Budroni, Marcello Antonio and Farris, Emmanuele and Zirulia, Andrea and Pisanu, Stefania and Filigheddu, Rossella Speranza and Rustici, Mauro (2014) Evidence for age-structured depensation effect in fragmented plant populations: the case of the Mediterranean endemic Anchusa sardoa (Boraginaceae). Ecological complexity, Vol. 20 , p. 142-150. ISSN 1476-945X. eISSN 1476-9840. Article.
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Depensation in a population growth rate, well-known as Allee effect, has dramatic implications for the dynamics and conservation of small or sparse populations, as it can drive low-density populations to extinction when their demographic size is below a critical threshold. Although rarely detected, depensation effects are believed to be common in nature. Here we present experimental evidence for Allee effect in one Mediterranean endemic plant: Anchusa sardoa. Depensation in the population growth rate is demonstrated through a density-based approach by showing the fingerprinting relationship which ties the population density to its per capita growth rate (pgr) during specific stages of the plant lifecycle. The pgr–density plots derived from observational data qualitatively compare with a general 2nd order polynomial function which features one of the peculiar trends underlying an Allee mechanism. We found strong evidence for depensation in the seedling and sapling classes, whereas no-depensation effect could be clearly observed in the adult classes. We also point out a characteristic demographic structure of A. sardoa (i.e. number of juveniles < number of adults) which reflects a not common life strategy with respect to Mediterranean endemic plants. By combining dynamical and demographic information, the results of this study suggest a possible scenario by which A. sardoa population could go extinct, and are discussed in the context of the increasing mass tourism in Mediterranean coastal environments.
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