Bottlenecks for plant recruitment in woodland remnants: An ornithochorous shrub in a Mediterranean ‘relictual’ landscape

Habitat fragmentation may lead to declines in plant populations and ultimately to extinction through a disruption of demographic processes, which may result in bottlenecks or even a collapse in regeneration. Nevertheless, very few studies have assessed the net effects of habitat fragmentation on plant recruitment integrating its multiple demographic processes. Using data from a four-year field study, we analyze how habitat fragmentation affects particular demographic processes and the overall magnitude of plant recruitment. We use as a case study the bird-dispersed shrub Myrtus communis in Mediterranean woodland patches within an extremely fragmented landscape (∼1% woodland cover). By means of observations and experiments, we quantified fecundity, fruit removal by frugivores, seed rain, post-dispersal seed predation by rodents and seedling emergence and survival. Within each patch, we quantified post-dispersal processes in different target microhabitats. We considered the life cycle to be a combination of consecutive life stages connected by transitional processes with specific probabilities. We calculated the overall probability of recruitment for each patch as the product of all of these probabilities. The demographic processes negatively affected by fragmentation were bird-generated seed rain and seedling emergence and survival, which were attributable, respectively, to lower fruit abundance and poorer habitat quality in the smaller patches. The negative effect of fragmentation on M. communis recruitment became stronger when all the demographic processes were integrated. Of all processes, seedling emergence and, above all, seedling survival were clearly bottlenecks for recruitment that were associated with habitat fragmentation. Results from our observations and experiments were consistent with natural patterns of regeneration given that we found higher seedling densities in larger patches and old population structures (with no saplings/juveniles) in some small patches. Our study shows that habitat fragmentation has serious negative effects on recruitment in M. communis due to demographic bottlenecks in seedling establishment. The available evidence (this and a companion study) suggests that the impoverishment of habitat quality associated with habitat fragmentation (edge effects, disturbances associated with management and microhabitat availability) can explain these results. Given that restoration at a landscape scale is likely to be extremely difficult, initial management actions should aim to improve habitat quality in the smallest woodland remnants.