The emergence of a novel infectious disease, chytridiomycosis, is now widely recognized as a major cause of amphibian declines and biodiversity loss across local and global scales. Amphibian mortalities caused by the pathogenic chytrid fungus, Batrachochytrium dendrobatidis (Bd) were first recorded in Iberia, Europe over a decade ago. In August 2009, hundreds of post‐metamorphic common midwife toads (Alytes obstetricans) were found dead in the water and margins of a pond in the Serra da Estrela Natural Park, north‐central Portugal. Histological and genetic analyses confirmed their infection with Bd. Given the likelihood of a new outbreak of chytridiomycosis, we evaluated the possible impacts of this disease on populations of A. obstetricans within the Park by conducting field surveys during 2010 and 2011. We compared the present distribution and abundance of A. obstetricans with historical records, and quantified the present prevalence and intensity of infection by Bd. Results showed that (1) A. obstetricans disappeared from 67% of the 1 × 1 km squares where it was recorded in the past, (2) breeding is currently limited to 16% of the confirmed breeding sites in the past, and that (3) larvae are now less abundant, as well as are highly infected by Bd in the remaining sites. These effects were most pronounced at altitudes above 1200 m. Our findings suggest that an outbreak of chytridiomycosis is responsible for the rapid decline of A. obstetricans in Serra da Estrela, and we believe that urgent conservation measures are needed to prevent local extinction of the species.
Propagule pressure is a leading determinant of population establishment. Yet, an experimental understanding of how propagule size and number (two principal parts of propagule pressure) determine establishment success remains incomplete. Theoretical studies suggest that the timing between introduction events, a component of propagule number, can influence establishment success. However, this dynamic has rarely been explored experimentally. Using
Escherichia coli
engineered with an Allee effect, we investigated how the timing of two introduction events influences establishment. For populations introduced below the Allee threshold, establishment occurred if the time between two introduction events was sufficiently short, with the length of time between events further reduced by reducing growth rate. Interestingly, we observed that as the density of bacteria introduced in one introduction event increased, the time between introduction events that allowed for establishment increased. Using a mathematical model, we provide support that the mechanism behind these trends is the ability of the first population to modify the environment, which can pave the way for establishment of the second population. Our results provide experimental evidence that the temporal distribution of introduction events regulates establishment, furthering our understanding of propagule pressure and may have implications in invasion biology and infectious disease.
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