Habitat and Prey Resource Overlap Between the Iriomote Cat
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Mammal Study 28: 47–56 (2003) © the Mammalogical Society of Japan Habitat and prey resource overlap between the Iriomote cat Prionailurus iriomotensis and introduced feral cat Felis catus based on assessment of scat content and distribution Shinichi Watanabe*, Nozomi Nakanishi and Masako Izawa Faculty of Science, University of the Ryukyus, Nishihara, Okinawa 903-0213, Japan Abstract. The Iriomote cat Prionailurus iriomotensis occurs only on Iriomote Island in the Ryukyu Archipelago of southern Japan. The population is estimated at 100 individuals and is on the decline. We examined resource overlap for prey and habitat between this species and introduced cats Felis catus by scat census and analysis of scat contents. The distribution of scats was completely different between the two species. The distribution of scats from Iriomote cats was associated with environmental factors such as vegetation types and terrain conditions, while the distribution of scats from feral cats mainly depended on locations of garbage dumps. Although the feral cat heavily utilized human rubbish, it also preyed upon thirteen species of native animals, ten of which were also used by Iriomote cats. From 1997 to 2001, the number of observed scats from Iriomote cats declined significantly, while feral cat scat became more common. Feral cats on Iriomote Island still depend on humans, but the expansion of their distribution into habitats of Iriomote cats may increase the competition for prey and habitat resources in the future. Key words: Felis catus, habitat use, interspecific competition, introduced species, Prionailurus iriomotensis. Introduced species often greatly affect local fauna and status or even extinction (Kitching and Jones 1981; King flora through predation, resource competition and 1984). In Okinawa and the Amami Islands of southern hybridization with native species (Elton 1958; Diamond Japan, mongooses introduced to control a venomous and Case 1986; Ecological Society of Japan 2002). Iso- snake (Habu Trimeresurus flavoviridis) and rats have lated fauna and flora with a small number of species are become established in vacant niches. As a result, many particularly vulnerable to invasive introduced species rare or endemic species of amphibians, reptiles, terres- (Elton 1958; Diamond and Case 1986; Frankel and Soulé trial birds, and mammals have been preyed upon and 1981). In recent history, 75% of extinctions of birds and threatened (Abe et al. 1991, 1998; Ogura et al. 2002; mammals have occurred on isolated islands, and 43% of Kawauchi and Sasaki 2002; Ministry of the Environment these were caused by introduced predators (Frankel and 2002a, b, c). Soulé 1981). Species are more susceptible in islands Introduced species often compete for prey and habitat because of restricted habitats and lack of experience with resources against native species, but the impacts of com- new predators, and are often at a competitive disadvan- petition are notably stronger on closely related species tage against aggressive invasive species (Frankel and (Diamond and Case 1986; Ecological Society of Japan Soulé 1981). Well-documented examples occurred in 2002). The Iriomote cat Prionailurus iriomotensis is a Australia and New Zealand, where unique fauna in iso- small felid and inhabits only Iriomote Island of the lated habitats were affected by introduced carnivores like Ryukyu Archipelago. This species is listed as endan- dingos Canis lupus dingo, feral cats Felis catus, and red gered because of its restricted habitat and small popula- foxes Vulpes vulpes. Predation on native marsupials and tion size, (IUCN 2000) estimated at approximately 100 terrestrial birds pushed many populations to threatened individuals (JWRC 1994). The population has declined *To whom correspondence should be addressed. E-mail: [email protected] 48 Mammal Study 28 (2003) Fig. 1. Location of the census route to collect scats on Iriomote Island in the Ryukyu Archipelago of southern Japan. A census route along a paved road six kilometers long and five meters wide was divided into 24 sections (a–x) at 250 m intervals. during the last decade due to habitat loss from develop- Study area ment and mortality from traffic accidents (Izawa and Doi 1991). While P. iriomotensis is the only native member The study was conducted on Iriomote Island (284 km2, of the order Carnivora on Iriomote Island, the introduced 24°20’N, 123°49’E) located in the southern Ryukyu feral cat F. catus distribution has been expanding away Archipelago, Japan. The climate is warm-temperate to from human settlements, often following new road subtropical with average monthly temperatures ranging construction. from 17°C in January to 29°C in July, and average Among carnivores, scat provides information about annual precipitation of approximately 2,300 mm at the individuals at specific sites (Macdonald 1980). The Iriomote Meteorological Station. Most of the island is Iriomote cat leaves scat on exposed surfaces such as dominated by highly folded mountains (the highest point on bare lands, rocks, and roads (Sakaguchi et al. 1986). = 469 m, Mt. Komi) and covered with subtropical ever- Sakaguchi et al. (1986) frequently observed scats from green broad-leaved forests dominated by Castanopsis Iriomote cats along streams, along the edges of forests sieboldii and Quercus miyagii. Flatlands occur only and grasslands. Scats were not present in the interior along the coast, where human activities, cultivated fields of cultivated fields, grasslands, or barren ground. Similar and villages are concentrated. About 2,000 people live patterns of habitat use were observed in radio-tracking on the island, mostly inhabiting twelve villages along the surveys (Watanabe 2001; Watanabe et al. 2001). Saka- coast. guchi et al. (1986) also suggested that the distribution The study site is in the eastern part of the island near of scat from Iriomote cats reflects both habitat use and two small villages (Fig. 1) with populations of 70 people specific preferences for latrine sites. In the current in Komi and 24 people in Mihara in January 2002. A study, we used scat locations to evaluate habitat use by paved road, 5 m wide and 50 km long was constructed in both Iriomote cats and feral cats. We also assessed prey 1976. We established a 6 km census route on the road preferences through fecal analysis of both species. divided into 24 sections (a–x) at 250 m intervals (Fig. 1). The route crosses three streams, and traverses subtropi- Watanabe et al., Resource overlap of native and feral cats 49 cal evergreen forests, rice fields, cultivated fields, man- containing a prey type/total number of analyzed scats) groves, and swampy forests. The two villages are along was calculated for each prey type and for each taxon sections h–j (Komi) and adjacent to sections w–x (mammals, birds, reptiles, amphibians, fishes, insects, (Mihara). Two garbage dumps are located 250 m away crustaceans, and others). from section i in a cane field and 50 m away from section u in a forest. Analysis of scat distribution According to the statistical reports of Taketomi Town Habitat utilization of P. iriomotensis is reported to be Office, while the population on Iriomote Island grew affected by environmental factors including terrain and from 1,892 people in 1995 to at least 1,976 people in vegetation (Watanabe 2001; Watanabe et al. 2001). In 2000, visitor numbers increased from 201,967 people in this study, we analyzed the relationships between scat 1995 to 285,080 people in 2000. locations for the two cat species and the surrounding environment by using a geographic information system Methods (GIS). Deflection of scat distribution was tested for each Scat census species by one-sample Kolmogorov-Smirnov tests. Scat Scats were collected by one or two observers walking distribution was compared between species by two- along both sides of the route at the end of each month sample Kolmogorov-Smirnov tests. from November 1997 to October 2001. Locations, cir- Multiple regression models were developed according cumstances, and conditions of scat were recorded during to quantification theory type I (Hayashi 1952) for the dis- these censuses. tribution of scat of the two species. The total number of scats collected in each section was the dependent Scat identification variable and six environmental factors were independent It was necessary to distinguish between the scats of variables. Three variables related to terrain: (1) mean the two cat species at the point of collection. Scat of P. elevation, (2) mean slope in each section, and (3) dis- iriomotensis can be easily distinguished by the specific tance from streams to the center of each section, were odors secreted from anal glands (Imaizumi et al. 1977; derived from digital elevation models (Digital Map 50 m Sakaguchi et al. 1986). Fecal odors withstand weather- mesh, Elevation published by Geographical Survey ing and degradation for longer periods than the interval Institute, Japan). Vegetation type (4) in each section between our censuses. Therefore, scats having the was categorized as forest, rice field and swamp, and specific odor of P. iriomotensis could be positively cultivated and bare land according to digital vegetation identified based on the odor. However, scat with no data, from the Natural Environment Information GIS characteristic odors could not be designated as F. catus (Environment Agency 1999). Variables related to human scat without further evidence. Since mammals ingest activities such as (5) distance from garbage dumps, and their own hair during grooming and pass it in their scat (6) distance from residential areas to the center of (Imaizumi et al. 1977), we characterized cat hairs in scat each section were calculated using GIS software, IDRISI at the time of collection and during lab analysis. version 32.2 (Clark Labs, The Idrisi Project). Multiple Although some feral cats have similar collar patterned regression analysis was performed using a macro in fur to that of Iriomote cats, hairs in scat were clearly Microsoft Excel. identified by hair-length, solidity, and color pattern when All independent variables were categorized, and each viewed under a stereomicroscope.