Harnessing Historic Records and Long-Term Monitoring Data to Evaluate Amphibian Extinction Dynamics

Understanding the nature and extent of global amphibian declines has been hampered because pre-decline data rarely exist, post-decline data can be incomplete and amphibian population fluctuations can be influenced by multiple factors. Importantly, data deficiencies have impeded assessments of the roles of the pathogen, chytrid fungus Batrachochytrium dendrobatidis, and co-occurring host or competitor species in amphibian population declines. We used dynamic occupancy models to examine the influence of chytrid upon two sympatric frog species and their interactions. Models combined sporadic historic data and more intensive post-decline data collected over a 55 -year period (1958-2012) for the threatened Spotted Tree Frog Litoria spenceri and nonthreatened Lesueur's Frog L. lesueurii. Our analysis indicated an approximate 50 % decline in L. spenceri site occupancy. This decline was most severe at sites when both chytrid and L. lesueurii were present. Further L. spenceri declines are predicted without intervention. In contrast, L. lesueurii tended to become more prevalent over time, and changes in L. lesueurii occurrence over time were not associated with chytrid or L. spenceri occurrence. Efforts to conserve L. spenceri when threatened by chytrid may be most effective at sites where L. lesueurii (a potential pathogen reservoir) is absent. Our work highlights the risk posed by sympatric pathogen reservoir species for susceptible species. This study not only demonstrates the value of long -term population monitoring, but how incorporation of sporadic, historic records may provide valuable insights into processes underlaying species decline and recovery.