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Comprehensive survey of Spider tortoise distribution in Madagascar gets underway in the north

The Madagascar spider tortoise (Pyxis arachnoides spp.), or Kapila as the species is referred to locally, has a carapace size of around 15cm, making it one of the world’s smaller tortoise species. With its intricate spider web type patterning on the carapace, it is arguably one of the world’s most beautiful and charismatic chelonia.

During 2008, the species’ IUCN Red List status was changed to Critically Endangered following the Red Listing and Action Planning Turtle Workshop organized in Madagascar by the IUCN/SSC Tortoise and Freshwater Turtle Specialist Group (TFTSG), the TSA and a number of other international conservation NGOs and local Malagasy agencies. Delegates at the meeting also agreed, as part of the Conservation Action Plan for Madagascar’s four, threatened, endemic tortoise species, that reliable data be collected on the exact population densities and extent of the remaining ranges of these rare animals. To date, most population predictions of Madagascar’s chelonian, with a few exceptions, have been made on dated information, or as in the case of P. arachnoides spp., nothing more than an educated guess! With the sustained threats of poaching for food and the illegal export trade, in addition to pressure from habitat destruction, the remaining wild populations of these four species are thought to have declined greatly during recent times. So for effective conservation to be implemented, we really need to know how many individuals we are dealing with and where these populations now still exist.

Historically, P. arachnoides spp. was through to inhabit a continuous strip of coastal dry forest; covering approximately 560 km of south western Madagascar’s coast line. P. arachnoides spp. is divided into three subspecies with P.a. brygooi inhabiting the northern extent of the range between Morombe and the Manombo River. P.a. arachnoides can be found further south as far as the Menarandra River, then the extreme southern reaches of the range is inhabited by P. a. oblonga.

The question this project hopes to answer is just how many spider tortoises does Madagascar have left in the wild and where are the last remaining strong holds across this species historical range? With financial support from the TSA, Turtle Conservation Fund, the UK’s Royal Geographic Society and the British Chelonia Group, a small team was assembled, comprised of both British and Malagasy field biologists, with the aim of traversing the coastal dry forest range of P. arachnoides spp. After three field seasons we hope to have sufficient data to answer this question. In addition to this we hope to revisit an old survey site close to the coastal fishing community of Anakoa and monitor the population that was first studied in 2002.

P. arachnoides spp. is probably one of the more difficult tortoise species to study in the wild. The cryptic coloration and behavior of the animal make it quite difficult to find, in addition to this the species is only really active for a limited period of the year during the region’s short wet season, which makes driving in the region impossible for all but the most skilled local drivers. The extreme heat of the southern Madagascan summer means the species has adopted crepuscular behavior and is only active in the early morning and late afternoon, plus the thick spiny vegetation can be a punishing environment to work in, in particular the thick Mikea forests of the north. Despite this, the team has just completed a successful first field season (January-March 2009), surveying the range of the northern spider tortoise (P. a. brygooi). We traversed the range of the subspecies on foot and by 4×4 concentrating our survey effort across areas of good forest habitat between Toliara and Morombe. Sadly, the subspecies seem to be confined to just three isolated areas now, and is far from inhabiting all of what was through to be its historical range.

The first population was discovered around the forests of Ifaty and Mangily, with sparse numbers stretching north to the Manombo River area. This population supported some individuals which displayed interesting morphological characteristics, not consistent with either P. a. brygooi or P. a. arachnoides, whereby some animals had the less domed shell of P. a. arachnoides but the ridged plastron hinge of P. a. brygooi. This small sub-group was also noticed by Rick Hudson and Michael Ogle of TSA during their visit to the region in January 2007. In addition to this population, a concentrated population was detected north of the Baie de Fanamotra, in an interesting marginal habitat that was a mixture of typical coastal dry scrub interspersed with mangroves. Finally a small population was recorded around the forests east of Morombe at the northern extremity of P. a. brygooi’s range.

The further north we traveled, the more communities we passed where people collect the tortoise for food. Until several hundred years ago the larger radiated tortoise (Astrocheys radiata), a species sympatric in its range with some of the more southern populations of P. arachnoides spp., was through to have been present in these more northern forests. However, the species’ demise in these areas was probably brought about by the Mikea tribe who has hunted the species to extinction within this area. Nowadays it appears that the local communities within this area have directed their collection efforts to the smaller spider tortoises.

Our return to the 2002 P. a. arachnoides study site in Anakao revealed a worrying decline in numbers, with a recorded decline of about 25% in the local population. However, the numbers and population size and structure was still in a healthier state than many of the populations of P. a. brygooi that we were encountering in the north. The tortoises in this area appeared not to be under pressure by local communities exploiting them for food, but instead the habitat had become more fragmented since my field trip to the area six years previously, as a result of charcoal production and livestock grazing.

The results of this work have already resulted in some positive outcomes for the conservation of the species. This work has come at a time when Madagascar has been attempting to increase its protected area coverage to three times its current total area, as the government has realized that Madagascar must protect its unique natural resources if it is to develop its fledgling tourism industry. Currently P. a. brygooi’s historical range failed to fall inside any formally recognized protected areas. However, there are two new protected areas proposed for the region; Ranobe PK32 and Northern Mikea Protected Areas. However, due to the now fragmented nature of the subspecies’ range none of the remaining populations fall within the boundaries of these newly proposed protected areas. But, the results of this work have stimulated discussion with World Wildlife Fund (WWF); the agency charged with gazetting and implementing the protected area expansion program. Currently, WWF are considering altering the boundaries slightly of at least one of the areas or introducing some kind of adaptive management to accommodate the nearby population of northern spider tortoises that we discovered.

These data will also be incorporated into the TSA’s GIS database. This database is creating a ‚Äòreal time’ map of the current distribution of the remaining spider tortoise populations. Our recent field work is to date, the only comprehensive survey of the distribution and population size for the northern spider tortoise that has ever been undertaken. But we still have a long way to go in understanding the current range and distribution of the whole species. This project aims to undertake a further two field seasons repeating this detailed survey for both P. a. arachnoides and P. a. oblonga. The project has made a positive start but we still have a lot of work to do. The TSA will continue fundraising efforts to support this range wide survey. Spider tortoises are considered a keystone of the TSA’s Madagascar program, both in and ex situ.

-Ryan CJ Walker
Nautilus Ecology, 1 Pond Lane, Greetham, Rutland, LE15 7NW, United Kingdom
Department of Life Sciences, Open University, Milton Keynes, MK7 6AA, United Kingdom

The TSA helped sponsor Ryan’s fieldwork in 2009 and is committed to helping complete the three year survey. GPS data collected during this project is being compiled into a GIS mapping program by TSA and Fort Worth Zoo affiliate Brian Jones.