	5.2	NWI4.jpg	Rock with algae, mussels, barnacles
5	1.50	4.9	NWI5.jpg	Rock with algae, mussels, barnacles
6	1.38	4.5	NWI6.jpg	Rock with algae, mussels, larger barnacles, small tidepool with coralline algae
7	1.51	5.0	NWI7.jpg	Rock with algae, mussels, larger barnacles, small tidepool with coralline algae
8	1.32	4.3	NWI8.jpg	Large barnacles, Anthopleura elegantissima
9	1.39	4.6	NWI9.jpg	Large barnacles
10	1.12	3.7	NWI10.jpg	Large barnacles
11	0.97	3.2	NWI11.jpg	Large barnacles
12	0.81	2.7	NWI12.jpg	Large barnacles, Fucus, coralline algae in tiny pool
13	0.61	2.0	NWI13.jpg	Fucus, barnacles, red algae
14	0.21	0.7	NWI14.jpg	Fucus. Ledge, barnacles
15	0.31	1.0	NWI15.jpg	Fucus, barnacles
16	0.32	1.1	NWI16.jpg	Fucus, tidepool
17	0.18	0.6	NWI17.jpg	Fucus, barnacles, Ulva, red algae
18	0.03	0.1	NWI18.jpg	Fucus, barnacles
19	-0.01	0.0	NWI19.jpg	Hedophyllum
20	0.25	0.8	NWI20.jpg	Fucus
21	0.04	0.1	NWI21.jpg	Rock with Fucus, barnacles, Ulva
22	-0.16	-0.5	NWI22.jpg	Rock with Fucus, coralline algae
23	-0.44	-1.4	NWI23.jpg	Hedophyllum, Mastocarpus tar spot, encrusting reds
24	-0.59	-1.9	NWI24.jpg	Codium, Ulva, Hedophyllum
Table 4: Species catalogued in the transects at Urchin Rocks and Northwest Island.

* = species observed at the sites but not counted in the transects.
Present at

Urchin Rocks	Present at

Northwest Island
Algae:
Fucus gardneri	X	X
Leathesia difformis	X
Hedophyllum sessile	X	X
Alaria marginata	'*	X
Nereocystis luetkeana	'*	X
Ulva fenestrata	X	X
Codium fragile	'*	X
Corallina vancouveriensis	X
Calliarthron tuberculosum	X
Lithothamnion phymatodeum	X	X
Pterothamnion pectinatum	X
Porphyra cuniformis	X
Chondracanthus exasperata	X
Mastocarpus papilatus	X	X
Mazzealla splendens	X
Endocladia muricata	X
Neorhodomela larix	X	X
Diatoms	X
Cnidarians:
Anthopleura elegantissima	X	X
Urticina crassicornis	'*	X
Crustaceans:
Balanus glandula	X	X
Semibalanus cariosus	X	X
Pagurus hirsutiusculus	X	X
Oregonia gracilis	X	X
Hemigrapsus nudus	X	X
Idotea wosnesenskii	X	X
Mollusks:
Lottia persona	X	X
Lottia digitalis	X	X
Tectura scutum	X	X
Acmaea mitra	X
Diodora aspera	X
Unknown limpet sp	X	X
Mytilus trossulus	X	X
Calliostoma spp	X	X
Nucella lamellosa	X	X
Nucella emarginata	X	X
Unknown whelk	X	X
Ceratostoma foliatum	X
Katharina tunicata	X	X
Mopalia muscosa	X	X
Tonicella lineata	X
Echinoderms:
Pisaster ochraceous	'*	'*
Leptasterias hexactis	X	X
Henricia sp	'*	'*
Strongylocentrotus droebachiensis	'*	'*
Cucumaria miniata	'*	'*
Eupentacta quinquesemita	'*	'*

Table 5: Comparison of algal abundance and diversity among high and low intertidal levels at Urchin Rocks and Northwest Island. Categories: "Site" = Urchin Rocks or Northwest Island. Height: "High" = an intertidal level of +3 feet and above, "Low" = below +3 feet. Macroalgae = brown + green + red algae. Statistically significant differences are printed in boldface.
.....................Mean values.....................	Significance (2-way ANOVA)
Urchin Rocks	NW Island
High	Low	High	Low	Site	Height	Interaction
% cover brown algae	9.1%	6.3%	4.7%	33%	.000	.000	.000
% cover green algae	0%	16%	3.6%	28%	.061	.009	.048
% cover red algae	4.6%	2.7%	7.3%	18%	.002	.127	.029
% cover macroalgae	14%	25%	12%	54%	.013	.000	.007
% cover diatoms	0%	30%	0%	0%	.001	.001	.001
Total algae % cover	14%	55%	12%	54%	.839	.000	.989
Algal species richness	1.1	2.9	1.7	3.2	.196	.000	.695

Table 6: Comparison of animal abundance and diversity among high and low intertidal levels at Urchin Rocks and Northwest Island. Categories: "Site" = Urchin Rocks or Northwest Island. Height: "High" = an intertidal level of +3 feet and above, "Low" = below +3 feet. Counts are per square meter. Barnacle relative size is obtained by dividing % cover by count x 1000 for each quadrat. Statistically significant differences are printed in boldface.
.....................Mean values.....................	Significance (2-way ANOVA)
Urchin Rocks	NW Island
High	Low	High	Low	Site	Height	Interaction
Cnidarian count	165	100	61.9	18.5	.005	.083	.724
Cnidarian % cover	13.6	13.8	8.00	3.85	.091	.661	.629
Barnacle count	7360	3940	4010	2560	.031	.026	.356
Barnacle % cover	48%	25%	50%	29%	.666	.001	.933
Barnacle relative size	9.7	10.2	16.4	15.6	.113	.977	.855
Motile crustaceans count	1.80	1.70	0.38	1.80	.419	.441	.374
Mussel count	.57	6.18	134	.07	.307	.304	.264
Motile mollusks count	332	178	168	113	.044	.065	.367
Total animal count	7859	4219	4426	2689	.029	.018	.386
Total animal % cover	62%	40%	58%	33%	.534	.008	.842
Total % cover

(algae + animal)	61%	89%	55%	86%	.701	.004	.868
Species richness	6.00	6.90	4.75	8.77	.682	.002	.045
Simpson's diversity	1.72	1.71	1.29	1.59	.081	.344	.312
Simpson's equitability	0.34	0.27	0.28	0.20	.157	.085	.847
Shannon's diversity	0.64	0.72	0.42	0.66	.153	.107	.402
Shannon's equitability	0.40	0.39	0.27	0.32	.096	.755	.626
Simpson's diversity

(barnacles omitted)	2.31	2.56	2.01	3.41	.421	.020	.096
Simpson's equitability

(barnacles omitted)	0.50	0.53	0.62	0.51	.387	.456	.234
Shannon's diversity

(barnacles omitted)	1.01	0.99	0.81	1.39	.439	.038	.028
Shannon's equitability

(barnacles omitted)	0.66	0.63	0.69	0.73	.326	.911	.605
Rarefaction diversity

(barnacles omitted)	2.78	2.85	2.44	3.80	.329	.026	.044
Table 7: Number of individual animals of selected species found in a concentrated search over approximately 400 m2 at Urchin Rocks and Northwest Island. Substrate was primarily undulating bedrock with a few boulders. Chi-square probabilities indicating a significant probability that the observed counts differ from a 1:1 ratio are in boldface.
Probability of equality

by Chi-square
Species	Urchin Rocks	Northwest Island	Chi-square

(df = 1)	P
Sponges: Haliclona sp	(several)	(several)	–	--
Cnidaria:
Anthopleura elegantissima	507	109	255.8	<.01
Urticina sp	6	32	16.4	<.01
Echinoderms:
Leptasterias hexactis	143	17	97.6	<.01
Pisaster ochraceous	1	0
Henricia sp	1	5	1.5	< .5
Strongylocentrotus droebachiensis	1	9	4.9	< .05
Cucumaria miniata	35	49	2.0	<.5
Eupentacta quinquesemita	10	12	.04	<.9

Figure Captions

Figure 1: Transect sites on Urchin Rocks and Northwest Island, Rosario Bay, Fidalgo Island, WA.

Map is adapted from NOAA. Composite aerial view is from WA Dept. of Ecology

Figure 2: Slope characteristics of the two sites. A: Urchin Rocks. B: Northwest Island

Figure 3: Photographic comparison of the community sampled at Urchin Rocks and Northwest Island

at characteristic tidal heights

Figure 4: Total percent cover for A: brown algae and B: green algae by tidal height at the two sites

Figure 5: Total percent cover for A: red algae and B: coralline algae by tidal height at the two sites

Figure 6: Total percent cover for A: macroalgae (brown, red, and green algae) and B: diatoms

by tidal height at the two sites

Figure 7: A: Percent cover for total algae (macroalgae + diatoms). B: Algal species richness by

tidal height at the two sites

Figure 8: A: Animal species richness and B: Animal diversity by the rarefaction method (sample

size = 9) by tidal height at the two sites

Figure 9: A: Simpson's diversity index and B: Simpson's equitability index for all animals by

tidal height at the two sites

Figure 10: A: Shannon's diversity index and B: Shannon's equitability index for all animals by

tidal height at the two sites

Figure 11: A: Simpson's diversity index and B: Simpson's equitability index for all animals except

barnacles by tidal height at the two sites

Figure 12: A: Shannon's diversity index and B: Shannon's equitability index for all animals except

barnacles by tidal height at the two sites

Figure 13: A: Total cnidarian percent cover and B: Total cnidarian count per square meter by tidal

height at the two sites

Figure 14: A: Total motile crustaceans and B: Total motile mollusks per square meter by tidal

height at the two sites

Figure 15: A: Total barnacle percent cover and B: Total barnacles per square meter by tidal

height at the two sites

Figure 16: A: Total animal percent cover and B: Total animal count per square meter by tidal

height at the two sites

Figure 17: Barnacle relative size (% cover/count x 1000) by tidal height at the two sites

Figure 18: Total mussel (Mytilus trossulus) count per square meter by tidal height at the two sites

Figure 19: Total percent cover (combining algae and animals) by tidal height at the two sites