Conflicting selection on Cneorum tricoccon (Rutaceae) seed size caused by native and alien seed dispersers

The disappearance of native seed dispersers due to anthropogenic activities is often accompanied by the introduction of alien species, which may to some extent replace the ecological service provided by the extinct ones. Yet, little empirical evidence exists demonstrating the evolutionary consequences of such alien ‘replacement’. Here, we document the conflicting selection exerted on seed size by two native lizards (Podarcis lilfordi and P. pityusensis) and an alien mammal species (Martes martes), all acting as legitimate seed dispersers of the Mediterranean relict Cneorum tricoccon. While lizards mostly exerted a negative directional selection on seed diameter, especially P. pityusensis, the much larger pine marten exerted positive selection on seed size. Our findings suggest that this among-disperser variation in the selection regimes, together with the occurrence of spatial variation in the presence of each of seed disperser, help to create the geographical variation observed for seed size of C. tricoccon. To our knowledge, this is the first empirical evidence showing opposing selective pressures between native and alien species in the seed dispersal process in an invaded ecosystem.

受人为活动影响，本地种子传播者（native seed dispersers）逐渐消失，与此同时外来物种不断被引入，后者可在一定程度上替代已灭绝本地物种所提供的生态系统服务（ecosystem service）。然而，目前鲜有实证研究能够证明这类外来物种“替代”行为所带来的进化后果。本研究针对地中海孑遗植物Cneorum tricoccon的三类有效种子传播者展开探究：两种本地蜥蜴（Podarcis lilfordi与P. pityusensis）以及一种外来哺乳动物松貂（Martes martes），三者对种子大小施加了相互冲突的选择作用。其中蜥蜴类主要对种子直径施加负向定向选择（negative directional selection），尤以P. pityusensis为甚；而体型更大的松貂则对种子大小施加正向选择（positive selection）。研究结果表明，不同传播者间的选择机制差异，加之各类种子传播者的分布存在空间异质性，共同促成了C. tricoccon种子大小所呈现出的地理变异格局。据我们所知，本研究首次提供实证证据，证明在被入侵的生态系统中，本地与外来物种在种子传播过程中施加了相互对立的选择压力。