Received: 15 th Feb-2014 Revised: 20 th March-2014 Accepted: 23 rd March-2014 Research article SPECIES DIVERSITY OF CAVE-DWELLING SPIDERS ON SIARGAO ISLAND, PHILIPPINES Michael Henry D. Cabili and Olga M. Nuñeza* Department of Biological Sciences, College of Science and Mathematics, MSU-Iligan Institute of Technology, Andres Bonifacio Avenue, Tibanga, 9200 Iligan City, Philippines *Corresponding author, email: olgamnuneza@yahoo.com ABSTRACT: Siargao Island is home to many caves that are interesting habitats because of their extreme environments. Spiders that are good indicators of biodiversity are also known to be common cave dwellers. However, studies on cave spiders in Siargao Island are poorly known. This study was conducted to determine the species diversity of spiders in Siargao caves. Sampling was done through modified cruising method. Eight species belonging to six families were recorded in 10 caves sites of Siargao. Caves 1 and 4 where rich guano material was present had the highest species richness. The dark zone of these caves was found to have more number of species and individuals than the entrance or twilight zone. Low species diversity with more or less even distribution was recorded in Siargao caves. It appears that the presence of guano material contributes to the higher number of spiders in caves especially in the dark zone. Physico-chemical factors such as temperature and relative humidity appear to play an important role in the distribution and abundance of spiders in the different cave zones. Keywords: Biodiversity, caves, dark zone, guano INTRODUCTION Caves are found world-wide, and are natural laboratories for the study of evolution [1]. A cave is considered to be an extreme environment [2] that is characterized by total darkness, almost constant air and water temperature, relative humidity approaching saturation and a relatively poor supply of nutrients [3]. Despite these characteristics, cave ecosystem harbors a variety of unique and sensitive organisms, many of which are cave obligates [4] including spiders with numerous troglobitic forms known from temperate and tropical caves [5]. Cave fauna are unique and constitute one of the important components of biodiversity [6]. They are also worth analyzing because all of these species have successfully invaded one of the harshest environments [7]. Spiders are common denizens of caves [5] with a wide range of physiological and morphological adaptation [8]. They are one of the most abundant predatory groups in the terrestrial ecosystems as they feed on insects and some other arthropods and thus, they can play an important role in pest control [9]. They are important food source for birds, lizards, wasps and other animals, and they are predators feeding on insects and small arachnids [10]. Spiders are also excellent indicators of environmental features of biodiversity [11]. There are approximately 40,000 spider species that have been described worldwide belonging to 109 families, [12] and new species are still being discovered. There are thousands of obligate subterranean animals and majority of them have been found in caves [13]. A study on the colonization of subterranean habitats by spiders in Central Europe showed a total of 161 spider species with the number of species declining with increasing habitat depth [14]. Several studies on spiders are on the comparative biology of cave-dwelling spitting spiders (araneae: scytodidae): parental care, cooperative prey-capture, cannibalism, natal dispersal and reproductive behavior [15] and the diversity of cave-dwelling spiders in Greece [16]. However, in the Philippines, there are only few studies on the diversity of cave fauna especially in Mindanao, the country’s second largest island. Recent studies were on the species richness of ants on Siargao Island [17], the species richness of cavernicolous species of crickets in selected cave sites in Mindanao [18] and the cave spiders in Mindanao [35]. This study aimed to provide information on the species richness, diversity, and the distribution of spiders within the caves on Siargao Island. International Journal of Plant, Animal and Environmental Sciences Page: 392 Available online at www.ijpaes.com