Persistence by longevity has been rarely considered as an alternative to regeneration by seeding for plants showing multiple demographic strategies. We propose a conceptual model of multiple demographic strategies for long-lived plants in stable habitats, shifting from regeneration by seeding to persistence by longevity and/or vegetative reproduction, along gradients of abiotic stress or interspecific competition. Regeneration by seeding would be promoted under low abiotic stress or under low competition, whereas persistence by longevity and/or vegetative reproduction would predominate at high levels of abiotic stress or competition. We test this model with two threatened species of the Mediterranean region, the shrub Juniperus communis, a widely distributed species which maintains relict populations in the Mediterranean mountains thanks to great adult longevity and Pinguicula vallisneriifolia, a palaeo-endemic herb relying on a perennial habit and vegetative reproduction under drought imposed stress or high competition at late successional phases. As a main consequence, multiple demographic strategies enhance a plant's ability to exploit environmental heterogeneity at different spatial (patches, localities, regions within the species' distribution area) and temporal (individual life span, glacial-interglacial cycles) scales. The potential of multiple demographic dynamics based on persistence and regeneration must be considered as a major ecological trait determining the long-term viability of peripheral populations of relict species as well as the inertia against extinction of many threatened endemisms, thereby contributing to the maintenance of the high plant diversity characterizing the Mediterranean region.
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Vol. 14 • No. 6