Relative Abundance of Lizards and Marine Toads on Saipan, Mariana Islands 1
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Pacific Science (1996), vol. 50, no. 3: 274-284 © 1996 by University of Hawai'i Press. All rights reserved Relative Abundance of Lizards and Marine Toads on Saipan, Mariana Islands 1 GARY J. WILES AND JESSE P. GUERRER0 2 ABSTRACT: Twelve species of lizards and the marine toad (Bufo marinus L.) were surveyed in six habitat types at three sites on the island of Saipan, Mar iana Islands, using visual censuses, hand captures, and adhesive traps. Toads were rare on each of the study sites. Anolis carolinensis Cuvier was most com mon in disturbed forests. Four species of geckos, Gehyra mutilata (Wiegmann), G. oceanica (Lesson), Lepidodactylus lugubris (Dumeril & Bibron), and Pero chirus ateles Dumeril, were most abundant in forests and abandoned build ings, and a fifth species, Hemidactylus frenatus Dumeril & Bibron, occurred most frequently on structures of all types and in open fields. The skink Carlia fusca Dumeril & Bibron was the most abundant diurnal lizard in all habitats. Emoia caeruleocauda de Vis occurred in all habitat types surveyed except open fields and was usually much less common than C. fusca. Emoia atrocostata (Lesson) was documented for the first time on Saipan, with a population found on a small offshore islet with scrubby strand vegetation. Lamprolepis smarag dina (Lesson) was relatively common at only one of three study sites, where it was seen primarily on large tree trunks. Varanus indicus (Daudin) displayed broad habitat use, but also was common in only one study area. At least five of these species are introductions, with C. fusca suspected of causing population reductions of other terrestrial skinks on the island. INCREASING AlTENTION has been focused on species, the skink Carlia fusca Dumeril & the diversity, abundance, and ecology of Bibron, has been implicated in the declines of reptile and amphibian communities in the native skinks elsewhere in the island chain Mariana Islands of Micronesia (Sabath 1981, (Rodda et al. 1991). Finally, most of Saipan Wiles et al. 1989, 1990b, Rodda et al. 1991, is occupied by highly disturbed vegetation, Rodda and Fritts 1992). However, no studies which likely has altered its lizard popula have examined the herpetofauna of Saipan, tions. Native plant communities currently which is the second largest and second most cover less than 8% of the island (Engbring populous island in the archipelago. A better et al. 1986). The goals of our study were understanding of the reptiles on the island is to document lizard and marine toad (Bufo desirable because of accelerated habitat loss marinus L.) abundance and habitat use at associated with recent economic growth and several locations on Saipan. the island's high risk of obtaining a breed ing population of brown tree snakes [Boiga Study Area irregularis (Merrem)]. Saipan's reptile fauna is also of interest because of the introduction Saipan (15 0 10' N, 145 0 45' E) is located in of several lizard species during the last few the central Mariana Islands and has a land 2 decades (Rodda et al. 1991). One of these area of 123 km • The east-central portion of Saipan, where this study took place, is domi nated by a series of rugged uplifted terraces and steep hillsides that culminate in a tall 1 Manuscript accepted 25 October 1995. 2 Division of Aquatic and Wildlife Resources, 192 ridge running north-south along the center Dairy Rd., MangiJao, Guam 96923. of the island. Maximum elevation is 466 m. 274 Herpetofauna of Saipan-WILES AND GUERRERO 275 Surveys were made at three sites, Hakmang island, Isleta Maigo Luao (= Forbidden Is (= Kagman) Peninsula, Laolao Bay, and land), occurs off the southeastern tip of the Kannat I Pitot, all located along or near the peninsula, separated by a narrow, shallow coast (Figure 1). Hakmang Peninsula con channel (about 15-20 m wide at low tide) sists of a large, low plateau bounded by strewn with boulders. The island is 3.2 ha in coastal escarpments on several sides. A small area and has steep sides with a flattened Saipan I) Isleta Maigo o Fahang Isleta Managaha _~r..o--O 0 1----: Isleta Maigo Luao N , km 1 o 3 FIGURE I. Map of Saipan, Mariana Islands, with locations of the three study sites. 276 PACIFIC SCIENCE, Volume 50, July 1996 summit at an elevation of 41 m. The locality obtusifolia R. Brown. The vines Flagellaria at Laolao Bay features a narrow coastal flat indica L., lasminum marianum, and Alyxia bordered by steep hills to the north. Kannat I torresiana Gaudichaud were also common. Pitot is composed of a gently sloping hillside Numerous rock outcrops and boulders typify cut by several small stream valleys. Eleva this habitat. tions of the study sites occur from 0 to 150 m. Weedy open fields frequently occupy re Soils are thin and derived from limestone cent agricultural lands. Weed growth may be rock, except for one area of volcanic earth on thick or fairly open, and 0.5-1.5 m high. the north side of Laolao Bay. Common plants are Sida acuta Burmann, Saipan's climate is tropical and temper fil., Mimosa invisa Martins, Mikania scan atures remain warm and relatively constant dens, Bidens alba (L.) DC., Lantana camara during the year, ranging from 22 to 33°C. L., Stachytarpheta sp., Cassia alata L., and Annual rainfall averages about 2000 mm, Chromolaena odorata. An area of savanna most of which falls from July to November with Miscanthus floridulus (Labill.) at Laolao (van der Brug 1985). A dry season occurs Bay was lumped with this habitat category. between January and May, when rain dimin Strand vegetation occurs as a narrow band ishes to a mean of about 90 mm per month. on limestone substrates along the shore ofthe Descriptions of plant communities on ocean. It is usually stunted and windswept, Saipan appeared in Fosberg (1960), Engbring being 0.5-3 m tall and commonly includes et al. (1986), and Falanruw et al. (1989). Six Scaevola sericea Vehl, Wollastonia bifiora (L.) habitat types were recognized for our study, DC., Pemphis acidula Forst., Zoysia matrella with secondary forest and tangantangan (L.) Merrill, Bikkia tetrandra Brongniart, [Leuceana leucocephala (Lam.) de Wit] forest and Ipomea pes-caprae (L.) Roth. Bare rock being the most common communities. Sec faces and boulders are often present. ondary forest is variable in nature and is Abandoned concrete and tin buildings composed largely of introduced species. The were a final habitat type. A few of these most numerous trees are Leucaena leuco structures were located in secondary forest, cephala, Acacia confusa Merrill, Albizia leb tangantangan, or open fields on each study beck (L.) Bentham, Cocos nucifera L., Carica area. None were lighted. papaya L., Barringtonia asiatica L. (Kurz), Most of the study sites exhibited little evi and Melanolepis multiglandulosa Reinw., and dence of cattle grazing, but a few fields and Mikania scandens (L.) Willd and other vines groves of tangantangan were apparently used are also common. The canopy ranges in regularly. This activity altered the amounts height from 5 to 20 m, and the understory and types of ground cover and therefore po may be dense or relatively open. tentially changed the lizard communities at a Large, nearly pure forests of tangantangan few locations. grow on sizable portions of the study area. Canopy height is usually 4-5 m. The vines lasminum marianum DC., Mikania scandens, and Passifiora suberosa L. are often present, MATERIALS AND METHODS and a dense herbaceous or grassy ground cover sometimes exists. Field surveys were made on Hakmang Limestone forest is characterized by sparse Peninsula from 11 January to 3 February to moderate undergrowth, a canopy 10-15 m 1991, at Laolao Bay from 15 February to 14 high, and scattered taller emergent trees. This April 1991, at Kannat I Pitot from 17 to 26 vegetation type was found primarily on hill January 1992, and at Isleta Maigo Luao on sides where clearing had not occurred in the 10 October 1992 and 20 August 1994. Count past. Common tree species include Cyno sites were selected to provide wide coverage metra ramifiora L., Barringtonia asiatica, of the common or unique natural habitats at Guamia mariannae (Safford) Merrill, Pisonia each location. One or two observers partici grandis R. Brown, Ochrosia mariannensis A. pated in surveys at each site. Observers cen DC., Pandanus dub ius Sprengel, and Premna sused lizards by walking slowly through each Herpetofauna of Saipan-WILES AND GUERRERO 277 site and counting the number of animals seen BUFONIDAE on the ground and in trees. Every lizard ob served was recorded by species, date, time, Bufo marinus L. locality, microhabitat, and in the case of Marine toads were rare at each study site, Emoia caeruleocauda de Vis, by the color of with only a few individuals seen per location. its tail and posterior portion of its body. Liz Toads were recorded in all habitats except ards not positively identified by sight during coastal strand and old buildings (Table 1). censuses were classified as being either un Hilly terrain and porous soils in the east known skinks or geckos. Animals were cap central part of the island probably result in a tured whenever possible to obtain accurate scarcity of breeding sites for this species. This identifications. Skinks were caught by shoot was despite the presence of several ephemeral ing them with a heavy rubber band; geckos were captured by hand. Species identities streams that ran through the Laolao Bay and Kannat I Pitot sites, which were expected to were determined with an unpublished identi retain pools of standing water long enough to fication key produced by G. H. Rodda and permit breeding and growth of tadpoles. B.