Threat and response: A decade of decline in a regionally endangered rainforest palm affected by fire and introduced animals David T. Liddle a,b, *, Barry W. Brook b , Janet Matthews b , Stephen M. Taylor c , Peter Caley d a Biodiversity Conservation, Department of Natural Resources, Environment and the Arts, Northern Territory Government, P.O. Box 496, Palmerston, Northern Territory 0831, Australia b School for Environmental Research, Charles Darwin University, Darwin, Northern Territory 0909, Australia c Department for Environment and Heritage, South Australian Government, 115 Maryvale Road, Athelstone, SA 5076, Australia d National Centre for Epidemiology and Population Health, Australian National University, Canberra, ACT 0200, Australia ARTICLE INFO Article history: Received 6 December 2005 Received in revised form 21 April 2006 Accepted 24 April 2006 Available online 14 June 2006 Keywords: Arecaceae Ptychosperma Exotic animal impact Fire impact Monsoon rainforest Groundwater ABSTRACT The wild population of the palm Ptychosperma macarthurii near Darwin, in monsoonal northern Australia, is regionally endangered and provides a focus to illustrate a range of issues pertinent to conservation of rainforest habitat. Surveys in 1990 found that several populations exhibited a polarised size class structure typified by large adults and small juvenile plants. Over the following decade, in the absence of wildfire and in a period of reduced disturbance from introduced buffalo (Bubalus bubalis), cattle (Bos indicus) and pig (Sus scrofa), sufficient small juvenile plants survived and grew so as to infill the intermedi- ate size classes. Three stage (bifid, juvenile and adult) transition matrix models character- ised the population as declining under all observed conditions (intrinsic rate of increase: unburnt + few animals 0.9850; unburnt + many animals 0.9584; burnt <1 year 0.8737; burnt 1–2 years 0.9146; burnt >2 years 0.9937). In the absence of fire, simulations conducted to explore management options revealed a positive rate of increase with exclusion of intro- duced animals. With only partial introduced animal control or supplementation with juve- nile plants, the median rate of increase remained negative. The regional population is at risk by more frequent and more intense fire due to the invasion of exotic grass species and land use changes in the catchment which result in an increased drying of the rainfor- est habitat. Ongoing decline is the most likely outcome in the absence of effective manage- ment intervention. Ó 2006 Elsevier Ltd. All rights reserved. 1. Introduction Rainforests in the monsoonal tropics of northern Australia typically occur as a mosaic of small patches embedded in an expanse of savanna woodland (McKenzie, 1991; Russell- Smith, 1991). In the Northern Territory of Australia, around 15,000 rainforest patches have been identified (Russell-Smith, J., and Lucas, D., unpublished data), compris- ing less than 0.5% of the land area (Price et al., 1999). Median patch size is 3.6 ha (Price et al., 1995) with many of the plant species commonly occurring as small populations, compris- ing less than 50 adult plants in individual rainforests (Rus- sell-Smith and Lee, 1992). The occurrence of small isolated populations has important implications for conservation, 0006-3207/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2006.04.028 * Corresponding author: Tel.: +61 8 89448492; fax: +61 8 89448455. E-mail address: dave.liddle@nt.gov.au (D.T. Liddle). BIOLOGICAL CONSERVATION 132 (2006) 362 – 375 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/biocon