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SOME ECOLOGICAL ASPECTS OF THE LAND CRAB, GECARCINUS PLANATUS STIMPSON, AT SOCORRO ISLAND, COLIMA, MEXICO Alfredo Ortega-Rubio, María Luisa Jímenez, Jorge Llinas and Gustavo Arnaud Centro de Investigaciones Biológicas del Noroeste Apdo. Postal No. 128. La Paz 23000, Baja California Sur, MCxico. ABSTRACT As a part of a study of the natural resources of Socorro Island, some ecologicai aspects of the land crab, Gecarcinus planatus, were studied. Density of crabs was establishedusing mark-recapturemethods. During February 1990, crabs density per hectare varied from 755 individuais (SD = 30.36) to 1,645.25 (SD = 45.56). Food habits were established by examining the gut contents of adult individuals collected in the field. Land crabs' food mainly consisted of vegetable matter (80%), arthropods 00%) and detritus (10%). General behavior was observed directly in the field during study periods. In the northern part of the island, crabs climbed trees of &S cotinifolia and foraged on its leaves even during the day. Predation on crabs was determined by anaiyzing excrements of the only posible major predators on the island, the feral house cat catus)and the yellowcrowned night-heron (N~ticoraxviolaceapravirost~s). A total of 46 cat scats and 26 yellowcrowned night-heron pellets samples were collected and anaiyzed. Only two of the cat scat samples in May showed crab's remains, which only represents 10%of the cat's t o d diet for this month. The land crab is a significant part (62.80%) of the food of the yellowcrowned night-heron. m INTRODUCTION Socorro Island biodiversity is severely endangered because of the growth of the ferai sheep (Ovis aries) population in the southern half of the island (Ortega et al. 1992). There are no sheep in the northem half, where the vegetation is denser and better conserved (Ortega-Rubio et al. 1991). The land crab, Gecarcinus lanat tus Stimpson, is an abundant species on the island. The world distribution of this species is along the Pacific Ocean from the southern coast of the Gulf of California, Tenacatita Bay, Marias Islands and Revillagigedo Archipelago in Mexico, to Clipperton Atoll, Gorgona, (von Prahl 1983) and Malpelo Island, Colombia, (Niaussat and Ehrhardt 1968). This crab ranges from sea level to the highest parts of Socorro Island, at 1050 m, and is the most conspicuous nocturnal inhabitant of the forest. As a part of a study of the natural resources of Socorro Island with the aim of promoting the legal protection of the island as a Biosphere Reserve, our research team has developed several studies concerning different elements of the ecosystem (Ortega et al. 1992). In agreement with Veitch (1990), Walter (1990), Wethje et al. (1993), the apparent abundance of this land crab observed during our expeditions to the island indicates that it is a species worthy of much study. MATERIAL AND METHODS Study area. --Socorro Island is located 460 km south of the tip of Baja Califomia Peninsula (18" 47' N and 110" 57' W) (Adem 1960). Socorro Island is the largest of four islands in the Revillagigedo Archipelago, with an area of 150 km2and a maximum height of 1050 m at the tip of Mount Evennann (Blazquez 1960). The climate is arid-tropical, with a monthly average temperature ranging between 19" and 28" C and an annuai average precipitation of 761 mm (Veitch, 1990). Socorro island is biologically important because of its high incidence of endemic species: 26% of the plants and 90% of the terrestrial avifauna are unique in the world at the level of genus, species or subspecies (Levin and Moran 1989, Brasttrom 1990). JOURNAL OF THE AREONA-NEVADA ACADEMY OF SCIENCE VOL. 30(1) SOCORRO ISLAND 1 Eroded areac SCALE 0 -re 1 2 3 4 5 l. Map of e d e d aras of Socorro Island. . K m ISSUE 1, 1997 FOREST RESOURCES OF TROPICAL DRY FOREST & RIPARIAN COMMUNlTIES, MEXICO 19 The natural vegetation ranges from coastal halophytes to shrubland associations and to forest; it is distributed along an altitudinal gradient and is related to the topography (Levin and Moran 1989). Vegetation has been strongly affected since 1869 by the overgrazing of feral sheep (Villa 1960) introduced 123 years ago (Levin and Moran 1989). An estimated 30% of the natural vegetation and soil of the island has been destroyed or eroded by sheep's activities (Figure 1). METHODOLOGY Density. -Crab density was established using the modified Petersen Index (Bailey 1952), with markrecapture data (Jimenez et al. 1994). Thus, during February 1990 we spent 48 hours during eight nights in forest comrnunities of the island recording the number of terrestrial crabs and their activities in two belt transects of 10x80 m established at 560 m of altitude. The transects were surveyed for crabs each night by four persons (JimBnez et al. 1994). Every unmarked crab was captured and marked with indelible ink. We recorded the location of al1 marked crabs within the transects. Food Habits. --Direct observations of feeding behavior of land crabs were made at night during two visits to the island, on May 1990 and August 1991. On our first visit, the crabs emerged from their burrows because of the first seasonal rain at the Ficus cotinifolia - Ilex socorroensis forest, located at 760 m elevation. During our second stay, the crabs were observed at the northem part of this island in mixed scrub vegetation in Cape Middleton and Playa Norte at approximately 15 m elevation. Also, 20 land crabs of different sizes and both sexes were collected in this forest. These samples were fixed in alcohol (70%) and taken to the laboratory where the guts were removed for the analysis of their contents. Behavior. -During al1 visits to the island we observed over a total of 64 hours the activity of the land crab individuals mostly during the night, but also during the day with binoculars. Predation on Crabs. --Walter 0990) reported the red-tailed hawk (Buteo jamaicensis socorroensi) as a predator on the land crab. In order to know the effect of other predators on the crab, we analyzed the diet of two more potential species. We collected and analyzed 26 pellets of yellow-crowned night-heron and 46 feral house cat scats, for the presence of the crab remains. An intense search throughout most of the island, for cat feces was made during the months of February, May and November of 1990, and the search for yellowcrowned night-heron pellets was accomplished in May 1990. RESULTS AND DISCUSSION Density. -At the study sites we found that crab density per hectare varies during the winter from 755 (SD = 30.36) to 1,645.25 (SD = 45.56). Such densities were associated directly to the degree of complexity and conservation of the forest (JimBnez et al. 1994): at the study site, forest tree average cover varies from 1600 m2 to 2300 m2. In zona where the vegetation structure and cover were more complex, crab densities were significantly higher: crab density in the structurally more complex forest was more than twice as high as in the structurally less complex forest (JimBnez et al. 1994). In the perturbed zones without vegetation cover, the land crabs do not occur. Food Habits. -From the analysis of gut contents, we found that the land crabs fed mainly on vegetable matter (80% of wet weight), remains of unidentifiable arthropods 00%) and detritus (10%). Our observations at the field indicate that crabs feed on fruits of Ficus cotinifolia, lichens, grasses and dead leaves. There was no evidence of any ingested seeds in our study, perhaps because the gastric mil1 of land crabs destroys it as Walcott (1988) had suggested. According to this author, feeding on seeds, either directly or on those embedded in feces or detritus may affect local plant species composition and thus represent an important barrier to invasion by exotics. Nevertheless, hard inedible seeds encased in edible pulp may be JOURNAL OF THE ARIZONA-NEVADA ACADEMY OF SCIENCE VOL. 30(1) transported by crabs (Walcott 1988) similarly to fruits of the endemic trees of Socorro Island. Ehrhardt and Niaussat 0970) mention that swarms of G. vlanatus reduce to a skeleton any living or dead animal, including members of its own species, on Clipperton atoll. We also had the opportunity to observe a big population of land crabs feeding on a dead feral sheep, but we did not see evidence of cannibalism in the fíeld as was observed for land crabs in Clipperton by Ehrhardt 0968), and by Ehrhardt and Niaussat 0970). Considering their abundance and the contents of their stomachs, it is possible that G. lanat tus could be the main degrader of organic matter at Socorro island, mainly during the rainy season, when they are most active. Behavior. --In the northern part of the island, crabs climbed trees of Ficus cotinifolia and foraged on leaves even during the day. This type of activity was observed in other parts of the island only at night. Niaussat and Ehrhardt 0968) and Ehrhardt and Niaussat 0970) have recorded this diurnal climbing behavior for S, planatus at the Clipperton Atoll. Land crabs may seek refuge in the branches of trees to avoid predation as G. latteralis does in the Bermuda Islands in areas where the vegetation is very complex (Bliss 1968, 1979). Land crabs can reach remarkable densities in the absence of heavy predation and, on islands with scarce mammalian and avian predators such as on the northern part of Socorro Island, they can become the dominant large animals; climbing trees maybe also be a means to avoid competition. Redation on Crabs. -Pelle& of 26 yellow-crowned night-herons were collected in May 1990 and were analyzed. From the total pellets samples, the most commonly found were crabs (62.80%); the least, crickets of the food of the yellow-crowned remains (9.30%) vable 1). The land crab represented a ~ignificant~part night-heron. On the other hand, of the 46 feral house cat feces were collected during February (N = 12), May (N = 19) and November (N = 15) and analyzed, 75% of them were collected from the southern part of the island in the grassland -Croton masonii shrub zone, close to the area which is being occupied by the Mexican Naval Garrison, and 25% were collected from Ficus cotinifolia forest, and mixed scrub zone. Table 1. Prey items recorded from 26 pellets of the yellow-crowned night-heron, collected in the southern part of the Socorro Island, Mexico, May 1990. Prev Number of items Percent Land Crab (Gecarcinus planatus) Grasshopper (Acrididae) Scorpions (Vaeiovis ianssi) Crickets (Gryllidae) Totals Only two house cat scats (collected in May in the Ficus cotinifolia forest and in the Croton masonii shrub zone) showed crab remains, representing only 1.88 % of the cat's total diet, both feces that contained crab's remains were collected in the Ficus cotinifolia forest, and in the Croton masonii shrub zone. On the other hand, remains of insects and mice CM_r-S musculu~)appeared frequently, representing 35.84 % and 31.13 %, of the cat's diet vable 2); in scats collected from the southern part of the island. ISSUE 1, 1997 FOREST RESOURCES OF TROPICAL DRY FOREST & RIPARIAN COMMUNTTIES, MEXICO 21 Table 2. Contents of 46 feral house cat scats collected in February, May and November 1990 in Socorro Island. Number of iterns Percent m 33 31.13 Birds 9 8.49 Prey House mouse musculus) Reptiles Insects Land crab (Gecarcinus planatus) Totals Our results indicate that the pressure of predation by the cat towards the crab is minimai since it is rarely consumed. The cat prefers mice and insects as prey, the food items most cornmonly consumed (Rodríguez-Estrella et d. 1991). CONCLUSIONS Because of their high densities, land crabs probably may play a vital role in the stmcturing and functioning of Socorro Island ecosystems. However, in order to precisely understand their ecologicai impact, more research must be done on the land crabs'.reproductive biology, longevity, symbionts and burrow types made by juveniles and adults. ACKNOWLEDGMENTS This study was sponsored by the Centro de Investigaciones Biológicas del Noroeste (CIBNOR), the Secretaría de Programación y Prekupuesto (SPP), the Consejo Nacional de Ciencia y Tecnología (CONACyT), the Fund for Studies and Research Ricardo J. Zevada from Mexico, and by The World Wildlife Fund (WWF) of the United States of America. We would like to express our thanks to the Secretaría de Desarrollo Urbano y Ecología (SEDUE) and the Armada de Mexico for their support to this work. To Donaid J. Pinkava and two anonymous reviewers for their helpful suggestions on an earlier version of this manuscript. To Aradit Castellanos, Armando Tejas, Yolanda Maya, Ricardo Aguilar, Federico Salinas, Tangaoxan Argueta, Tom Van Eyck and Waiter Whetje for their help in the field. And to Lolita Vázquez and Veronica Hiraies who provided expert secretarid assistance. JOURNAL OF THE ARIZONA-NEVADA ACADEMY 0 F SCIENCE VOL. 30(1) LITERATURE CITED ADEM, J. 1960. Introduccion. Pp. 9-12. b:A. Adem, E. Cobo, L. Blásquez, F. Miranda, A. Viiialobos, T. Herrera, B. ViUa y L. Vdzquez. La Isla Socorro. Archipidlago de las Revillagigedo. Monograña del Instituto de Geofísica, Universidad Autónoma de Mexico, M6xico. BAILEY, N. T. 1952. 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