Biological Conservarion 69 (1994) 9- 13

ECOLOGICAL ASPECTS OF THE LAND planatus (: ) IN SOCORRO ISLAND, MEXICO

Cecilia Jiménez, Alfredo Ortega-Rubio, Sergio Alvarez-Cárdenas & Gustavo Arnaud Centro de Investigaciones Biológicas de Baja California Sur, Apartado Postal No. 128, La Paz 23000 BCS, Mexico

(Received 29 November 1990; revised version received 27 October 1992; accepted 30 April 1993)

Abstract rostris (J. Llinas, pers. comm.). In spite of its possible The results of a study are presented on the Socorro Is- importance, very little is known about the ecological land crab Gecarcinus planatus, a species inhabiting this factors influencing the crab's density and behavior in oceanic island in a wide range of communities and alti- the island, only a few reports of its distribution and tudes. In February 1990, two transects in the forest com- (Rathbun, 19 18; Villalobos, 1960). munity at Socorro Island were sampled and al1 The aim of this work is to report the crab's density captured were recorded. Through mark-recapture meth- and home range in two different areas of the forest as- ods the population density was estimated, and the individ- sociation influenced by sheep overgrazing, and to cal1 ual home ranges were determined by means of the attention to the extinction process occurring now at Component Bivariate Model. The structure and phenol- Socorro Island. ogy of the vegetation was recorded in the sume two tran- sects. It seems that crab density is related to cover of STUDY AREA trees, particularly to Ficus cotinifolia, Guettarda insu- laris, and Bumelia socorrensis. In addition, the phenol- Socorro is the largest of the Revillagigedo Islands. This ogy of G. insularis also seems to be important to crab archipelago is located 450 km from the tip of Baja Cal- distribution. Home range of G. planatus is inversely as- ifornia Peninsula (18" 20' and 19" 20' N; 110" 45' and sociated to crab population density. Vegetation commu- 114" 50' W; Fig. 1). Socorro Island is 140 km2 in area nities in Socorro Island are endangered by sheep and its summit, at mount Evermann, is 1200 m high overgrazing, so densities and home ranges of the crab in (Medina, 1978). The climate is arid-tropical, with an this forest muy be affected in the future. To determine average annual temperature of 24.8 "C and annual av- the real impact of exotic species on the structure and erage precipitation of 327.7 mm. The lowlands are dry junction of the communities present in Socorro Island while the highlands are engulfed by cloud. The (such as feral sheep) it will be necessary to study the re- archipelago is biologically important because of its lationships between vegetation and the different high incidence of endemic species: 26% of the plants populations in the areas not strongly disturbed. and 90% of the terrestrial avifauna are unique in the world to the level of genus, species or subspecies (Levin Keywords: Mexico, Socorro Island, land crab, Gecarcinus & Moran, 1989; Brasttrom, 1990). planatus. Vegetation varies from coastal halophytes 'to shrub- land associations and to forest distributed along an al- titudinal gradient and related to the topography (Levin INTRODUCTION & Moran, 1989). Since 1869 the vegetation has been Gecarcinus planatus Stimpson is a land crab widely dis- strongly affected by the overgrazing of feral sheep tributed from Mexico to Panama. In Mexico it inhabits (Villa, 1960). An estimated 30% of the natural vegeta- the west coast and the adjacent islands, from Baja Cali- tion and soil of the island has been destroyed or eroded fornia to Guerrero States (Rathbun, 1918). On Socorro by sheep grazing (Fig. 1). island the crab occurs in a wide range of communities and at altitudes from sea level to 1,200 m. It has been METHODOLOGY reported previously in shrub communities of the island (Rathbun, 1918), but we found it also in the high for- During February 1990 we spent eight nights in the for- est. Because of their high density, we think that land est community recording the number of land crabs and crabs in Socorro Island could be a key factor in the their activities in two transects previously established at structure and function of these ecosystems, i.e. it could 560 m above sea level. Both transects were located in be the principal prey of some birds, including the yel- areas with similar slope, but differing vegetation. The low-crowned night heron Nycticorax violacea gravi- area of each transect was 800 m2 (10m X 80 m). Eigh- C. Jiménez et al.

Fig. 1. Socorro lsland showing vegetation communities and eroded areas (E). G, grassland; F, Ficus forest; C, Croton scrub; M, mixed scrub; B, Bumelia--Guettarda forest; P, prairie. Mount Evermann is located at P. teen numbered plastic stakes were placed inside each coln-Peterson Index) was chosen because of its simplic- transect at intervals of 10 m. The transects were ity, accuracy and efficiency. surveyed for crabs during eight consecutive nights Home range was based on individuals with a mini- by four persons. Every new sighted crab was captured mum of three sightings. Coordinates of capture and and marked with indelible ink at first capture. When recapture within the transect were transferred to we traversed the transects we marked al1 crabs not scale maps. Home range was then estimated using the previously labeled. Observations of labeled crabs statistical Component Bivariate Model (Koepl et al., were recorded and mapped on a grid. Because the 1975) modified by Aguirre et al. (1984). This statistical recapture was only visual disturbance to crabs was method was chosen because the Component Bivariate minimized. Model is not sensitive to the number of recapture The modified Petersen Index (Bailey, 1952) was used points used, in contrast to poligonal methods of home for estimate the size of the population using the recap- range analysis (Mohr & Wilson, 1947; Stickel, 1954; ture data: Harvey & Barbour, 1965; Southwood, 1966). In addi- tion, unlike other statistical methods, the Component Bivariate Model is conservative and minimizes the ex- pected home range area (Hayne, 1949; Dice & Clark, where N = population size, M = marked individuals 1953; Calhoun & Casby, 1958, Jenrich & Turner, (first sample), n = total number of individuals in the 1969). Other statistical methods assign a probability of second sample, and m = recaptured marked individuals. finding an individual where it never was observed. The Petersen's index (also called Lincoln or Lin- Moreover, it has been shown that the Component Bi- Land crab ecology, Socorro Island, Mexico 11 variate Model is the best predictor of home range size, Table 2. Main features of the vegetation structure of transects at least for severa1 lizard species such as Sceloporus studied scalaris and S. grammicus (Gutiérrez & Ortega, 1985). Attributes Transect 1 Transect 2 Habitat characteristics such as structure (cover, den- sity, height), diversity and phenology of vegetation F. cotinifolia density were measured in each transect (Mueller-Dombois & F. cotinifolia height (m) Ellenberg, 1974) and the number of crab burrows F. cotinifolia cover (m2) below each tree was also recorded. F. cotinifolia with fruits F. cotinifolia relative dominance (%) F. cotinifolia foliage (%) RESULTS Average crab's burrows in F. cotinifolia Crab density and home range G. insularis density We recorded a total of 72 crabs active during our sur- G. insularis height (m) veys. Fifty-eight (80.55%) were marked, and 32 (55.17%) G. insularis cover (m2) G. insularis with fruits of these were visually recaptured at least twice. G. insularis relative dominance (%) Density, home range and number of burrows for G. insularis foliage (Ola) each transect are presented in Table 1. Crab density in Average crab's burrows in the structurally more complex forest (transect 2) was G. insularis more than twice as high as in the structurally less com- B. socorrensis density B. socorrensis height (m) plex forest (transect 1) (t=8.17; d.f.=4; p<0.001). In B. socorrensis cover (m2) contrast, individual home ranges in the structurally less B. socorrensis with fruits complex forest were larger compared with those of in- B. socorrensis relative dominance (%) dividual~from transect 2 (t=5.81; d.f.=22; p<0.001). B. socorrensis foliage (%) Numbers of burrows were 169 and 226 in transects 1 Average crab's burrows in B. socorrensis and 2, respectively. 1. socorroensis density 1. socorroensis height (m) Forest community structure 1. socorroensis cover (m2) The sampled area did not contain either herbaceous or 1. socorroensis with fruits juvenile plants, and the shrub medium stratum was also 1. socorroensis relative dominance (Oh) absent. Transects 1 and 2 comprised the same tree 1. socorroensis foliage (%) Average crab's burrows in species, but transect 1 contained 45 trees while 58 were 1. socorroensis present in transect 2 (Table 1). P. socorrense density This forest comprised the following dominant P. socorrense height (m) species: Bumelia socorrensis (Sapotaceae), Ficus cotini- P. socorrense cover (m2) folia (Moraceae), Guettarda insularis (Rubiaceae), Ilex P. socorrense with fruits P. socorrense relative dominance (%) socorroensis (Aquifoliaceae) and Psidium socorrense P. socorrense foliage (%) (Myrtaceae) (Table 2). All, except F. cotinifolia are So- Average crab's burrows in corro Island endemics. P. socorrense At the community level, only small differences in di-

Table 1. Main features of the vegetation structure, number of versity, tree density, average tree height, and average burrows, population density and average home range for the crown height were detected. Table 2 presents the main two transects studied. The numbers in parenthesis are standard differences in relative dominance between the transects. deviations For example, F. cotinifolia represents 28.77% of the Attributes Transect 1 Transect 2 total cover in transect 1, while in transect 2 it represents 36.57% (,$= 174.01; d.f.= 1; p<0.001); B. socorroensis Number of trees represents 32.28% and 30.72% of the total cover in tran- Tree density (no. per m2) sects 1 and 2 respectively (,$=51.60; d.f.=l: p<0.001); Tree diversity Average tree height (m) and C. insularis represents 28.77% and 19.99% in tran- sects 1 and 2 respectively (y=17.19; d.f.= 1; p<0.001). Average shift height (m) Phenological characteristics, such as percentage of foliage and numbers of trees containing fruits, were Average crown height (m) also different for both transects, both being higher in Average tree cover (m2) the structurally more complex forest (i.e. C. insularis trees with fruits, 2=15.06; d.f.=l; p<0.001) (Table 2). Number of Crab burrows Crab density (no. per 800 m2) DISCUSSION Crab Home Range (m2) Socorro Island crab density seems to be influenced by the structure of the vegetation and its phenological at- 12 C. Jiménez et al. tributes, even within the same forest community. Also, in which the land crab could survive; (2) to determine the home range of G. planatus in this island could be the rate of loss of the natural vegetation of the island; affected by the density and phenology of the trees. and (3) to start a restoration program of the natural The Socorro Island crab feeds mainly on plant and vegetation of the island in order to protect aH its natu- animal debris in the soil. Crab density is apparently as- ral resources. sociated with total tree cover of F. cotinifolia, G. insu- laris, B. socorroensis. The last two seem to be the ACKNOWLEDGEMENTS principal food sources for the crab in this community. We observed some crabs feeding on G. insularis fruits This study was sponsored by the Centro de Investiga- and their density was higher under G. insularis with ciones Biológicas de Baja California Sur (CIB), the Sec- mature fruits, which also could suggest that home retaría de Programación y Presupuesto (SPP), the ranges may shift seasonally. In addition, the number of Consejo Nacional de Ciencia y Tecnología (CONA- burrows under the roots of F. cotinifolia and B. socor- CyT), the Fund for Studies and Research Ricardo J. roensis was very large, indicating that the root architec- Zevada from México and the World Wildlife Fund ture of these trees may provide good refuges. (WWF) of the United States of America. In spite of the similarities in structure between both We would like to thank the Secretaría de Desarrollo transects, their vegetation differences can affect the Urbano y Ecología (SEDUE) and of the Armada de crabs' activities. Lower cover, reduced total foliage and México for their support for this work; to Aradit presence of mature fruits may affect microhabitat char- Castellanos, Yolanda Maya and Walter Whetje for acteristics such as soil humidity, food availability and their help in the field; Jorge Llinas and Ricardo Ro- antipredator protection. These parameters can explain driguez for the comments on an early version of the the lower density, the larger home range and the manuscript and to Lolita Vazquez for typing it. Fi- smaller number of burrows found in transect 1 in con- nally, an anonymous reviewer made very valuable sug- trast to transect 2. gestions which greatly improved the manuscript. The home range size of G. planatus seems to be in- versely related to the same structural and phenological REFERENCES characteristics of the community that enhance its popu- Aguirre, G., Adest, G. & Morafka, D. (1984). Home range lation density. The increase in density increases the and movement patterns of the Bolson tortoise Gopherus likelihood of agonistic intraspecific interactions that flavomarginatus. Acta Zool. Mex., (NS), 1, 1-28. could reduce the home range (Ortega & Gutiérrez, Bailey, N.T. (1952). Improvements in the interpretation of re- 1987). Also, individuals who live in better habitats can capture data. J. Anim. Ecol., 21, 120-7. range over smaller areas to fulfil their requirements, ex- Brasttrom, B.H. (1990). Biogeography of the Islas Revil- lagigedo, Mexico. J. Biogeogr., 17, 177-83. hibiting smaller home ranges than those who live in Calhoun, J.B. & Casby, J.U. (1958). Calculation of home marginal or less desirable habitats. Additionally, in range and density of small mammals. US Public Health poor habitats, wider crab movements increase the risk Monograph, No. 55. of predation. Dice, L.R. & Clark, P.J. (1953). The Statistical concept of The daily activity patterns of crabs are also affected home range as applied to the recapture radius of the deer mouse Peromyscus. Contrib. Lab. Vert. Biol. Univ. Mich., by the vegetation structure: cover, height, density and 62, 1-15. dominance. In the northwest side of the island, where a Gutiérrez, A. & Ortega, A. (1985). Comparación de métodos very dense forest exists, we found crabs active during para calcular el área de actividad de Sceloporus scalaris. the entire day. This diurna1 activity has not been previ- Acta Zool. Mex. (NS), 12, 1-12. ously reported in the literature. Harvey, J.M. & Barbour, R.W. (1965). Home range of Mi- crotus ochrogaster as determined by a modified area Our results suggest that tree cover, particularly that method. J. Mammal., 46, 398406. of F. cotinifolia, G. insularis and B. socorrensis, is the Hayne, D.W. (1949). Calculation of size of home range. J. key structural factor linked to crab population size. Mammal., 30, 1-18. The latter two tree species are endemic to Socorro Is- Jenrich, R.I. & Turner, F.B. (1969). Measurement of noncir- land. cular home range. J. Theoret. Biol., 22, 227-37. Koepl, J.W., Slade, N.A. & Hoffman, R.S. (1975). A bivari- The forest studied must be considered to be disap- ate home range with possible application to ethological pearing due to lack of demographic replacement, as no data analysis. J. Mammal., 56, 81-90. regeneration of trees, shrubs, or herbs was observed. Levin, G.A. & Moran, R. (1989). The vascular flora of Isla This lack of regeneration is caused mainly by sheep Socorro, Mexico. Society of Natural History, San Diego. overgrazing (Ortega et al., 1992), and will lead ulti- Medina, G. (1978). Memoria de la Expedicion Cientijca a las Islas Revillagigedo. Universidad de Guadalajara. mately to home range size demands incapacitating the Mohr, C.O. & Wilson, M.F. (1964). Table of equivalent pop- ability of Socorro land crab to adjust. ulations of North American small mammals. Amer. Midl. We cal1 attention to this systematic extinction pro- Nat., 37, 22349. cess on Socorro Island and the need to study the func- Mueller-Dombois, M. & Ellenberg, H. (1974). Aims and tional mechanisms and specific relationships between methods of vegetation ecology. John Wiley, New York. Ortega Alfredo & Gutierrez, América (1987). Evaluación de the vegetation structure of Socorro Island and this diferentes métodos para estimar el área de campeo en dos crab. We consider that the top priorities should be: (1) especies de iguknidos simpátridos. Doñana Acta Verte- to determine the minimum natural vegetation structure bruta., 14, 133-8. Land crab ecology, Socorro Island, Mexico 13

Ortega, Alfredo et al. (1992). Estudio de los recursos natu- Stickel, L.F. (1954). A comparison of certain methods of rales de la isla Socorro, Revillagigedo, México. Ciencia., measuring ranges of small mammals. J. Mammal., 35, 1-15. 43, 175-84. Villa, B. (1960). Vertebrados terrestres. La isla Socorro. Rathbun, M.J. (1918). The grapsoid crabs of America. Smith- Monogr. Inst. Geojis. Univ. Nac. Autonoma Mex., 2 203-16. sonian Inst. Bull., No. 97. Villalobos, F.A. (1960). Notas acerca del aspecto hidrobio- Southwood, T.R. (1966). Ecological methods. Methuen, logico de la parte sur de la Isla. Monogr. Inst. Geojis. Univ. London. Nac. Autonoma Mex., 2 153-80.