Marine Environmental ReJearch 7 (I 982) 279-293

A QUANTITATIVE ASSESSMENT OF HUMAN TRAMPLING EFFECTS ON A ROCKY INTERTIDAL

COMMUNITY

K. A. BEAUCHAMP & M. M. GOWING

Center for Coastal Marine Studies, University o f California, Santa Cruz, California 95064, USA

(Received: 1 May, 1982)

ABSTRACT

The density and dirersity of algae and im'ertebrates in the rocky marine intertidal were studied at three sites differing in degree of human trampling. Quantitatit, e sampling showed that (1) a general pattern of higher density and diversity occurred at the less trampled sites, (2) the densities of the mussels and barnacles and the dh'ersity of algae were unaffected and (3) at the most trampled site, the brown alga Pelvetiopsis limitata was absent and the small bivalves Lasaea spp. were found in lower densities.

INTRODUCTION

Declines in the density and diversity of intertidal biota have been attributed to the use of their habitat by humans, as reported in studies of molluscs (Smith & Gordon, 1948), algae (Widdowson, 1971 ; Boalch et al., 1974; Emerson & Zedler, 1978) and pollution (Nicholson & Cimberg, 1971 ; Nicholson, 1972; Littler & Murray, 1975). These effects have recently been summarised by Gordon (1974) and Carefoot (1977). However, there is no published work in which human usage has been quantified. Woodland & Hooper (1977) reported effects of experimental trampling of coral reefs in Australia, but there are no comparable studies for north temperate zones.

Human use has been suggested as a cause of long-term algal changes along permanent transects. Boalch et al. (1974) resurveyed an area originally sureeyed by Colman (1933) and found a general decrease in fucoid cover, possibly due to human trampling. Thom & Widdowson (1978) resurveyed several intertidal sites surveyed by Dawson (1965) and documented changes in algal forms, with larger species becoming less common, and crustose and turf forms increasing in abundance, especially in areas near large cities and public parks.

279 Marine Environ. Res. 0141-1136/82/0007-0279/$02.75 © Applied Science Publishers Ltd, England. 1982 Printed in Great Britain

280 K . A . BEAUCHAMP, M. M. GOWING

Terrestrial studies of human trampling are common (reviewed by Liddle, 1975), and the methods for assessing damage are fairly standardised. Organisms are usually counted along transects perpendicular to trails or paths. People do not use paths to explore the intertidal, so a different sampling method is required. In contrast to terrestrial studies, Gonor & Kemp (1978, p. 1) noted that 'many methods in current use for studies of the impact of human activities on intertidal environments are found only in unpublished reports of limited circulation'.

In this paper we present the results of a quantitative field study using randomly selected quadrats. This study was designed to compare species diversity and density of organisms in an intertidal area with three distinct levels of usage by humans, and to identify organisms that appear susceptible to human trampling.

MATERIALS AND METHODS

The study area (36 °, 57', 05"N; 122 °, 04', 45"W) is located in Santa Cruz, California, USA, adjacent to Natural Bridges State Park, the De Anza Mobile Estates and the Joseph M. Long Marine Laboratory ofthe University of California at Santa Cruz (Fig. 1). The three sampling sites within the study area were located in the rocky mid-intertidal, a zone characterised by dense mussel beds interspersed with sparsely populated patches (hereafter called bare rock). Site S (trampled, probably stressed) is easily accessible from the State Park to the east and is used by educational tours as well as casual visitors (Fig. 2). Site U (untrampled) is located on a small island (8 m x 17 m) 15 m west of site S and surrounded by a deep channel at low tide. The island is difficult to climb onto, and is infrequently visited. Site I (intermediate) is intermediate between sites S and U in accessibility and human use. The average elevation of each study site is as follows: Site S, 1.8 m above MLLW, Site U, 1.5 m above MLLW and Site I, 1.7 m above MLLW. The sites were sampled on three consecutive days in December, 1977 and again on three consecutive days in June, 1978. At each site, within a 10m 2 area, twenty randomly selected squares, 10 cm on a side, were scraped bare with a putty knife and the organisms collected. Of the twenty samples, ten were from the mussel bed and ten from the bare rock; tidepools were not sampled.

Specimens were preserved in borate-buffered 10 ~ formalin. Macroscopic (> I mm) animals and algae were identified to species whenever possible, using keys in Smith & Carlton (1975) and Abbott & Hollenberg (1976), respectively. The algal genera Cladophora, Ralfsia, Ulva and Enteromorpha were not identified to species because either immature specimens were present or the condition of the specimens obscured the necessary key characteristics. Two dominant invertebrates were not identified to species. The accurate identification of numerous young acorn barnacles (Balanus spp. and Chthamalus spp.) was too time-consuming and uncertainty in species descriptions (M. Keen, personal communication; Haderlie & Abbott, 1980)

HUMAN TRAMPLING EFFECTS ON A ROCKY INTERTIDAL COMMUNITY 281

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in the bivalve genus Lasaea did not permit certain identification of L. subt'iridis and L. cistula. The numbers of individual invertebrates, the wet weights of algae and shell lengths of Mytilus californianus (to the nearest 1.0 mm) were recorded.

Peh'etiopsis limitata, about 4-8 cm in length, is a visually dominant brown alga. Specimens were found to be too large to be adequately sampled using 100 cm 2 quadrats. Thus, twenty-four random l / 4m 2 quadrats were photographed (using Ektachrome 200 film) in June, 1978 and in December, 1978 in an area 8 m by 10 m including the study sites and the high mid-intertidal zone (see Fig. 1). These

282 K. A. BEAUCHAMP, M. M. GOWING

Fig. 2. View of study sites S (left side of channel with people) and U (right side of channel).

quadrats included both mussel bed and bare rock. The approximate percentage cover of P. limitata was determined by projecting the slides on paper, tracing the outline of the algal covering and weighing the paper cutouts.

To estimate the degree of human use, counts of people visiting the sites were recorded between the autumn of 1978 and the spring of 1979 during low tides. Although the human counts and the ecological study were not concurrent, we have observed over several years that the relative numbers of people visiting the sites are reflected in our counts. The dates and times of low tides for counts were selected randomly during times when tidepool tours were and were not scheduled, and included week days, week-ends and holidays. Seventy counts were made over 21 days, encompassing a total period of 9.6 h. The mean length of time per day was 27 min, with a count made every 5-15 min.

Dominance diversity curves (Whittaker, 1965) were constructed for each site and season for numbers of animals and wet weights of algae. A Shannon-Wiener diversity index (Krebs, 1972)* was calculated for each sample, using only numbers of animals that had been identified to species. Many species were found in insufficient densities for statistical comparison of sites, but these organisms were included in diversity calculations. Samples of algae from the mussel bed and bare rock and samples of animals from the bare rock contained too few species for unbiased use of

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and p~ = the proportion of the sample belonging to the ith species.)

HUMAN TRAMPLING EFFECTS ON A ROCKY INTERTIDAL COMMUNITY 283

the Shannon-Wiener diversity index (Bechtel & Copeland, 1970): the number of species in these samples was recorded.

Statistical analyses were performed using the SPSS (Statistical Package for the Social Sciences, Nie et al., 1975; Hull & Nie, 1979) and SAS (Statistical Analysis System, SAS Institute, 1979) subroutines. We selected either parametric or non- parametric tests using the criteria of Sokal & Rohlf (1969).

RESULTS

Number of human visitors The Natural Bridges State Park opened in 1933. From 1974 to 1978, approx-

imately 1,524,000 people visited the park, and visits have been increasing at the average rate of 67,000 a year since 1974 (Natural Bridges State Park Office, personal communication). In recent years participants in the Ocean Education Project, sponsored by the University of California, Santa Cruz Internship Program, have conducted tidepool tours in the study area for schools in the local counties. These tours emphasise minimal disturbance and allow no collecting. From January, 1977 to August, 1979 approximately 15,400 people have taken these tours (J. Anderson, personal communication).

A total of 538 people in the study area was recorded from our seventy counts. No one was seen on site U or on the island. Seven ( + 1 SE) people per count were seen on, or in close proximity to, site S and one ( +0.24 SE) per count on, or near, site I.

Densio' of organisms Totals of 67 species of invertebrates and 14 species of algae were identified from the

samples. Winter and spring densities of most taxa in the mussel bed samples were not significantly different at the three sites. In the instances where there was a difference, densities were usually higher in winter. The bare rock samples showed the same general pattern.

In the mussel bed, the mean number of animals and weight of algae were greater at the less trampled sites, U or I, in the spring (Table 1), but there was no significant difference among sites in the winter. On the bare rock there were no significant differences among the sites. The numbers of acorn barnacles (Balanus spp. and Chthamalus spp.) on the bare rock and in the mussel bed did not differ significantly among sites (Table 2). In the winter there were also no differences among sites in numbers of the mussel Mytilus ealifornianus, which is the most abundant organism in terms of biomass. Mussels were least dense at the least trampled site in the spring. Littorina spp. (Littorina scutulata plus a few Littorina keenae) were equally common at all sites on the bare rock. Numbers of the bivalves Lasaea spp. were much higher at the less trampled sites. There were more site effects for algae in the mussel bed than on the bare rock and Pelvetiopsis limitata (Table 3) was

TABLE 1 MEAN NUMBERS OF ANIMALS AND ALGAL WEIGHTS ( -1- SE) PER 100 cm l FOR WINTER AND SPRING SAMPLES FROM EACH SITE IN THE MUSSEL BED AND ON BARE ROCK. DIFFERENCES IN RANKS OF SITES WERE EVALUATED WITH THE KRUSKALL-WALLIS TEST AND SITES ARE LISTED IN ORDER OF DECREASING RANKS, A COMMA INDICATES NO SIGNIFICANT DIFFERENCE (P > 0"05) IN RANKS BETWEEN IMMEDIATELY ADJACENT PAIRS AS EVALUATED WITH THE MANN-WHITNEY U TEST. NS = NO SIGNIFICANT DIFFERENCE (P > 0'05) AMONG THE THREE RANKS. SAMPLE SIZE WAS 10 FOR

EACH SITE IN BOTH WINTER AND SPRING

Taxon Season Mussel bed Differences Site S Site 1 Site U among ranks

Animals (Number) W 3536+278 3065 + 342 3939 +422 NS S 1 5 1 3 + ! 9 7 1737+128 2662+321 U > I , S

Algae (weight) W 0.8 + 0.3 0.7 + 0-2 I "9 _ 0.7 NS S 0.2+-0-1 1 .6+0 .4 1 .3+0.8 I , U , S (I > S )

Taxon Season Bare rock Differences Site S Site 1 Site U among ranks

Animals (Number) W 269 + 78 249 + 25 233 + 52 NS S 2 4 8 + 5 7 3 6 2 + 5 3 4 1 4 + 6 9 NS

Algae (weight) W 2.6 + 0-9 0.4 + 0-1 2.2 + 0.8 NS S 0 . 3 + 0 . 3 0 . 3 + 0 . 2 0 . 4 + 0 . 2 NS

TABLE 2 MEANS ( -1- SE) OF NUMBERS PER i 00 ¢m 2 OF DOMINANT ORGANISMS IN THE MUSSEL BED AND ON BARE ROCK FOR EACH SITE AND SEASON. NUMBERS OF INDIVIDUALS ARE SHOWN FOR ANIMALS AND WET WEIGHTS IN GRAMS FOR ALGAE. DIFFERENCES AMONG RANKS WERE EVALUATED AS IN TABLE 1. SAMPLE SIZE -- 10 FOR EACH SITE

AND EACH SEASON. W = WINTER 1977. S m SPRING 1978

Mussel bed Taxon Season Site S Site 1 Site U Differences

among ranks

Mytiluscalifornianus W 112-t-16 1 1 5 + 7 8 9 + 6 NS S 143+21 231-1-27 112+12 I > S > U

Lasaeaspp. W 1 0 + 7 37+_18 364+ 103 U > I > S S 0 1 4 + 7 751+381 U > I > S

Balanus spp. and Chthamalus spp. W 3066 + 247 2759 +- 325 3406 + 423 NS S 1308+ 171 1393+ 138 1777+- 163 NS

Rhodophyta W 0.64 + 0.35 0.60 _+ 0.26 0.40 + 0.10 NS S 0 .17+0 .09 1 .50+0.40 0 .50+0 .20 I > U > S

Phaeophyta W 0.01 + 0.01 0.03 +- 0.02 1-74 + 0-70 U > I, S S 0.01 -t- 0.01 0.01 + <0.01 0.80 + 0-80 NS

Bare rock

Taxon Season Site S Site I Site U Differences among ranks

Balanusspp. and Chthamalusspp. W 2 1 8 + 7 9 1 7 0 + 2 0 174+41 NS S 179 + 39 235 + 35 294 + 57 NS

Littorina spp. W 13 + 4 27 + 4 36 + 11 NS S 54 + 19 73 ___ 19 76 + 23 NS

Rhodophyta W 0-34 + 0-14 1-40 + 0-80 0.27 + 0-25 NS S <0.01 0 -29+0 .15 0 .14+0 .12 I , U , S ( I > S )

Chlorophyta W 0 .13+0-05 0 .40+0 .20 0 .14+0-05 NS S 0 .26+0-26 0 .02+0-02 0-15+0.11 NS

HUMAN TRAMPLING EFFECTS ON A ROCKY INTERTIDAL COMMUNITY 285

T A B L E 3 MEAN PERCENTAGE COVER OF Pelvetiopsis limitata. TWENTY-FOUR PHOTO QUADRATS WERE EVALUATED IN THE WINTER AND THE SPRING OF

1978 AT EACH SITE AND INCLUDE BOTH MUSSEL BED AND BARE ROCK.

DIFFERENCES AMONG RANKS WERE EVALUATED AS IN TABLE 1

Season Site S Site 1 Site U Difference among ranks

Winter 0 3 - 3 + 0 . 7 7 . 8 + 1 . 5 U > I > S Spring 0 7 . 7 + 1 . 4 8 - 4 + 2 ' 1 I , U > S

conspicuously absent from the most-trampled site. In the spring there was no significant difference in percentage cover by P. limitata on sites U and I, but in the winter, cover was significantly greater at site U than at site I.

Diversity of organisms Dominance-diversity curves were similar in shape for the three sites for both

animals and algae, demonstrating the same general community structure. Representative curves are shown in Fig. 3 for animals at the three mussel bed sites

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286 K. A. BEAUCHAMP, M. M. GOWING

T AB L E 4 MEAN SHANNON-WIENER DIVERSITY INDEX ( + S E ) FOR MUSSEL BED ANIMALS. THE SAMPLE SIZE WAS 10 FOR EACH SITE FOR EACH SEASON.

DIFFERENCES AMONG RANKS WERE EVALUATED AS IN TABLE l

Season Site S Site 1 Site U Difference among ranks

Winter 1 .98±0.14 2 .56±0 .06 1 .65±0.20 I > S , U Spring 1.32±0.11 1 .48±0.09 1 .94±0.25 NS

T A B L E 5 MEAN ( ± S E ) NUMBER OF SPECIES OF ANIMALS AND ALGAE AT THE THREE SITES DURING BOTH SEASONS.

DIFFERENCES AMONG RANKS WERE EVALUATED AS IN TABLE l

Mussel bed Taxon Season Site S Site 1 Site U Differences

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Animals (Number of species) W S

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for the winter and spring samples, and indicate communities with large numbers of rare species. Shannon-Wiener diversity indices for animals in the mussel bed differed in the winter and spring (Table 4). In the spring, there was no significant difference in mean diversity index among the three sites. In the winter, however, diversity was significantly highest at the untrampled site.

The number of animal species was higher at the intermediate site than at the trampled site, in both winter and spring (Table 5). In the spring, there was no significant difference between the number of species of animals at sites U and I. There were fewer significant site differences in numbers of algal species and, when differences were significant, more algal species were found at sites U and I than at site S.

Size distribution of Mytilus californianus In all samples the majority of mussels were between 1 and 9 mm in length and the

distributional pattern was similar for all sites (Fig. 4). Mean mussel size (Table 6) during winter and spring was greatest at site U, intermediate at site S and least at site

HUMAN TRAMPLING EFFECTS ON A ROCKY INTERTIDAL COMMUNITY 287

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288 K. A. BEAUCHAMP, M. M. G O W I N G

T A B L E 6 MEAN SIZE IN MILLIMETERS ( "b SE) OF Mytilus californianus FROM MUSSEL BED SAMPLES ( N -- SAMPLE SIZE). DIFFERENCES AMONG RANKS WERE EVALUATED AS

IN TABLE I AND WERE SIGNIFICANT AT THE 0"001 LEVEL

Winter Spring Mean + SE (N) Mean + SE (N)

Site S 1 6 . 6 + 0 . 5 1221 1 5 . 0 + 0 . 4 1433 Site I 1 5 . 3 + 0 . 6 1134 1 0 . 0 + 0 - 3 1963 Site U 1 8 - 4 + 0 - 6 935 1 8 . 9 + 0 - 5 1119

Differences among ranks U > S > 1 U > S > I

T A B L E 7 COMPARISONS OF NUMBERS OF LARGE AND SMALL Mytilus californianus AT THE THREE SITES. DIFFER- ENCES IN MEANS WERE EVALUATED WITH ANALYSIS OF VARIANCE OR THE KRUSKALL-WALLIS TEST. RE o SULTS SHOWN ARE SIGNIHCANT AT THE 0"05 LEVEL UNLESS OTHERWISE NOTED (NS = NOT SIGNI- HCANT). DIFFERENCES AMONG MEANS ARE AS IN

TABLE 1

Size Season Differences among means

l - 9 m m Winter I , S , U ( I > U ) Spring I > S > U

> 9 mm Winter NS Spring NS

I. Table 7 shows a comparison of large ( > 9 mm length) and small (1-9 mm) Mytilus californianus at the three sites. There is a significant difference in the number of small mussels among the three sites in both seasons, but no difference in the number of large mussels.

DISCUSSION A N D CONCLUSIONS

There is a marked gradient of human usage in these adjacent rocky intertidal areas (23 m from site S to site I and site U in between). It is unlikely that a gradient of overriding unmeasured physical factors exists among the sites. The three sites face the bay at a similar angle, have the same slope, appear to receive the same wave impact and the least-trampled site, site U, is bracketed by the other two sites. Large- scale physical disruption of the intertidal habitat along this stretch of the Pacific coast by random events such as crashing logs or boulders is rare. Harbour seals, which may cause changes in algal and animal composition (Boal, 1980), do not haul out here. While it is possible that undetected differences in water movement may

HUMAN TRAMPLING EFFECTS ON A ROCKY INTERTIDAL COMMUNITY 289

influence one site more than another, we believe the most likely cause of the observed differences is human trampling.

Measures of species diversity are commonly used in evaluating effects of stress on aquatic communities (Wilhm & Dorris, 1966; Sager & Hasler, 1969; Bechtel & Copeland, 1970; Cairns et al., I971 ; Littler & Murray, 1975; Botton, 1979 and Loi & Wilson, 1979). Excessive stress generally results in decreased diversity. In our study, animal diversity in the winter mussel bed samples was significantly greater for the least trampled site. Numbers of species of animals were consistently high at the intermediate trampled site for both seasons. These results are in agreement with Connelrs (1978) recent argument that diversity may be higher when disturbances are intermediate.

The greater similarity of numbers of species of algae among the three sites may be due to a combination of factors. Intertidal algae have been under stress due to human activities since the early 1900's (Nicholson, 1972) and in disturbed areas in Southern California there has been a change in algal types from larger species to turf-like forms (Littler & Murray, 1975; Widdowson, 1971; Thorn & Widdowson, 1978). The turf-form algae Endocladia muricata, Cladophora spp. and Polysiphonia pacifica were common in this study at all three sites. Thus, more intensive stress may be required to diminish species diversity. Alternatively, algal diversity may be controlled by herbivore populations (grazing molluscs), which were similar in species composition ( Collisella pelta, C. digitalis, C. scabra, C. strigatella, Tegula funebralis, Littorina scutulata, and L. planaxis) at all sites and in abundance at sites I and S. Lubchenco (1978) found that the effect of an herbivorous snail on algal species diversity depended on the relationship between the herbivore's food preference and the competitive abilities of the plants. Unless there is a site-specific change in herbivore species or densities, the number of algal species may remain similar at all sites.

Disturbed areas are often characterised by high densities of tolerant organisms (Patrick, 1949). Our results showed a different pattern. When differences in densities of animals among the sites were significant, highest densities occurred in the less trampled sites (Tables 1 and 2). Densities of several common organisms, such as Collisella scabra, Tegula funebralis, errant polychaetes, and Decapoda, were the same in the three sites. The algae showed the same general pattern: where there were site differences, weights were highest at sites I or U. No alga was significantly more dense at site S. These results suggest that the human stress at site S has not reached a sufficient intensity or duration to select for a few resistant species.

Another measure of disturbance is the decrease or loss of an organism from an area. The most striking difference between the three sites is the absence of Pelcetiopsis limitata at site S. Little is known of the ecology of P. limitata, but this alga can have a patchy local distribution (T. DeCew, personal communication). However, since a few plants did grow on the edge of the shelf near site S, and the surface of site S did not appear different than at sites U or I, we believe that human

290 K . A . BEAUCHAMP, M. M. GOWING

trampling is responsible for the absence of P. iimitata at site S. Although this large alga looks relatively tough, we have observed that pieces easily break off under foot, especially when the plants are growing on the edges of depressions in the rock. Reduction or absence of an alga as large as P. limitata may affect other organisms and alter habitats and heterogeneity of the mid-zone. The turf-forms that often replace arborescent algae in disturbed areas are thought to provide less habitat heterogeneity (Littler & Murray, 1975). Perhaps this phenomenon is reflected in the total number of algal plus animal species at the three sites, which was significantly less at site S than at sites U and I in winter and significantly less at site S than at site I in the spring.

Lasaea spp. are small (l-4mm), byssus-producing bivalves that, in turn, live among the byssus threads secreted by Mytilus californianus. They also live in other microhabitats such as sandy crevices, occasionally on the holdfasts of algae, as well as in empty barnacle shells. These clams were numerically dominant in the mussel bed samples and were found in significantly higher densities at site U in both spring and winter; they were absent from site S in the spring samples. One might expect that Lasaea would be less affected by human trampling because of its seemingly protected habitat. Human traffic may have a more subtle effect, such as removing suitable algal holdfasts (i.e. Pelvetiopsis limitata), or perhaps toot pressure is transmitted to the base of the mussels, weakening attachment of Lasaea to the byssal threads or to the substratum. Since this population of Lasaea spp. are brooding hermaphrodites (Beauchamp, unpublished observations), once they are lost from an area, they do not have the same potential for rapid recolonisation that is typical of organisms with planktonic larvae.

Numerically dominant organisms that did not show pronounced differences in density among the sites were Mytilus californianus and the barnacles Balanus spp. and Chthamalus spp. Because mussels are directly subjected to foot pressure, the increased numbers of people walking at site S might be expected to weaken the byssus attachment to the rocks, thus causing more mussels to be torn out by waves and storms. Open space could either cause the mussel bed to be less dense, and consist of predominantly smaller young mussels, or allow surviving mussels to grow larger. We found that mean mussel size was smallest at the intermediate site for both seasons. Densities of mussels were similar at all sites in the winter and higher at site S than at U in the spring (Table 2). Therefore, Mytilus californianus densities and sizes appear to be unaffected by trampling.

The large numbers of mussels in the smallest size class in two seasons (Fig. 4) are consistent with continuous spawning (Young, 1942; Suchanek, 1978) and recruit- ment via planktonic larvae throughout the year. Mytilus californianus larval settlement requires secondary space; that is, space covered by barnacles, byssus threads, or certain algae (Dayton, 1971). Limpets reduce barnacle and algal recruitment (Dayton, 1971). In our study, barnacle densities did not differ at the three sites and limpets greater than 5 mm long were equally abundant at sites S and I

HUMAN TRAMPLING EFFECTS ON A ROCKY INTERTIDAL COMMUNITY 291

in both seasons. Thus, the greater number of small mussels at site I (Table 7) may be due to algal species present, byssus threads, or a combination of both. Although site I has an advantage in recruitment, densities of mussels larger than 9 mm did not differ significantly among sites (Table 7).

Barnacles also might be sensitive to trampling because many grow on mussel shells and would be subjected to abrasion. There was, however, no significant difference in the densities of Balanus spp. plus Chthamalus spp. among the three sites for either season in the mussel bed samples. On the bare rock, where barnacles are stepped on directly, there were also no site differences in density. We have observed that the effect of foot pressure on barnacles depends on where the barnacle is growing. Barnacles on the top edges of mussel shells often are sheared off when stepped on, whereas many barnacles are protected from trampling by uneven topography. A footstep on a patch of barnacles may have no effect; however, twisting motions of the foot will destroy the barnacles that are taller than their neighbours.

The effect of trampling on bare rock can be considered in two alternative hypotheses: (1) as a naturally stressed area, bare rock would be less sensitive to human traffic than a mussel bed or (2) with less physical structure and fewer organisms to cushion foot pressure, bare rock would be more adversely affected than the mussel bed. It was not possible to accept or reject either hypothesis because of the lack of consistent trends in species richness and density site differences (Tables 1 and 5). However, the algae Ralfsia spp. and Codiurn setchelli were found only in bare rock samples; no animals were unique to the bare rock.

In summary, we have examined the diversity and density of organisms in three closely adjacent rocky intertidal sites subjected to widely differing amounts of human use. There was a general pattern of higher densities and species richness at the less trampled sites. This was more pronounced in the mussel bed than on the bare rock. Numbers of species of algae were more similar at all sites than numbers of species of animals, and turf-form algae were common. Of the dominant organisms, acorn barnacle densities and Mytilus californianus densities and sizes were unaffected by human traffic. The absence of the brown alga Peh'etiopsis limitata at site S accounted for the striking visual difference among the three sites. This alga, along with the small bivalve Lasaea spp., may be sensitive indicators of human traffic.

ACKNOWLEDGEMENTS

We appreciate the assistance given by a number of people throughout this study: Dr J. Pearse, Dr R. Hinegardner, D. Lindberg, Dr M. Silver, Dr L. Fox, D. Beauchamp, Dr D. Coder, Dr D. Garrison, Dr W. Doyle, Dr L. Goff, Dr P. Dayton and Dr R. Spies. The De Anza Mobile Estates kindly allowed us to count people from their property. This project was supported in part by Environmental Studies and Marine Studies at the University of California, Santa Cruz.

292 K. A. BEAUCHAMP, M. M. GOW1NG

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