Short-term effects of experimental trampling on polychaetes of a rocky intertidal substratum ( Asinara Island MPA , NW Mediterranean )

The wide range of anthropogenic disturbances influencing marine coastal assemblages is of increasing concern. Rocky intertidal communities are particularly sensitive to disturbance from indirect agents, such as sewage and industrial effluents (Littler and Murray, 1975), and to more direct agents, such as harvesting and trampling (Duran and Castilla, 1989; Underwood and Kennelly, 1990; SCIENTIA MARINA 70S3 December 2006, 179-186, Barcelona (Spain) ISSN: 0214-8358

Trampling is an important ecological phenomenon on many shores, and its effects, when intensive, may significantly contribute to local changes in the composition of marine communities.On rocky shores, trampled areas generally have lower species diversity and density of sessile organisms than in less disturbed areas.Several types of impact are associated with trampling such as direct mortality or dislodgment of organisms, weakening of algal holdfasts and structural damage, resulting in both increased vulnerability of rocky communities to other abiotic (e.g.dessication) or biotic (e.g.predation) factors, and habitat loss, as sessile organisms are crushed or removed (Brosnan and Crumrine, 1994;Brown and Taylor, 1999).
Most of the research has been focused on large conspicuous organisms (e.g.Povey and Keough, 1991;Brosnan and Crumrine, 1994;Keough and Quinn, 1998).However, smaller cryptic targets may also deserve attention due to their high abundance (Hicks, 1986) and productivity (Edgar and Moore, 1986), and their utility as food for higher trophic levels (Coull and Wells, 1983;Jones, 1988).In rocky intertidal habitats, highest densities of small animals are typically found on macro-algae (Gibbons and Griffiths, 1986), which provide epifauna with a range of resources, such as food, and refuge from predation and dessication (Gibbons, 1991).Several studies carried out in shallow water revealed that canopy-forming macroalgae may be badly damaged by human trampling, showing a rapid decrease in canopy cover (Povey and Keough, 1991;Brosnan and Crumrine, 1994;Schiel and Taylor, 1999;Milazzo et al., 2002).Epifaunal abundance is therefore likely to be reduced where human trampling reduces the biomass of their hosting algae.
The Asinara Island Marine Protected Area (MPA) (41°03'58''N 8°15'57''E) in Sardinia (Italy) was a prison island until 1997 and public access has been forbidden for nearly a century.This has led to the maintenance of a specific Mediterranean flora and fauna and has prevented serious damage to the coastal marine assemblages (Villa et al., 2002).For this reason, in recent years, this MPA has become a tourist attraction and human presence, where allowed, may conflict with conservation goals (Eckrich and Holmquist, 2000).
In the present study we examined the immediate effects of experimental human trampling on polychaetes inhabiting intertidal hard-bottom habitats covered by algae within two "no-entry, no-take" sites at the Asinara Island MPA.

MATERIALS AND METHODS
The study was carried out in late August 2002 within the integral reserve of the Asinara Island MPA, on two eastern rocky shore locations called Cala Arena and Cala Sant'Andrea (Fig. 1).At each location, six experimental areas within a platform covered by photophilous mixed erect algal assemblages were randomly chosen between 0.2-0.4m water depth.
Two different experimental intensities of trampling (60 and 120 steps/20x20 cm quadrat) and controls (no trampling) were applied to the quadrats.Estimates of trampling intensities to apply in the experiment were derived from observation of visitors at the MPA during the summer peak period (Casu, 2004).Each non-control quadrat was entirely trampled by a person ~ 60 Kg in weight and wearing rubber-soled shoes.At each location, two replicate quadrats were randomly assigned to the three trampling levels, and two replicate samples were collected within each quadrat.Replication of the experiment as well as the size of the corer used for sampling were decided on the basis of a pilot study and a cost-benefit analysis (Casu et al., 2004).One day after the experimental trampling, benthic samples were collected using plastic cylinders of 40 mm inner diameter, corresponding to 12.56 cm 2 in surface scraped area (Brown and Taylor, 1999).In the field algae and benthic invertebrates were removed down to the basal crust using a metal scraper and preserved in 4% formalin in seawater.In the laboratory, the polychaetes were sieved through a 100-mm mesh, counted, and identified.
Analysis of similarities (1-way ANOSIM, Clarke, 1993) was performed on overall abundance of polychaete species to test for differences among the three different levels of trampling.To identify all "important" taxa, the percentage contribution of each polychaete species to the average dissimilarity among treatments was calculated using SIMPER (Clarke, 1993).Non-metric multidimensional scaling (nMDS) was used to produce two-dimensional ordination plots to show similarities among polychaete assemblages at different trampling intensities at the two sites.Analysis was performed using the Bray-Curtis similarity coefficient (Bray and Curtis, 1957) on untransformed data.All analyses were carried out using the PRIMER 5 programme (Plymouth Marine Laboratory, UK).
Three-way ANOVAs were performed to test hypotheses about differences in the abundance of polychaetes among different levels of trampling.These analyses were preceded by Cochran's test for homogeneity of variances (Winer et al., 1991Underwood, 1997).'Location' (Cala Arena and Cala Sant' Andrea) was treated as random and 'Trampling' was treated as fixed factor (0 which served as control, 60 and 120 footfalls/quadrat): they both were orthogonal, while 'area' was random and nested in location (2 levels).Cochran's test was used to check for the homogeneity of variances (Winer, 1971).Whenever necessary data were transformed.Student-Newman-Keuls (SNK) test was used for a posteriori comparison of the means when appropriate (Underwood, 1997).This analysis was carried out using the GMAV 5.0 software (University of Sydney).

RESULTS
Overall, 1,146 polychaetes, belonging to 37 species and 13 families (Table 1), were collected.Dominant families were Syllidae (569), Sabellidae (191) and Opheliidae (162).As is usually the case in hard intertidal and upper subtidal bottoms (Abbiati et al., 1987)   physical disturbance.The analysis of similarity revealed significant differences in abundance of polychaetes among the different levels of trampling (ANOSIM, R=0.384 p=0.1%).This is also indicated by the spread of the replicate samples for each site in the nMDS ordination plot (Fig. 3).The pair-wise comparisons (Table 2) showed lower abundance of polychaetes in trampled plots than in controls independently of intensities (0-60 p=0.6%; 0-120 p=0.1% 60-120 p=5.5%).SIMPER analysis identified polychaete species that mostly contributed to the average dissimilarities (Table 3).Analyses of variance on overall polychaete abundance identified a significantly higher abundance in controls than in trampled plots (Table 4 and Table  5).Among the most important polychaete species contributing to the average dissimilarities among treatments, Polyophthalmus pictus, Exogone naidina, Salvatoria clavata, Syllis bouvieri, Syllis prolifera, Platynereis dumerilii and Amphiglena mediterranea were more abundant in controls than in tram-   pled areas (Table 4 and Table 5).The null hypothesis was that the abundance of polychaete species collected was the same at different trampling intensities and controls, however, between the two trampling levels (60 and 120), an alternative hypothesis (SNK Tests) could not be identified (Table 5).

DISCUSSION
Polychaetes occurring in Asinara Island MPA shallow waters revealed high vulnerability to experimental human trampling, which caused immediate declines in abundance of the total number of individuals.Trampling reduces densities of algaedwelling animals due to the direct effect of the crushing impact of the footsteps.Also, algal loss due to fragmentation may increase susceptibility of the remaining organisms to other abiotic and biotic factors such as dessication and predation (Brosnan and Crumrine, 1994;Schiel and Taylor, 1999).
Moreover, if direct impact of trampling caused a subsequent shift in community composition, vulnerability of individual taxa would be related to their morphology and ecology (Brosnan and Crumrine, 1994;Brown and Taylor, 1999).Differences between similar species such as the two small sabellids Fabricia stellaris, which did not respond to trampling, and Amphiglena mediterranea, which, conversely, is significantly reduced in abundance by experimental trampling, are probably related to the different ecological requirement of these two species (Gillandt, 1979;Tena et al., 2000).However, effects of trampling on mobile polychaetes are difficult to assess in this experiment due to the small spatial scale: these polychaetes could have easily moved away from the trampled quadrats after trampling and before sampling.Other trampling studies have demonstrated little direct reduction of mobile animals such as gastropods (Povey and Keough, 1991).
Subtidal habitats adjacent to sampled platforms have mostly sandy bottoms.Wave action, depending on site exposure and wind direction, traps the sedi-  Cala S. Andrea 0=60=120 0>60>120 0>60=120 0>60=120 ment on platforms among algae thus contributing to the substratum, allowing for the settlement of interstitial polychaete species, such as Parapionosyllis spp.(Abbiati et al., 1987, Giangrande 1988;Somaschini et al., 1994).In such a case, it is more likely that trampling effects are indirect, through changes to the habitat itself.Algal vulnerability to trampling is likely to depend on their morphology (Milazzo et al., 2002).This feature can also be the reason for spatially significant differences in some polychaete species, found at both spatial scales (locations and areas).
Tolerance and recovery capabilities are crucial features in any disturbed benthic assemblage and are strictly related not only to the community structure but also to the type, magnitude and frequency of the occurring disturbance (Underwood, 1989;Schiel and Taylor, 1999).The present experiment, with trampling performed only once at two different intensities, could be referred too as a "pulse" disturbance, whereas other studies are designed to accomplish a "press" disturbance, where species are repeatedly impacted over long periods (Beauchamp and Gowing, 1982;Castilla and Duran, 1985;Povey andKeough, 1991, Brosnan andCrumrine, 1994).Recovery is likely to be related not only to the reduction or cessation of pulse disturbances but also to the time necessary to allow recruitment and growth processes (Schiel and Taylor, 1999).Pulse and press disturbances are highlighted by changes in polychaete assemblages because the presence of mobile and sessile organisms, generally results in a decrease in abundance and diversity.
The biodiversity of rocky intertidal zones, especially in MPAs, also depends on a wide range of other human recreational activities.The risk is that, without proper management, those activities could increasingly contribute to degradation of intertidal areas.MPAs could play a major role in safeguarding resident flora and fauna by establishing specific controlled areas, where many rocky intertidal organisms could benefit from limited human access (Agardy, 1997;Murray et al., 1999) FIG. 1. -Map of the study locations.Delimited sectors along the coastline identify the zones at different protection level within the Asinara MPA FIG. 2. -Mean abundance (+SE) of overall polychaetes and each of the important polychaete species sampled at control (0) and at different trampling intensities (60 and 120 steps/quadrat); areas (n=2).
RESUMEN: EFECTOSA CORTO PLAZO DE UN PISOTEADO EXPERIMENTAL SOBRE LAS POBLACIONES DE POLIQUETOS QUE HABITAN UN SUSTRATO ROCOSO INTERMAREAL EN LA ISLA DE LA ASINARA (AREA MARINA PROTEGIDA DEL Syllidae was the richest family, in both numbers of species (19) and individuals.

TABLE 1 .
-Taxonomic list of the recorded polychaete species

TABLE 3 .
-Relative contribution of polychaete species to dissimilarities among trampling intensities at each site (cut off for low contributions 90%).

TABLE 4 .
-Results of ANOVAs of the effects of location, trampling (0, 60 and 120 steps/quadrat) and area on the overall abundance of polychaetes and each of the important polychaete species.*p<0.05

TABLE 5 .
-Results of SNK tests on trampling intensities for overall abundance of polychaetes and each of the important polychaete species.T= Trampling, L= Location, A= Area.