Predicting the potential distribution of an endangered cryptic subterranean mammal from few occurrence records

Abstract

Knowledge of geographical distributions and habitat preferences are central to the conservation and management of threatened species. Ecological niche models can be used to map potentially suitable habitat, making them valuable tools in conservation biology. These models can assist in searching for new populations of species with poorly known distributions and can also be used to guide area selection for systematic biodiversity planning. These models have traditionally required ten or more occurrence records for calibration and this has limited the use of these tools in conservation biology, since many threatened species are known from fewer records. However, recently models have been successfully calibrated using few records. To illustrate this, we developed an ecological niche model to map the potential distribution of the endangered Juliana's golden mole (Neamblysomus julianae) in South Africa so that it could be used in searching for unrecorded populations and for area selection in systematic biodiversity plans. A model was calibrated in Maxent using only four occurrence records. Predictor variables included various bioclimatic, soil and vegetation types. The model identified limited suitable habitat within the map region. The first model facilitated the identification of two previously unrecorded populations. A second model was calibrated using the two additional occurrence records. This increased the proportion of correctly predicted presence records. Jackknife analyses indicated that the models were successful at predicting known presences as suitable. This paper demonstrates the use of ecological niche modelling to conservation of a cryptic endangered species with a poorly known distribution and discusses issues that are relevant for the application of this approach to other species.

Introduction

Knowledge of geographical distributions and habitat preferences are central to the conservation and population management of threatened species (Guisan et al., 2006, Thomaes et al., 2008). Ecological niche models have been widely applied in conservation biology to predict the potential distributions of species using correlative models that exploit a species–environment relationship (Anderson et al., 2009, Chefaoui et al., 2005, De Siqueira et al., 2009, Peterson, 2001, Raxworthy et al., 2007, Rushton et al., 2004, Titeux et al., 2007). Ecological niche models require occurrence records and a set of environmental predictor variables in order to predict a species’ potential distribution (Gaubert et al., 2006, Peterson, 2001). Many species of conservation concern are cryptic or range-restricted and often very few occurrence records are available for these species (Pearson et al., 2007). Recently success has been achieved in calibrating ecological niche models with fewer than ten occurrence records (De Siqueira et al., 2009, Pearson et al., 2007). In view of this, ecological niche models have the potential to make a contribution to conservation in two important ways, especially for species with few occurrence records. First, they can be used to direct field surveys that are designed to search for new populations of rare and poorly known species by identifying potentially suitable areas (Bourg et al., 2005, De Siqueira et al., 2009, Guisan et al., 2006). Second, they can be used in conservation area selection for systematic biodiversity plans (Cowling et al., 2003, Loiselle et al., 2003, Wilson et al., 2005). This planning process identifies parcels of land that have high biodiversity value using the principles of systematic biodiversity planning (Driver et al., 2003, Margules and Pressey, 2000). These biodiversity plans require a set of targets to be defined, which are usually based on several elements of biodiversity (Driver et al., 2003) but can include species distributions and particularly threatened or range-restricted species (Loiselle et al., 2003, Wilson et al., 2005). In this way species of conservation concern will gain a level of protection from threatening processes by being part of a network of conservation areas, whilst the biodiversity of the region will simultaneously be conserved.

The endangered Juliana's golden mole (Neamblysomus julianae; Chrysochloridae) is a small (35 g), habitat-specific mammal that is particularly difficult to study due to its subterranean lifestyle (Jackson et al., 2008, Jackson et al., 2009). Despite occurring less than 15 km from the centre of South Africa's capital city, Pretoria, this species was only described in 1972 (Bronner & Bennett, 2005). Although recent studies have investigated certain aspects of its biology (Jackson et al., 2007, Jackson et al., 2008, Jackson et al., 2009), it is still relatively poorly known. The distribution range of the species is highly restricted. It is known to occur on the Bronberg Ridge (BR) in eastern Pretoria, 120 km north in the Nylsvley Nature Reserve (NNR), and 350 km east in the south western Kruger National Park (KNP) (Bronner & Bennett, 2005). Based on dental characteristics (Bronner, 1995) and recent genetic evidence (Maree, pers. comm.), animals from the KNP population differ significantly from the other two populations and evidence suggests that the KNP population could be treated as a separate species. Juliana's golden mole thus appears to be restricted to the BR and NNR areas.

The BR is less than 15 km in length and the suitable sandy soils are on average less than 1 km in width (Bosch, 2004). The NNR population is described from the reserve where approximately 800 ha constitutes suitable habitat (Jackson, 2007). Based on the current understanding of its distribution, the geographic areas occupied by these two populations are very small and not adequate to ensure the long-term persistence of the species. The small size and fossorial lifestyle means that this species has poor dispersal capabilities. Unlike mole-rats that are powerful diggers, excavating soil with their large teeth, golden moles use only their forelimbs and have far weaker tunnelling capabilities that restricts them to softer soil types. They are thus extremely susceptible to habitat fragmentation, as even relatively minor obstacles such as roads can constitute impenetrable barriers. These factors in combination with small geographical ranges have resulted in ten of the 21 golden mole species appearing on the IUCN red list, with an additional two species listed as data deficient. Of these species, five are listed amongst the ten most endangered mammal species in South Africa (IUCN, 2004).

Recently a thorough ecological assessment that specifically documented the species’ habitat requirements was undertaken (Jackson et al., 2008). Using this information we aimed firstly to make use of ecological niche modelling to produce a potential distribution map for Juliana's golden mole that could be used to search for new populations. Secondly, we aimed to produce a model that could be used for area selection in systematic biodiversity planning.