A quantitative assessment of human trampling effects on a rocky intertidal community

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<ul><li><p>Marine Environmental ReJearch 7 (I 982) 279-293 </p><p>A QUANTITATIVE ASSESSMENT OF HUMAN TRAMPLING EFFECTS ON A ROCKY INTERTIDAL </p><p>COMMUNITY </p><p>K. A. BEAUCHAMP &amp; M. M. GOWING </p><p>Center for Coastal Marine Studies, University of California, Santa Cruz, California 95064, USA </p><p>(Received: 1 May, 1982) </p><p>ABSTRACT </p><p>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. </p><p>INTRODUCTION </p><p>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 &amp; Gordon, 1948), algae (Widdowson, 1971 ; Boalch et al., 1974; Emerson &amp; Zedler, 1978) and pollution (Nicholson &amp; Cimberg, 1971 ; Nicholson, 1972; Littler &amp; 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 &amp; Hooper (1977) reported effects of experimental trampling of coral reefs in Australia, but there are no comparable studies for north temperate zones. </p><p>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 &amp; 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. </p><p>279 Marine Environ. Res. 0141-1136/82/0007-0279/$02.75 Applied Science Publishers Ltd, England. 1982 Printed in Great Britain </p></li><li><p>280 K.A . BEAUCHAMP, M. M. GOWING </p><p>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 &amp; 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'. </p><p>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. </p><p>MATERIALS AND METHODS </p><p>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. </p><p>Specimens were preserved in borate-buffered 10 ~ formalin. Macroscopic (&gt; I mm) animals and algae were identified to species whenever possible, using keys in Smith &amp; Carlton (1975) and Abbott &amp; 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 &amp; Abbott, 1980) </p></li><li><p>HUMAN TRAMPLING EFFECTS ON A ROCKY INTERTIDAL COMMUNITY 281 </p><p>_z3~ ,z],,dz,,7 L </p><p>lg </p><p>map </p><p>- .gltrt S </p><p>/ ? t6 </p><p>Fig. 1. Natural Bridges intertidal platform, Santa Cruz County, California, USA (36 , 57', 5"N; 122 , 04', 45"W), showing location of study sites S, U and I, December, 1977 to June, 1978. </p><p>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. </p><p>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 </p></li><li><p>282 K. A. BEAUCHAMP, M. M. GOWING </p><p>Fig. 2. View of study sites S (left side of channel with people) and U (right side of channel). </p><p>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. </p><p>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. </p><p>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 </p><p>s </p><p>* Shannon-Wiener formula: H = - ~ (pl)(log~ p~), where H = species diversity, s = number of species </p><p>and p~ = the proportion of the sample belonging to the ith species.) </p></li><li><p>HUMAN TRAMPLING EFFECTS ON A ROCKY INTERTIDAL COMMUNITY 283 </p><p>the Shannon-Wiener diversity index (Bechtel &amp; Copeland, 1970): the number of species in these samples was recorded. </p><p>Statistical analyses were performed using the SPSS (Statistical Package for the Social Sciences, Nie et al., 1975; Hull &amp; Nie, 1979) and SAS (Statistical Analysis System, SAS Institute, 1979) subroutines. We selected either parametric or non- parametric tests using the criteria of Sokal &amp; Rohlf (1969). </p><p>RESULTS </p><p>Number of human visitors The Natural Bridges State Park opened in 1933. From 1974 to 1978, approx- </p><p>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). </p><p>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. </p><p>Densio' of organisms Totals of 67 species of invertebrates and 14 species of algae were identified from the </p><p>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. </p><p>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 </p></li><li><p>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 &gt; 0"05) IN RANKS BETWEEN IMMEDIATELY ADJACENT PAIRS AS EVALUATED WITH THE MANN-WHITNEY U TEST. NS = NO SIGNIFICANT DIFFERENCE (P &gt; 0'05) AMONG THE THREE RANKS. SAMPLE SIZE WAS 10 FOR </p><p>EACH SITE IN BOTH WINTER AND SPRING </p><p>Taxon Season Mussel bed Differences Site S Site 1 Site U among ranks </p><p>Animals (Number) W 3536+278 3065 + 342 3939 +422 NS S 1513+!97 1737+128 2662+321 U&gt;I ,S </p><p>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 &gt;S) </p><p>Taxon Season Bare rock Differences Site S Site 1 Site U among ranks </p><p>Animals (Number) W 269 + 78 249 + 25 233 + 52 NS S 248+57 362+53 414+69 NS </p><p>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 </p><p>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 </p><p>AND EACH SEASON. W = WINTER 1977. S m SPRING 1978 </p><p>Mussel bed Taxon Season Site S Site 1 Site U Differences </p><p>among ranks </p><p>Mytiluscalifornianus W 112-t-16 115+7 89+6 NS S 143+21 231-1-27 112+12 I&gt;S&gt;U </p><p>Lasaeaspp. W 10+7 37+_18 364+103 U&gt;I&gt;S S 0 14+7 751+381 U&gt;I&gt;S </p><p>Balanus spp. and Chthamalus spp. W 3066 + 247 2759 +- 325 3406 + 423 NS S 1308+ 171 1393+ 138 1777+- 163 NS </p><p>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&gt;U&gt;S </p><p>Phaeophyta W 0.01 + 0.01 0.03 +- 0.02 1-74 + 0-70 U &gt; I, S S 0.01 -t- 0.01 0.01 + </p></li><li><p>HUMAN TRAMPLING EFFECTS ON A ROCKY INTERTIDAL COMMUNITY 285 </p><p>TABLE 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. </p><p>DIFFERENCES AMONG RANKS WERE EVALUATED AS IN TABLE 1 </p><p>Season Site S Site 1 Site U Difference among ranks </p><p>Winter 0 3 -3+0.7 7 .8+1.5 U&gt;I&gt;S Spring 0 7 .7+1.4 8 -4+2'1 I ,U&gt;S </p><p>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. </p><p>Diversity of organisms Dominance-diversity curves were similar in shape for the three sites for both </p><p>animals and algae, demonstrating the same general community structure. Representative curves are shown in Fig. 3 for animals at the three mussel bed sites </p><p>1~ooo </p><p>e, ooo </p><p>~.ooo </p><p>4.ooo </p><p>~+ooc </p><p>Iooo </p><p>ao+ </p><p>l + </p><p>i a| . +. 4 %. </p><p>+ </p><p>+%1 </p><p>&lt; </p><p>I I I </p><p>W </p><p>I /~00o </p><p>- - &amp;~oo </p><p>- - ~000 </p><p>- - 800 </p><p>- - 600 </p><p>- - ~ ~'oo </p><p>1 </p><p>1 </p><p>80 </p><p>6o </p><p>1, 8 </p><p>,4 </p><p>Z </p><p>I I </p><p>- $ </p><p>X </p><p>I x </p><p>i / </p><p>1 </p><p>I </p><p>- -e </p><p>- - + </p><p>x </p><p>1% +.."s,K~4~.~t.%~'.~Y,:: . . . . I ~; o m 2O 3O 40 </p><p> S'PEC/E~ SE~O'E,~.,'CL" </p><p>Fig. 3. Dominance diversity curves for each site in the mus~l bed winter (W) and spring (S) samples. + = site S (most trampled),. = site 1 (intermediate level of trampling), x = site U (least trampled). </p></li><li><p>286 K. A. BEAUCHAMP, M. M. GOWING </p><p>TABLE 4 MEAN SHANNON-WIENER DIVERSITY INDEX (+SE) FOR MUSSEL BED ANIMALS. THE SAMPLE SIZE WAS 10 FOR EACH SITE FOR EACH SEASON. </p><p>DIFFERENCES AMONG RANKS WERE EVALUATED AS IN TABLE l </p><p>Season Site S Site 1 Site U Difference among ranks </p><p>Winter 1.980.14 2.560.06 1.650.20 I&gt;S,U Spring 1.320.11 1.480.09 1.940.25 NS </p><p>TABLE 5 MEAN (SE) NUMBER OF SPECIES OF ANIMALS AND ALGAE AT THE THREE SITES DURING BOTH SEASONS. </p><p>DIFFERENCES AMONG RANKS WERE EVALUATED AS IN TABLE l </p><p>Mussel bed Taxon Season Site S Site 1 Site U Differences </p><p>among ranks </p><p>Animals (Number of species) W S </p><p>Algae (Number of species) W S </p><p>20.91.1 29-10-9 23.20.9 I&gt;U,S 15.31.5 20.00.6 17.81.4 I ,U ,S ( I&gt;S) 2 .80.2 4 .60.5 5 .50.4 U , I&gt;S 2 .00.5 2-50.4 2-10.3 NS </p><p>Taxon Season Bare rock </p><p>Site S Site I Site U Differences among ranks </p><p>Animals(Numberofspec ies) W 9 .31.0 S 7.31-1 </p><p>A lgae(Numberofspeeies) W 4 .60.6...</p></li></ul>

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