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1 June 2013 Monitoring the ungulate prey of the Komodo dragon Varanus komodoensis: distance sampling or faecal counts?
Achmad Ariefiandy, Deni Purwandana, Graeme Coulson, David M. Forsyth, Tim S. Jessop
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Abstract

Monitoring the abundances of prey is important for informing the management of threatened and endangered predators. We evaluated the usefulness of faecal counts and distance sampling for monitoring the abundances of rusa deer Rusa timorensis, feral pig Sus scrofa and water buffalo Bubalus bubalis, the three key prey of the Komodo dragon Varanus komodoensis, at 11 sites on five islands in and around Komodo National Park, eastern Indonesia. We used species-specific global detection functions and cluster sizes (i.e. multiple covariates distance sampling) to estimate densities of rusa deer and feral pig, but there were too few observations to estimate densities of water buffalo. Rusa deer densities varied from from 2.5 to 165.5 deer/km2 with coefficients of variation (CVs) of 15-105%. Feral pig densities varied from 0.0 to 25.2 pigs/km2 with CVs of 25-106%. There was a positive relationship between estimated faecal densities and estimated population densities for both rusa deer and feral pig: the form of the relationship was non-linear for rusa deer, but there was similar support for linear and non-linear relationships for feral pig. We found that faecal counts were more useful when ungulate densities were too low to estimate densities with distance sampling. Faecal count methods were also easier for field staff to conduct than distance sampling. Because spatial and temporal variation in ungulate density is likely to influence the population dynamics of the Komodo dragon, we recommend that annual monitoring of ungulates in and around Komodo National Park be undertaken using distance sampling and faecal counts. The relationships reported here will also be useful for managers establishing monitoring programmes for feral pig, rusa deer and water buffalo elsewhere in their native and exotic ranges.

Spatial and temporal variation in the abundances of primary prey can have major demographic effects on predators (Dale et al. 1994, O'Donoghue et al. 1997, Ramakrishnan et al. 1999, Karanth et al. 2004). Large declines in prey abundances may reduce the viability of endangered predator populations (Karanth & Stith 1999), and predators on islands may be especially vulnerable to declines in prey abundances (Frankham 1998). A key feature of many predator-prey systems is the large prey base required to sustain the predator (Karanth et al. 2002a, 2004). Hence, apex predators typically occur at low densities, making robust inference about changes in the abundance of the predator difficult because of the impracticality of obtaining even modest sample sizes. In such systems, monitoring prey abundances may be particularly useful for providing an early warning of changes in predator populations (Karanth et al. 2002b, Lovari et al. 2009).

The Komodo dragon Varanus komodoensis is the world's largest lizard. Adult male dragons can reach 3 m in length and weigh up to 87 kg (Jessop et al. 2006). Currently, the Komodo dragon is listed in Appendix I of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) and is classified by the International Union for the Conservation of Nature (IUCN) as ‘vulnerable’ due to its demographic decline and limited distribution (World Conservation Monitoring Centre 1996, CITES 2011). The Komodo dragon is endemic to five small islands in eastern Indonesia, with four island populations in Komodo National Park (KNP) and several fragmented populations on Flores (Ciofi & De Boer 2004). The Komodo dragon is an apex predator, with three ungulate species dominating the diet of adults: rusa deer Rusa timorensis, feral pig Sus scrofa and water buffalo Bubalus bubalis (Auffenberg 1981, Jessop et al. 2006; Fig. 1). Previous work has shown the distributions and abundances of these three species to be important determinants of the demography of Komodo dragon populations (Jessop et al. 2006; but see Laver et al. 2012).

Anthropogenic threats such as habitat loss (from illegal logging and agriculture) and illegal hunting could reduce the densities of ungulates (Groom 2006, Steinmetz et al. 2010), and hence Komodo dragon populations, inside and outside KNP (Ciofi et al. 2007). For example, a reduction in the density of deer caused by illegal hunting was considered the major cause of the extinction of the Komodo dragon on the island of Padar within KNP during the early 1980s (Ciofi & De Boer 2004). Regular patrols by rangers since 2000 are thought to have reduced illegal hunting of deer and feral pigs within KNP. However, outside KNP, especially on the island of Flores, illegal hunting of deer still occurs and may be affecting the local distribution and abundance of the Komodo dragon (Ciofi & De Boer 2004). Despite these concerns, no attempt has been made to implement monitoring of the distribution and abundance of the ungulate prey species of the Komodo dragon.

A wide variety of methods have been used to estimate the abundance of ungulates (reviews in Thompson et al. 1998, Mayle et al. 1999). Faecal counts are a commonly used indirect sampling method for estimating the relative abundance of ungulates, particularly in wooded habitats where animals are difficult to observe directly (Mayle et al. 1999). The faecal count method is a relatively inexpensive method that is easy for park rangers to use and can be implemented in a wide range of habitats (Forsyth et al. 2003, Månsson et al. 2011). However, as for any index of animal abundance, it is desirable that the relationship between faecal density and animal density be validated for the species and area of interest: the relationship should be positive and linear (e.g. Forsyth et al. 20