Biodivers Conserv (2011) 20:1973–1985 DOI 10.1007/s10531-011-0069-1 ORIGINAL PAPER
Impacts on biodiversity at Baltic Sea beaches Marc Schierding • Susanne Vahder • Laura Dau • Ulrich Irmler
Received: 31 January 2011 / Accepted: 5 May 2011 / Published online: 13 May 2011 Ó Springer Science+Business Media B.V. 2011
Abstract Sandy and shingle beaches were investigated in 2009 and 2010 along the coast of the Baltic Sea in northern Germany with the purpose of assessing biodiversity gradients from shingle to sandy beaches, from beach to primary dunes, and the impacts of tourism on biodiversity. On nine beach sites, ranging between 100% shingle and 99% sand without shingle, Carabidae, Staphylinidae, and Araneae were studied. Two of the six sandy beaches were open and four were closed to tourists. Additionally, trampling effects from tourists, species richness of plants, and plant cover were investigated on sixteen beaches. According to results, primary dunes showed higher species richness in carabids and spiders, but not in staphylinds. Shingle beaches exhibited lower species richness in Staphylinidae and Araneae, but not in Carabidae. As estimated by the Jackknife II method, shingle beaches were the lowest in total species richness. Trampling intensity ranged from 0 footprints m-2 day-1 on closed beaches, up to a maximum of 30 footprints m-2 day-1. On ‘‘intensively’’ used beaches (12 footprints m-2 day-1, on average), reduction of plant cover was more pronounced than on ‘‘extensively’’ used beaches (7 footprints m-2 day-1, on average). Both plant cover and plant species richness were lower on intensively and extensively used beaches than on closed beaches. In arthropods, only staphylind and spider species richness was significantly lower on open beaches than on closed beaches, but no differences were found in carabids. Referring to our results, trampling effects from tourists have high impact on species richness of sandy beaches, on both intensively and extensively used sites. Keywords
Baltic Sea Biodiversity Human impact Sandy beach Shingle Tourism
Introduction Coastal habitats have been subjected to increasing impacts by urban populations. Sandy beaches, in particular, are of high economic and social value. They also demonstrate special ecological features and contain a distinctive biodiversity that is generally not M. Schierding S. Vahder L. Dau U. Irmler (&) Institute for Ecosystem Research, University of Kiel, Olshausenstrasse 40, 24098 Kiel, Germany e-mail:
[email protected]
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recognised (Schlacher et al. 2007). Tourist activity on beaches has increased dramatically in the last decades (Jedrzejczak 2004; Schlacher et al. 2008). On most intensively used beaches, beach-cleaning and armouring have serious impacts on beach plant and faunal communities (Llewellyn and Shackley 1996; Brown and McLachlan 2002; Dugan et al. 2008). Negative effects on birds breeding on shores have already induced administrations in most European countries to close parts of the beaches to tourism, usually only during the breeding season. However, persistent damage has been found to populations of year-long animal residents, e.g. sandhoppers (Weslawski et al. 2000; Veloso et al. 2008). In order to evaluate human impacts on beach biodiversity, Schlacher et al. (2008) proposed to study the biodiversity of undisturbed beach, first, to reveal the natural variance of biodiversity of beaches. Although the fauna of beaches is well known in many European countries (e.g. Speybroek et al. 2008), the impacts on species richness by human activities are poorly known, in particular at the coasts of the Baltic Sea. In contrast to tidal coasts, the Baltic Sea coast reveals no tides, or tides within ranges of mere centimetres, and is reflected by a wide distribution of terrestrial organisms near the sea line. As arthropods provide the highest contribution to total species richness in the world, it can be expected that they also reflect best the actual situation of species richness at beach habitats. Beside measures of footprint densities on beaches and their impact on plant species richness and cover, the present study selected two coleopteran families and spiders to study the following questions: (i) how species richness changed in the vertical gradient from beach to primary dunes and how it changed in the horizontal gradient from shingle to sandy soil conditions; (ii) whether individual species prefer specific beach habitats; (iii) how species richness and individual species react to tourist activities?
Sites and methods The investigation was performed in 2009, from April 9th to August 20th, at nine locations along the Baltic Sea coast of Schleswig–Holstein in northern Germany, where tidal ranges are extremely low, less than 10 cm (Fig. 1). At these nine locations, sites were selected that include primary dunes at the upper elevation of beaches, sandy beaches at 3–4 m closer to the sea line from the primary dune sites, and shingle beaches at the base of cliff coasts (Table 1). As sandy beaches were the main focus of the investigation, six sandy beaches were selected, in contrast to four sites within primary dunes and shingle beaches. Two of the six sandy beaches were open to tourists, but with only extensive use by tourists. Carabidae, Staphylinidae, and Araneae were recorded by means of pitfall traps. Up to eight replicate pitfall traps were installed at each beach site to compensate trap losses caused by tourists. For the final analysis, in order to have equal numbers of traps per site, only four pitfall traps were recorded per site. Pitfall traps in the single locations were installed in 25 m distance to avoid pseudo-replications. They had an opening of 5.6 cm diameter, were filled with 10% vinegar and a detergent liquid, and protected against direct precipitation by a transparent shelter. Site sand content was determined by sieving, using a 0.063 mm sieve after oxidising the organic matter by H2O2, and shingle content by sieving a larger soil sample in the field. Finer silt and clay material was derived by subtracting (Schlichting et al. 1995). To assess trampling effects on beach soil, sixteen sandy beaches were investigated in 2010 along the Flensburger Fo¨rde on both German and Danish sides. For this investigation, on each beach, five replicate 1 m2 plots were marked and footprints were counted after one
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1975
Danmark
BalticSea
23 45
1 Schleswig-Holstein
6
7
Kiel
North Sea
Hamburg
LowerrSaxony
Mecklenburg -Vorpommern
Fig. 1 Investigated locations at the Baltic Sea coast: Lindho¨ft (1), Da¨nisch-Nienhof (2), Stohl (3) Stakendorf and Hohenfelde (4), Hubertsberg (5), Behrensdorf and Lippe (6), Weißenhaus (7); the ellipse indicate the area where footprint density and vegetation was measured Table 1 Habitats at the investigated locations, abbreviations of locations, sand, and gravel content with standard deviation, and situation to tourists on sandy beaches Location/habitat (abbreviation)
Shingle beach Primary dune Tourism on Sandy beach sandy Sand (%) Gravel (%) Sand (%) Gravel (%) Sand (%) Gravel (%) beach
Behrensdorf (BD) Da¨nisch-Nienhof (DN)
Closed
Hubertsberg (HB)
90 ± 8
4±6
–
–
–
–
–
–
0±0
100 ± 0
–
–
–
–
38 ± 16
62 ± 16
–
–
Hohenfelde (HF)
Open
81 ± 11
17 ± 11
–
–
80 ± 18
17 ± 19
Lippe (KB) Lindho¨ft (LH)
Closed
94 ± 9
0±0
–
–
91 ± 7
7±7
Open
94 ± 3
1±2
–
–
–
–
Stakendorf (SD)
Closed
79 ± 22
19 ± 22
–
–
98 ± 1
0±1
Stohl (ST) Weißenhaus (WH)
Closed
–
–
23 ± 18
77 ± 18
–
–
99 ± 0
0±0
18 ± 17
82 ± 17
99 ± 0
0±0
day. This procedure was performed on two different days on each of the sixteen beaches. The beaches were divided into ‘‘intensively’’ used and ‘‘extensively’’ used sites: intensive = [ 9 footprints m-2 day-1; extensive = [ 0–9 footprints m-2 day-1; control = closed to
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tourists, meaning 0 footprints m-2 day-1. In addition, plant species richness and plant cover were determined for a 60 m long beach section which was separated into three subsections of 20 m length. Thus, for each of the sixteen beaches, three replicate data were available. Statistical analysis was performed using the program STATISTICA (StatSoft Inc 2004). Parametric and non-parametric distribution of data was tested using the Kolmogorov–Smirnov test. The normally distributed data were compared using ANOVA with a subsequent LSD post hoc test. Differences of species richness or abundance between two habitats were tested by U-test or t-test, correlations between environmental parameters and species richness by Pearson correlation. The program ‘‘PAST Version 2.04’’ (Hammer et al. 2001) was used for the following analyses. To estimate total species richness in the habitats Jackknife II species richness was calculated. The second order of Jackknife estimator seems to be the most accurate for estimating total species richness (Schoeman et al. 2008). For the multivariate ordination of sites detrended correspondence analysis (DCA) was performed. According to ter Braak (1987) a distinct ordination of assemblages can be expected at eigenvalues higher than 0.5. For the comparison of species richness between beaches open or closed to tourists a sample rarefaction was calculated.
Results Effect of natural beach heterogeneity In total, 836 carabid specimens, represented with 72 species, 2310 staphylinid specimens, with 100 species, and 2208 spider specimens, with 96 species, were recorded. Including all investigated arthropods, site species richness ranged between 22 and 80 species (Table 2). The results of the DCA shows that beach habitats and primary dunes are separated by the first axis, which accounts for the highest eigenvalue with 0.56 (Fig. 2). This means that elevation from the sea level represents the strongest gradient of the investigated beach sites. In contrast, the separation between sandy and shingle beaches corresponds with the second axis with an eigenvalue of only 0.35, which means that the shingle––sand gradient is less important for the overall variance in the beach community. Some sites primarily attributed to the beach sites connect primary dunes and both sandy and shingle beaches. Total species richness was highest in primary dunes for all arthropod groups (Table 2). Referring to species richness per trap, staphylinid beetles revealed significantly higher numbers of species on sandy beaches and dunes, whereas the two other groups showed higher numbers of species in primary dunes, only (results of ANOVA at DF 2: Carabidae: F = 7.9, P = 0.001; Staphylinidae: F = 10.5, P \ 0.001; Araneae: F = 19.4, P \ 0.001). This was also true when total species richness was regarded as a result of Jackknife II estimation. The relationship between soil conditions of beaches and species richness is supported if the shingle or sand content at the individual traps of all beaches are regarded (Fig. 3). Since sand content and shingle content are negatively correlated, only the relationship between sand content and species richness is displayed (sand content = 97–0.96 * shingle content; r = 0.99, P \ 0.001). According to these results, sand content contributes to 37% (r2 = 0.37) of the total species richness on beaches. The species richness of the three arthropod groups show similar relationships to sand content as total species richness, with the exception of Carabidae (Carabidae: r = -0.03, P = 0.9; Staphylinidae: r = 0.63, P \ 0.001; Araneae: r = 0.42, P = 0.008).
123
29
36
12
16
9
12
30
22
21
15
20
18
19
21
19
HB
34
29
17
27
15
4
KB
14
18
11
LH
23
30
28
26
22
11
SD
16
13
6
ST
33
33
11
28
22
5
19
16
6
WH
70
74
58
35
99
75
70
33
100
52
41
28
a
b
15a
12
7c
9a
10
4b
7a
4
b
5b
4.0
4.7
2.2
4.0
4.2
3.3
3.2
2.0
2.1
143a
90b
59c
88b
116a
61c
96a
77b
57c
5.1
6.5
3.4
5.3
6.6
5.5
5.9
5.1
4.8
SD
Mean
Mean
SD
Jackknife II
Per trap
Total
HF
BD
DN
Species richness
Localities
Exponents indicate significant differences at P \ 0.05
Primary dune
Sandy beach
Shingle beach
Araneae
Primary dune
Sandy beach
Shingle beach
Staphylinidae
Primary dune
Sandy beach
Shingle beach
Carabidae
Habitat type
133
76
52
76
102
49
84
66
47
-95
153
104
66
100
130
73
108
88
67
?95
Table 2 Species richness in the investigated habitats and Jackknife II species richness to show likely total species richness with standard deviation (SD) and lower (-95) and upper (?95) confidence limits, different exponents indicate significant differences according to ANOVA or t-test with Bonferroni correction in Jackknife II means
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540
LB1
Sandy beaches
2nd axis (eigenvalue: 0.35)
480
SB1
420 WB2
360 WP2 WS4
WB1
SB3
SS2
SG2
SB4 SG4
SP1
LB4 LB3 SG1 KB3
LB2
SP2 KB2 KP3 WB3BB2 SG3 KP2 BB3 WP4 KB1 KP1 KP4 SP3 240 WP3 BB1 KB4 SP4 WP1 HP1 HB2 180 HP3 HB3 Shingle beaches BB4 HP2 HP4 HB1 HG2 120 HG3 HB4 Primary dunes HG1 WG4 DG4 WG2 HG4 60 DG2 DG3 DG1 WG3 WG1 0 0 60 120 180 240 300 360 420 480 540
300
1st axis (eigenvalue: 0.56) Fig. 2 Results of detrended correspondence analysis with all pitfall traps used; first character refers to first character of local abbreviations; second character refers to habitat for primary dunes (P), sandy beach (B), and shingle beach (G); numbers refer to the four pitfall traps
45
95% Confidence interval Species (n) = 13.9 + 0.14 * Sand content r = 0.60; p < 0.001
40 35
Species (n)
30 25 20 15 10 5 0
0
20
40
60
80
100
Sand content (%) Fig. 3 Relationship between sand content and species richness on the beach habitat
Impacts on individual species Most individual species were too rare to analyse their attribution to one of the three habitats. But sixteen species were abundant enough to perform a Kruskal–Wallis ANOVA with
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Table 3 Median values of individual species with preference for one the three habitats and results of Kruskal–Wallis ANOVA with subsequent U-test taking Bonferroni correction into consideration Species Number of samples
Primary dune 16
Sandy beach 24
Shingle beach 16
H
P
Araneae Xysticus kochi
2.0*
0.0
0.0
14.4
0.001
Arctosa perita
2.0*
1.0
0.0
22.6
\0.001
Erigone arctica
0.0
6.0*
0.5
10.8
0.005
Philodromus fallax
0.0
1.0*
0.0
13.9
0.001
Arctosa cinerea
0.0
1.0
0.0
7.1
0.030
Pardosa agricola
2.0
2.0
14.0*
19.7
0.001
Araeoncus crassiceps
0.0
0.0
0.5*
14.2
0.001
Staphylinidae 36.0*
11.5*
0.0
24.2
\0.001
Polystomota grisea
0.0
1.0*
0.0
16.4
\0.001
Polystomota punctatella
0.0
0.5*
0.0
13.8
0.001
Phytosus spinifer
0.0
0.5*
0.0
12.2
0.002
Cafius xantholoma
0.0
0.0
1.0*
12.0
0.003
Harpalus affinis
0.5
0.0
0.0
6.4
0.040
Calathus fuscipes
0.5
0.0
0.0
18.6
\0.001
Broscus cephalotes
0.0
0.0
1.5
6.1
0.040
Bembidion saxatile
0.0
0.0
0.5
6.7
0.030
Bembidion tetracolum
0.0
1.0*
2.0*
10.2
0.006
Aleochara bipustulata
Carabidae
* Significantly different from values without asterisk
subsequent U-tests (non-parametric, considering Bonferroni correction) to find significant preferences (Table 3). Among Carabidae, only Bembidion tetracolum seems to avoid dune habitats, since abundances were significantly lower than in the two beach habitats. ANOVA found significances for four species: two with higher abundance in primary dunes and two with higher abundance in shingle beaches, but the results could not be supported by the post hoc test. Two spider and one staphylinid species were found, inhabiting mainly the primary dune sites. According to the statistical results, the higher abundance of the staphylinid species Aleochara bipustulata on primary dunes compared to sandy beaches was not significant. This species seems to inhabit both habitats with similar success. Sandy beaches were preferred by two spider species and three staphylinid species. It seems that Arctosa cinerea, also, prefers sandy beaches, because ANOVA results were significant, but subsequent U-tests revealed no significance for the higher abundance on sandy beaches. Three species, i.e. the two spiders Pardosa agricola and Araeoncus crassiceps, and the staphylinid Cafius xantholoma, were significantly more abundant on shingle beaches. Effects of trampling intensities on plant species richness and plant cover of sandy beaches The trampling investigation showed that the mean number of footprints was significant for each of the three groups (F = 31.7, P \ 0.001) (Table 4). In the intensively and
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Table 4 Results of the investigations of tourist trampling on 16 sandy beaches Group
Footprints m-2 day-1 n
Intensive Extensive Control
Plant species
Plant cover (%)
Mean
SD
n
Mean
SD
n
Mean
SD
50
12.1a
3.6
15
7.5a
3.6
15
27.0a
17.7
100
b
7.0
1.3
30
7.5
a
4.0
30
45.5b
29.6
10
0.0c
0.0
3
12.7b
0.6
3
79.8c
19.7
Exponents indicate significant differences at P \ 0.05
extensively used beaches, footprint density ranged from 5 to 30 footprints m-2 day-1, and from 0 to 14 footprints m-2 day-1, respectively. In spite of significantly varying trampling intensities, plant species richness did not vary in all beaches open to tourists. Only the closed beaches provided a significantly higher species richness (F = 4.2, P = 0.04). In contrast to species richness, plant cover reflected the trampling situation. All three groups differed significantly, with lowest cover on intensively used beaches and highest cover on closed beaches (F = 24.7, P \ 0.001). Effects of tourism on arthropod diversity of sandy beaches Sandy beaches closed to tourists have significantly higher arthropod species richness than beaches open to tourism (Table 5). In regard to species richness per trap, Carabidae revealed higher species richness on open beaches, whereas staphylinds and spiders revealed nearly twice as many species on beaches closed to tourists. None of the species were found only on beaches open to tourists. Thus, half of the species of closed beaches are absent from open beaches. Species richness differences between the two beach types are still more obvious when Jackknife II species richness is regarded. In this case, species richness of carabid beetles was not higher on open beaches than on closed beaches. According to sample rarefaction that includes all species of the three arthropod groups, the two curves of beaches open and closed to tourists show clearly different slopes with
Table 5 Species richness results of sandy beaches under closed and open conditions to tourists with results of t-test with significant differences indicated by * Situation of beach
Traps
Species per trap
n
Mean
SD
Jack-knife II t/p
Total
S.D.
P
0.5
-95
?95
Carabidae Closed Open
16
3.4
1.9
-2.2
45
3.1
8
4.8*
1.8
0.04
45
3.8
39
51
36
54
96
122
48
64
88
112
24
40
Staphylinidae Closed Open
16 8
11.6*
4.2
3.1
109*
6.3
6.8
1.8
0.005
56
3.4
\0.001
Araneae Closed Open
16 8
14.1*
2.4
5.2
100*
5.6
6.9
4.4
\0.001
32
3.4
-95 lower confidence limit, ?95 upper confidence limit
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\0.001
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1981
160 Closed
140
Open
No. species
120 100 80 60 40 20 0 0
2
4
6
8
10
12
14
16
18
No. samples Fig. 4 Sample rarefactions of beaches open to tourists and closed to tourists with all species of beetles and spiders; vertical bars indicate standard deviation
increasing number of samples (Fig. 4). Even after 16 pitfall traps, no steady state was found for the curve of the closed beaches. Both curves pass a more or less divergent shape, which reveals an increasing difference with increasing number of samples. At 8 pitfall traps that limits the comparison of the two curves, both 75 and 102 species were calculated for beaches open and closed to tourists, respectively. If the correlation analysis between sand content and species richness is performed omitting the two beaches open to tourists, the relationships between both parameters are much closer than under inclusion of the open beaches. Regarding this regression, species richness = 14.1 ? 0.16 * sand content (r = 0.69, P \ 0.001) with a coefficient of determination at 0.49. This means that higher sand content is to 49% responsible for the high species richness on sandy beaches, if the tourist effect is excluded, which is distinctly higher than the 37% responsibility when open beaches are included. Effects of tourism on individual arthropod species For only a few species that were abundant on sandy beaches the effect of tourism on abundance could be tested (Table 6). Among spiders and staphylinid beetles, all tested species showed higher abundance on beaches closed to tourists. Even Cafius xantholoma, generally preferring shingle beaches, was found in higher abundance on closed sandy beaches than on open sandy beaches. This relationship was also found for Pardosa agricola, but its range of abundance between both beach types was not significant according to the U-test. Noteworthy is that the three species specific to sandy beaches, i.e., Arctosa cinerea, Polystomata grisea, and Phytosus spinifer, reacted significantly negatively to tourist activities. P. punctatella was also more abundant on closed beaches, though not significantly so. Among the ground beetles, only Bembidion tetracolum was more abundant on beaches open to tourists. All other ground beetle species on sandy beaches were too rare to perform a statistical analysis (see also Table 3).
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1982 Table 6 Median abundances of species under closed and open conditions on sandy beaches with results of U-test
Biodivers Conserv (2011) 20:1973–1985
Species/n
Closed 16
Open 8
Z
P
Araneae Arctosa cinerea
1.5*
0.9
2.0
0.040
Erigone arctica
9.5*
1.0
2.7
0.006
Philodromus fallax
1.0*
0.0
2.7
0.006
10.5*
1.0
3.6
\0.001
3.5
0.0
1.8
0.060
Oedothorax apicatus Pardosa agricola Staphylinidae
28.0*
2.5
3.7
\0.001
Polystomota grisea
2.0*
0.0
2.6
0.009
Polystomota punctatella
1.0
0.0
0.6
0.500
Phytosus spinifer
0.5*
0.0
2.2
0.030
Cafius xantholoma
0.5*
0.0
2.3
0.020
0.0
2.5*
-3.5
\0.001
Aleochara bipustulata
Carabidae * Significant results
Bembidion tetracolum
Discussion Poorness of knowledge concerning beach biodiversity can be attributed in part to the low abundance of species and their below ground live that allow rare observation. Although beaches seem at first glance species-poor, their soils are inhabited by arthropods so abundant that they serve important roles in stabilization of sand particles (Koehler et al. 1995). Ecological investigations on beaches are further complicated by difficulties in sampling and measuring of species under conditions of intense beach tourism activity and trampling. Assemblages at Baltic Sea coasts are clearly differentiated by species composition. Investigations of spider fauna of six habitats along the German Baltic Sea coast, including lagoon and cliff habitats, revealed a clear separation of spider assemblages with regard to the six habitats (Vahder and Irmler 2010). Highest species richness was found in dunes and lagoons. Lagoons, however, provided no specific spider species, whereas, on beaches, several beach-specific species were found. Even cliffs had more specific species than coastal lagoons. Furthermore, primary dunes and sandy beaches had more than twice as many endangered species as all other habitats. The distinctly separated assemblages of coastal habitats at small scale were also stressed by Schaefer (1970) in his investigation of dunes, salt marshes, and dry grassland of the Baltic Sea coast, where he found highest species richness in salt marshes, with 23 spider, 19 ground beetle and 5 rove beetle species. Both dry grassland and dunes had distinctly lesser number of species with 14, 9, 13 and 22, 8, 9 for spiders, ground beetles, and rove beetles, respectively. According to Irmler et al. (2002), both ground beetle and spider species richness in salt marshes bordering the Baltic Sea ranged between 8 and 12, 24 and 45 species per trap, respectively. Thus, according to this investigation, only spiders revealed higher species richness in salt marshes than in the primary dunes or in the two beach types investigated in the present study. As in the present investigation, Italian beaches located at the non-tidal Mediterranean Sea also showed significantly distinct assemblages on sandy and shingle beaches and distinctly higher species richness at sandy beaches (Fanini et al. 2009).
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According to Schaefer (1974), habitat stability is the main factor responsible for specific species richness, particularly in coastal habitats. Habitat stability affected by wave and wind energy was also stressed as a main factor in meiofaunal and macrofaunal species richness of tidal coast beaches (Kotwicki et al. 2005; Jaramillo and McLachlan 1993). The higher stability of primary dunes, due to higher-than-shoreline elevations and thus lower susceptibility to storm and wave disturbance, can explain its higher species richness results of the present investigation. This explanation is also supported by investigations in Baltic Sea salt marshes, where species richness of spiders increased in the elevation rise of 20–40 cm above sea level, and no increase was found between 40 and 150 cm (Irmler et al. 2002). The vertical gradient from beach to primary dunes was also the main factor for the variance in the community investigated in the present study. This is supported by investigations of Schlacher et al. (2011) for the spatial variation in vegetation of Australian beaches. They found that the sea-land gradient was more important than the effect by camping behind foredunes. Low species richness of shingle coasts can be explained by the low habitat stability, too. There, sand particles have been eroded as effects of high wave and wind energy (Packham and Willis 1997). The increase of species richness from shingle to sandy beaches seems to be a general effect, shown by species census results from geographically diverse coasts and taxonomically diverse animal groups (Jaramillo and McLachlan 1993; Fanini et al. 2009). Thus, primary dunes and sandy beaches seem to be ‘‘hot spots’’ of biodiversity at Baltic Sea coasts, in both total species richness and numbers of habitat-specific and endangered species. Nevertheless, shingle beaches provide habitat for specific species of spiders and rove beetles that are not or rarely found on sandy beaches. According to our results, tourist trampling greatly impacts the species richness of animals and plants. Extensively used open beaches revealed significantly lower species richness than closed beaches. In our investigation, we classified intensive use by a footprint density higher than nine footprints m-2 day-1 during the main tourist season in July/ August. According to Kammler and Schernewski (2004), tourist activities change in relation to seasons, weather conditions, and day of the week. On average, tourist density on investigated Baltic Sea beaches ranged on a summer day between 7.6 and 84 m-2 person-1. Tourist densities between both 30 and 80 and between 7 and 12 m-2 person-1 were described as ‘‘extensively’’ and ‘‘intensively’’ used beaches, respectively. As Kammler and Schernewski (2004) did not determine trampling and we did not determine tourist density, it is not clear whether the two classifications of ‘‘extensive’’ and ‘‘intensive’’ are corresponding with our classification or not. Trampling impacted intertidal fauna of South African sandy beaches on experimental areas with controlled trampling (Moffett et al. 1998); where between 60 and 70% of some species were damaged after 200 human passages. According to Veloso et al. (2008), sandhoppers could not compensate high-intensity human pressure, even when it was only during one seasonal period. In our investigation, only carabids seem to suffer less under tourist activities compared to other arthropods. This may be explained by their stronger cuticula and their running behaviour. Moreover, species specific reaction to disturbed habitats, i.e. Bembidion tertacolum, can reveal higher abundance on open beaches than on closed beaches. On the mainland, this species exhibits highest abundances on highly disturbed agricultural fields (Irmler and Gu¨rlich 2004). Its preference for highly disturbed habitats may be also responsible for its higher abundance on shingle beaches than on sandy beaches. Nevertheless, individual carabid species do suffer under human trampling of sandy beaches. The tiger beetle, Cicindela maritima, for example, is nearly extinct along the Baltic Sea coast due to high tourist pressure (Irmler 2010). Its large home range
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(in only narrow closed beach stretches) and trampling-sensitive larvae are responsible for its extreme population decrease within the last 30 years. According to an investigation by Irmler (2010), 100 specimens require a one-kilometre stretch of dune and beach habitat without tourist impacts. Tourist pressure was not only found to halve the abundance, i.e. of ghost crabs (Ocypode ssp.), on beaches exposed to vehicle traffic in Australia, but also significantly compressed their home ranges (Schlacher and Lucrezi 2010). Staphylinids and spiders are less sclerotic than carabids and may be therefore more sensitive to mechanical disturbance (i.e., human trampling) than carabids. This can be also the cause for their lower species richness on shingle beaches than on sandy beaches; species richness of spiders, in particular, on open beaches is only one-third that of closed beaches. Overall, the results of plant and animal species richness showed that even extensively used beaches suffer dramatically under tourist activities. Thus, our investigation allows the conclusion that only beach stretches maintained under year-long closure to tourists will conserve the specific species compositions of beaches. Nevertheless, closing beach stretches may be restricted to upper elevation zones, while lower elevation zones can be left open for tourist activities.
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