Uncovering biodiversity deficits: Experimental evidence for dispersal limitation of woody species in Mediterranean forest patches

Abstract Deforestation and habitat disturbance are major drivers of local species extinctions within remnant forest patches. In plants, seed dispersal plays a key role in recolonising patches following disturbance. However, habitat fragmentation may hinder seed arrival to suitable but unoccupied patches, resulting in dispersal limitation and, thus, in prolonged biodiversity deficits. We assessed dispersal limitation in Mediterranean forest patches by experimentally introducing plant species from the regional pool. To do so, we sowed 61,000 seeds from 10 woody species and compared their establishment between occupied and unoccupied patches, assessing whether absences were driven by habitat unsuitability or dispersal limitation. We included five dry‐ and five fleshy‐fruited species to test whether dispersal mechanisms influenced dispersal limitation. Additionally, we evaluated major biotic (vertebrate granivores and herbivores) and abiotic (soil properties and moisture) factors that could act as ecological filters in unoccupied areas. Finally, we planted saplings of four species to assess survival at later life stages and test the effectiveness of planting as a restoration strategy. On average, both dry‐ and fleshy‐fruited species exhibited similar establishment rates in occupied and unoccupied patches, indicating that species absences were primarily due to dispersal limitation rather than to habitat unsuitability. Vertebrate antagonists had similar effects across patch occupancy levels, overall reducing establishment. Soil properties and moisture did not differ in ways that would limit establishment in unoccupied patches. Finally, planted saplings had similar or higher survival in unoccupied patches, indicating habitat suitability also in later life stages. Our experiments uncover diversity deficits of woody plants in forest patches driven by dispersal limitation, irrespective of the species' dispersal mechanisms. In general, neither biotic nor abiotic filters significantly constrained the establishment of introduced species in unoccupied patches more than in occupied ones. Synthesis and applications . Mediterranean forest patches in highly altered landscapes may harbour greater woody plant diversity than is currently observed due to dispersal limitation. Assisted colonisation through plant introductions can effectively restore such plant diversity deficits.