Adult proximity and frugivore's activity structure the spatial pattern in an endangered plant

Abstract

Seed dispersers play a key role in shaping the spatial patterns of plant populations. After their disappearance, we expect a substantial increase in plant aggregation, which can ultimately cascade into high plant competition. We used data of fully mapped distribution of four populations of the shrub Daphne rodriguezii from Menorca Island (Balearic Islands, W Mediterranean Sea), one in which D. rodriguezii coexists with its only disperser, the lizard Podarcis lilfordi, and three populations where this lizard went extinct long ago. We used spatial point-pattern analysis to better understand the lizard's role on the spatial distribution of D. rodriguezii. To this end, we used specific point-process models that represented our main hypotheses on the impact of (i) habitat suitability, (ii) disperser activity, (iii) proximity of adults on the distribution of juvenile and adult shrubs and (iv) the impact of adult proximity on juvenile survival. Plants were aggregated in all populations, and aggregation at short distances was stronger in populations without lizards. The observed spatial pattern of juveniles was better explained by a combination of hypotheses (ii) and (iii). The density of juveniles below adults was similar in all populations, but density far from adults was higher in the population with dispersers. In populations without dispersers, juveniles suffered higher mortality below adults. Overall, our results suggest that the effects of lizard extinction were subtle. In the absence of lizards' dispersal, plants self-replace in favourable locations, but a decline in density, is evidenced near adults attributed to higher competition. Lizards, which often defecate under fruiting plants, create also a tight juvenile-adult association, but allow juveniles to escape from the immediate proximity of adults attributed to longer-distance movements. Our study shows how the analysis of the spatial pattern of plants can leads to detailed hypotheses on the underlying mechanisms structuring plant populations. © 2012 The Authors. Functional Ecology © 2012 British Ecological Society.