Journal of Animal Ecology 2007 76, 711–721 © 2007 The Authors. Journal compilation © 2007 British Ecological Society Blackwell Publishing Ltd Quantifying the disease transmission function: effects of density on Batrachochytrium dendrobatidis transmission in the mountain yellow-legged frog Rana muscosa LARA J. RACHOWICZ*† and CHERYL J. BRIGGS* *Department of Integrative Biology, University of California, Berkeley, CA 94720-3140, USA; and †Resources Management and Sciences, Yosemite National Park, 5083 Foresta Road, PO Box 700, El Portal, CA 95318, USA Summary 1. Chytridiomycosis is an emerging infectious disease of amphibians, caused by the fungal pathogen Batrachochytrium dendrobatidis, which has been implicated recently in population declines and possible extinctions throughout the world. 2. The transmission rate of this pathogen was quantified in the mountain yellow-legged frog Rana muscosa through laboratory and field experiments, and a maximum likelihood approach was used to determine the form of the transmission function that was best supported by the experimental data. 3. The proportion of R. muscosa tadpole hosts that became infected increased with the number of previously infected R. muscosa tadpoles to which they were exposed, as would be expected in an infectious disease. 4. The laboratory experiment revealed some support for a transmission function in which the transmission rate levels off as the density of infected individuals increases. However, there was not enough power to distinguish between a frequency-dependent form and several other asymptotic forms of the transmission function. 5. The impacts of crowding and temperature on transmission were also investigated; however, neither of these factors significantly affected the transmission rate. Key-words: amphibian decline, disease transmission, extinction, host–pathogen interaction, Sierra Nevada. Journal of Animal Ecology (2007) 76, 711–721 doi: 10.1111/j.1365-2656.2007.01256.x Introduction Chytridiomycosis, an emerging infectious disease of amphibians caused by the chytrid fungal pathogen Batrachochytrium dendrobatidis (Berger et al . 1998; Longcore, Pessier & Nichols 1999), has been implicated in the dramatic population declines, and even complete extinctions, of numerous amphibian species throughout the world (Daszak, Cunningham & Hyatt 2003). In mountain yellow-legged frogs Rana muscosa Camp, B. dendrobatidis has been associated with the extinction of hundreds of local populations in the Sierra Nevada mountains of California in recent years (Rachowicz et al . 2006). Although the potential for disease to impact, or even regulate, host population dynamics has long been recognized (Anderson & May 1992; Hudson et al . 2002), simple disease models predict that disease on its own cannot cause the complete extinction of a popu- lation or species (Anderson & May 1992; de Castro & Bolker 2005). The reason for this is that simple models of directly transmitted pathogens generally assume that disease transmission takes on a density-dependent (mass action) form. Density-dependent disease trans- mission leads to host density thresholds below which the disease cannot invade or persist (McCallum, Barlow & Hone 2001; Lloyd-Smith et al . 2005), such that the disease is predicted to die out before the host population drops to extinction. However, if pathogen transmission takes on a frequency-dependent, rather than a density-dependent form, disease-induced extinction of the host is possible because frequency-dependent transmission results in no such host density threshold (Getz & Pickering 1983; McCallum et al . 2001). There- fore, quantification of the disease transmission process is extremely important for understanding the potential Correspondence: L. J. Rachowicz, Department of Integrative Biology, University of California, Berkeley, CA 94720-3140, USA. E-mail: lara.rachowicz@nps.gov