impact of human exploitation on populations of the intertidal southern bull-kelp durvillaea...

Biological Conservation 52 (1990) 205-220

Impact of Human Exploitation on Populations of the Intertidal Southern Bull-kelp Durvillaea antarctica

(Phaeophyta, Durvilleales) in Central Chile

R. H. Bustamante* & J. C. Castilla

Estaci6n Costera de Investigaciones Marinas (ECIM), Las Cruces, Departamento de Ecologia, Universidad Cat61ica de Chile, Casilla 114-D, Santiago, Chile

(Received 3 December 1988; revised version received 3 July 1989; accepted 4 August 1989)

A B S T R A C T

The effect of small-scale harvesting on populations of Durvillaea antarctica (Chamisso) was studied at four localities in central Chile. Density, standing crop and mean individual size were compared between populations protected from harvesting and those subjected to repeated cropping. Populations on islands, where access by collectors is restricted, were also assessed. Populations of D. antarctica at harvested mainland sites did not differ in abundance (density and standing crop) or mean size. At non- harvested sites standing crops were twice as high as at harvested sites. However, plant density at harvested sites was double that at non-harvested sites. Mean algae size at harvested sites was significantly smaller than at non-harvested sites, because collectors select the largest individuals, usually those with a holdfast diameter of greater than 4 cm. A significant positive correlation was found between wet biomasses present on the islands and those at adjacent harvested and non-harvested mainland sites. Availability o f potential space for settlement was also correlated with the density of algae, both on the mainland and on islands. Islands appear to act as seeding grounds or refugial areas, supplying recruits to the adjacent mainland sites. Differences in abundance of D. antarctica between the different

* Present address: Marine Biology Research Institute, Zoology Department, University of Cape Town, Rondebosch 7700, Republic of South Africa.

205 Biol. Conserv. 0006-3207/90/$03-50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain

206 R. H. Bustamante, J. C. Castilla

localities could therefore be expla&ed by the level of exploitation and by the local geomorphology, particularly the presence or absence of nearshore islands.

INTRODUCTION

Traditional studies of abundance of marine intertidal animal and plant species have often ignored or underestimated man as an interacting species. In the last decade the role of man as a key component, i.e. a high level and efficient predator of some species, has been assessed (Castilla & Paine, 1987; Hockey et al., 1988). Important contributions have recently been made at the population, community or multi-specific level, particularly in relation to functional modifications of the intertidal community and landscape under human predation. In several recent studies, intertidal communities protected from anthropogenic interference have been compared with those where man has exerted persistent influence on several organisms, often key species or species which interact strongly with others within their communities (Castilla & Durfin, 1985; Hockey & Bosman, 1986; Moreno, 1986; Oliva & Castilla, 1986; Bosman et al., 1987; Castilla & Paine, 1987; Catterall & Poiner, 1987). Man's influence on marine communities is exerted directly on the depredated species, and also indirectly on other organisms, affected by the removal of true target species. Such indirect effects are most obvious when interactions between species are strong, resulting in cascade or ripple effects (Guiller, 1984; Moreno, 1986; Durfin et al., 1987).

The impact of subsistence collectors of shellfish and algae, known in Chile as 'mariscadores de orilla' (Durfin et al., 1987) has not been fully evaluated. In central Chile, these collectors operate at a rudimentary artisanal level on Durvillaea antarctica. The plants are harvested during low tide by cutting the stipe at the base, thus leaving the holdfast attached to the substratum. Dried fronds and the stipes are used for human consumption and the crop is sold at local markets. Castilla & Bustamante (1989) demonstrated that at one locality in central Chile the protection of D. antarctica from collectors increased the density two- to four-fold over a c. 7-year period when compared with populations subjected to continuous harvesting. These authors suggest that inaccessible rocky islands situated close to the mainland (within 100 m) provide refuges or 'buffer zones' (sensu Castilla & Schmiede, 1979) for D. antarctica populations. These islands may act as important reproduction sites, allowing the colonization of the harvested mainland areas.

In this study we assess the effects of human exploitation on the population abundance of D. antarctica at several sites subjected to differing intensities of exploitation in central Chile.

Exploitation of Durvillaea antarctica 207

MATERIALS A N D METHODS

The study was carried out at Montemar, Curaumilla, Las Cruces and Pelancura, in central Chile (32 ° 57' S-33 ° 45' S, Fig. 1). Populations of D. antarctica were sampled on four occasions: March 1985, November 1985, March 1986 and March 1987. On statistical analysis, sampling dates were considered as the variable Time and different sites as the variable Localities. Sampling was done at a range of more or less accessible sites along a stretch ofc. 150 km of coastline. 'Accessibility' was ranked in relation to the distance between the study sites and the nearest urban centres" collectors concentrate their activities around such urban centres, because of a lack of any public transport system. Sampling was done at sites subjected to exploitation and within coastal reserves at Las Cruces (Estaci6n Costera de Investigaciones Marinas ECIM) and Montemar (Instituto de Oceanologia de Montemar IOM). These reserves have not been disturbed for 5 and 10 years, respectively, during which periods they have been continually patrolled to exclude collectors (Castilla, 1986).

Mainland sites easily accessible to collectors consisted of a continuous continental rocky shore and were termed Mainland. Small nearshore islets, mostly under 100m 2 in size and situated no more than 100m from the

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mainland, were termed Islands. These were inaccessible to the mariscadores for most of the time, with only rare exceptions during some spring tides. On the mainland and at several islands at each locality, three to six permanent linear transects, each parallel to the shoreline and 50 m long, were randomly positioned in the algal band formed by Lessonia nigrescens (Bory) and Durvillaea antarctica. The primary space of this belt is dominated by L. nigrescens in terms of cover, biomass and number of individuals (Guiler, 1959; Santelices et al., 1977, 1980; Castilla, 1981; Ojeda & Santelices, 1984). The area (m 2) occupied by the D. antarctica and L. nigrescens was calculated by multiplying the mean width of the band (measured at at least 10 randomly chosen points) by the total length of the transect (see details in Castilla & Bustamante, 1989). The total density (no. of live plants per m 2) and standing crop (kg of wet plants per m 2) were determined for each transect. Plant weight was determined according to the equation:

Wet-weight (kg)= 0.0021 x DH25119; (R 2 - 0.95, p < 0.001)

where DH = diameter of holdfast in cm (Castilla & Bustamante, 1989). The size structure, calculated from measurement of holdfast diameter,

was estimated both in harvested and non-harvested sites. Differences between size structure in harvested and non-harvested sites were tested with a Smirnov 1-tailed test (Conover, 1980). The size structure of harvested plants was assessed by measuring the diameter of their holdfasts remaining attached to the rock. To determine the validity of this technique, the survival of 24 holdfasts was monitored for 14 months after the plants were harvested. Table 1 shows that the holdfasts survive for up to 14 months, with a half-life of 6 months, and that no statistically significant changes take place in their size.

The wet biomasses of D. antarctica on islands were statistically correlated

TABLE 1 Survival and Sizes (Diameter) of 24 Tagged Durvillaea

antarctica Holdfasts after Harvest ing

Date Number % Mean size SD (cm)

October 1986 24 100-0 17-3 + 4.5 December 1986 23 95"8 17.5 _ 5'7 February 1987 21 87'5 19.3 + 4.9 April 1987 19 79.2 17'4 + 4.5 June 1987 11 45-8 17.5 ___ 4"9 Augus t 1987 6 25.0 18'5 _ 4-2 October 1987 4 16-7 17.3 + 3.1 December 1987 1 4.2

Exploitation of Durvillaea antarctica 209

with those of adjacent mainland sites by using values estimated for the different localities and sampling times. The perimeter of each island was measured and multiplied by the mean width of the algal band to obtain an estimate of the total area occupied by D. antarctica and Lessonia nigrescens (see details in Castilla & Bustamante, 1989). This area was defined as Potential Settlement Area Available on Islands (PSI) for D. antarctica, and was standardized for 1 km of shoreline. The PSI was separately correlated with the density of D. antarctica on the mainland and islands, for all sites sampled.

Mean density, standing crop, and sizes were compared between sites using Tukey's studentized range test (Conover, 1980; SAS, 1986). Using analysis of variance the significance of the variables Exploitation (harvested versus non-harvested), Time (four sampling dates), and Localities (4 sites) was tested for the following response variables: mean density, mean standing crop and individual mean size (Underwood, 1981).

RESULTS

Size structure, density and standing crop

Plants at harvested sites were smaller (mean holdfast diameter 9.51 cm _ 5.95 SE) than those at non-harvested sites (mean 15.34 cm ___ 8.28), as shown in Fig. 2. The size structure distributions were significantly different (Smirnov 1-tailed test, p < 0.001, Conover, 1980). A complete size range of D. antarctica (1-42cm holdfast diameter) was found in non- harvested sites. If the size distribution of plants removed from harvested sites is combined with that of extant plants, size structure differed only marginally from that of non-harvested populations (Smirnov 1-tailed test, p = 0.05). Mean holdfast size of the harvested fraction was 12-94 c m _ 4.45 in diameter, and no plants under 4 cm were found.

Mainland Exploitation had no clear-cut effect on plant density, although the highest recorded density was at a harvested site, and the lowest density was at a protected site (Fig. 3). Only at Curaumilla was density significantly different from other mainland sites (Tukey's test, p < 0"05). Standing crops were highest at the non-harvested sites at ECIM and IOM, with 0.233kg m -2 and 0-100 kg m-2, respectively. The standing crop of D. antarctica at ECIM was significantly larger (p < 0.05) than that found in any other locality. Similarly, mean holdfast sizes were significantly larger (p < 0-05) in non-harvested sites than in harvested sites (Fig. 3). In harvested mainland


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sites, mean holdfast diameter was less than 7.56 cm + 1-35, and did not differ significantly between sites (p > 0.05).

Islands Islands at Curaumilla supported a significantly higher density of plants than other island sites (1.055 plantsm -2, p < 0.05, Fig. 4). The greatest mean standing crop was also found at Curaumilla (0"535 kg m-2), significantly higher (p < 0.05) than other harvested sites (Fig. 4). The lowest standing crop (p < 0.05) was found at harvested islands at Las Cruces (0.205 kg m - 2). The largest plants occurred on islands at Las Cruces (both within and outside the ECIM reserve), but the mean sizes at Las Cruces were not significantly different (p > 0.05) from those elsewhere.

Abundance relationships

Figure 5 shows the estimated values of wet biomass of D. antarctica present on islands compared with those at adjacent mainland sites, in different

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localities and at different sampling times. A significant positive correlation (r = 0.606, p < 0"0046) was found. Figure 6 shows the mean density _ SE on the mainland and islands, at both harvested and non-harvested shores, in relation to PSI. A positive significant correlation was found between mean density on the mainland and the respective PSI for adjacent islands (r = 0-963, p < 0"0084). Plant density at islands was also correlated with PSI, and was consistently higher than that on the mainland (r=0-993, p < 0"0006).

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Table 2 shows the results of three ANOVA models, where only live populations of D. antarctica in harvested and non-harvested mainland shores were included. Differences in mean density were related to sampling times (p < 0.001) and site-specific effects (p < 0"001), but were not influenced by exploitation (p > 0.09). On the other hand, differences in standing crop and size reflect exploitation (p < 0.005) (Table 3). Variations in density, standing crop and size at islands sites were a consequence of site-specific effects and were unrelated to exploitation (i.e. accessibility) or sampling times (Table 3).

DISCUSSION

Predation by man on intertidal invertebrates often leads to decreasing prey abundance and mean prey size (Branch, 1975; Moreno et al., 1984; Castilla & Dur~n, 1985; Siegfried et al., 1985; Hockey & Bosman, 1986; Moreno, 1986; Oliva & Castilla, 1986; Hockey, 1987; Ortega, 1987). Exploited populations of the macroalga Durvillaea antarctica also contain smaller individuals than protected populations, but do not exhibit a predictable reduction in density (Figs 3 and 4). Populations vary in demography and abundance between sites, but these variations cannot always be explained in terms of harvesting pressure (Tables 2 and 3). Alternative explanations have been proposed.

Early studies in central Chile (Guiler, 1959) predicted that spatial competition between D. antarctica and Lessonia nigrescens, under undisturbed (unexploited) conditions, would result in D. antarctica populations dominating the primary space of the lower fringe on exposed rocky shores, provided there was no harvesting by humans. Guiler's prediction was based on the idea that D. antarctica is better adapted to live and grow within an environment characterized by high surge and surf. He suggested that exploitation of D. antarctica may allow L. nigrescem to become dominant. Santelices et al. (1980), in testing Guiler's prediction, concluded that L. nigrescens dominates exposed rocky shores, not because of human predation, but rather because L. nigrescens is less susceptible to wave action and is more long-lived than D. antarctica. Despite this D. antarctica is seldom completely excluded from such habitats due to its rapid growth and high colonization rate. Guiler's prediction could prove correct in habitats with intermediate wave action.

In this work we do not examine the competitive process between these two brown algae, but preliminary results (Bustamante & Castilla unpublished data) have shown that during primary colonization the result of space dominance by D. antarctica or L. nigrescens is determined by the season in

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which patches become available, and the supply of recruits is related in turn to the presence of parental stocks nearby (within metres).

In addition to factors such as competition and wave action, it is now evident that human predation has a determining influence on the abundance of D. antarctica populations (Castilla & Bustamante, 1989; and present data). Because of this, coastal geomorphology, which influences the accessibility of plants to human exploitation, can create refuges for plants potentially exploited by man. In particular, the number and size of inaccessible nearshore islands appear to be an important determinant of the consequences of exploitation. In areas where many, or large, islands are present within 100 m of the mainland, biomass and densities of D. antarctica on the adjacent mainland are high (Figs 5 and 6). This suggests that such islands form a reproductive refuge and are important for the repopulation of nearby exploited areas (Castilla & Bustamante, 1989). The importance of nearshore islands to local D. antarctica populations is further highlighted by differences in the abundance and standing crop of algae on the mainland at the two reserve areas of Las Cruces and Montemar. Although Montemar has been protected for longer (10 versus 5 years), the density and standing crop of D. antarctica are greater at Las Cruces. These sites differ geomorphologically in that there are several nearshore islands at Las Cruces which support substantial D. antarctica populations, but such islands are absent at the Montemar reserve (Fig. 6).

Although the short-term consequences of intertidal exploitation are fairly well understood, the prediction of long-term effects is more problematic and requires an understanding of the distribution of refuge (unexploited) populations and of the dispersive ability of exploited species. Extensive stretches of the Chilean coast are heavily exploited (Castilla, 1981; Moreno et al., 1984, 1986; Castilla & Durfin, 1985; Oliva & Castilla, 1986; Castilla & Paine, 1987; Durfin et al., 1987) and the importance of refuge areas (buffer zones) for some species has been demonstrated (Castilla & Schmiede, 1979; Moreno, 1986; Castilla & Bustamante, 1989). Local refuges appear to be important in maintaining stocks of D. antarctica in exploited areas. However, if effective conservation measures are to be implemented on the Chilean coasts, it is necessary to assess the 'sphere of influence' of a buffer or refuge area. This requires an understanding of the dispersal range and settlement dilution of key species, at present a largely unknown field of intertidal ecology.

ACKNOWLEDGEMENTS

This study was supported by the Fondo Nacional de Desarrollo Cientifico y Tecnol6gico of Chile through the projects FONDECYT 86/1100 and

Exploitation of Durviilaea antarctica 219

F O N D E C Y T 88/0432. We thank Drs G. M. Branch, M. George- Nascimento, P. A. R. Hockey, F. P. Ojeda and A. L. Bosman for criticising the manuscript. We are also deeply grateful for the field help of our friends E. Ortiz, A. Jullian and M. Bobadilla, and our colleagues and friends at ECIM for their hard work at Las Cruces.

R E F E R E N C E S

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Branch, G. M. (1975). Notes on the ecology of Patella cochlear and Cellana capensis and on the effects of human consumption on limpet populations. Zool. Afr., 10, 75-85.

Castilla, J. C. (1981). Perspectivas de investigacibn en estructura y din~imica de comunidades intermareales rocosas de Chile central, II. Depredadores de alto nivel tr6fico. Medio Ambiente, 5, 190-215.

Castilla, J. C. (1986). Sigue existiendo la necesidad de establecer parques y reservas maritimas en Chile? Ambiente y Desarrollo, 2, 53-63.

Castilla, J. C. & Bustamante, R. H. (1989). Human exclusion from intertidal rocky shore of Las Cruces, central Chile: effects on Durvillaea antarctica (Phaeophyta, Durvilleales). Mar. Ecol. Prog., Set., 50, 203-14.

Castilla, J. C. & DurUm, R. (1985). Human exclusion from the intertidal zone of central Chile: the effects on Concholepas concholepas (Gastropoda). Oikos, 45, 391-9.

Castilla, J. C. & Schmiede, P. (1979). Hip6tesis de trabajo sobre la existencia de zonas maritimas tampones en relaci6n a recursos marinos bentSnicos (mariscos y algas) en la costa de Chile continental. In Serninario-Taller sobre desarrollo e investigacibn de, ed. V. A. Gallardo. pp. 147-67.

Castilla, J. C. & Paine, R. T. (1987). Predation and community organization on eastern Pacific, temperate zone, rocky intertidal shores. Rev. Chil. Hist. Nat., 60, 131-51.

Catterall, C. P. & Poiner, I. R. (1987). The potential impact of human gathering on shellfish populations, with reference to some NE Australian flats. Oikos, 50, 114-22.

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impact of human exploitation on populations of the intertidal southern bull-kelp durvillaea...

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