Native seed dispersal by rodentsis negatively influenced by an invasive shrub

Abstract

[EN] Refuge–mediated apparent competition is the mechanism by which invasive plants increase pressure on native plants by providing refuge for generalist consumers. In the UK, the invasive Rhododendron ponticum does not provide food for generalist seed consumers like rodents, but evergreen canopy provides refuge from rodent predators, and predation and pilferage risk are key factors affecting rodent foraging and caching behaviour. Here we used a seed removal/ seed fate experiment to understand how invasion by an evergreen shrub can alter seed dispersal, seed fate and early recruitment of native trees. We used seeds of four species, small and wind–dispersed (sycamore maple Acer pseudoplatanus and European ash Fraxinus excelsior) and large and animal–dispersed (pedunculate oak Quercus robur and common hazel Corylus avellana), and monitored seed predation and caching in open woodland, edge habitats, and under Rhododendron. In the open woodland, wind–dispersed seeds had a higher probability of being eaten in situ than cached seeds, while the opposite occurred with animal–dispersed seeds. The latter were removed from the open woodland and edge habitats and cached under Rhododendron. This pattern was expected if predation risk was the main factor influencing the decision to eat or to cach a seed. Enhanced dispersal towards Rhododendron cover did not increase the prospects for seed survival, as density of hazel and oak saplings under its cover was close to zero as compared to open woodland, possibly due to increased cache pilferage or low seedling survival under dense shade, or both. Enhanced seed predation of ash and sycamore seeds close to Rhododendron cover also decreased recruitment of these trees. Rhododendron patches biased rodent foraging behaviour towards the negative (net predation) side of the conditional rodent / tree interaction. This effect will potentially impact native woodland regeneration and further facilitate Rhododendron spread due to refuge–mediated apparent competition.

[ES] Un arbusto invasor influye negativamente en la dispersión de semillas autóctonas por roedores. La compe-tencia aparente mediada por refugio es el mecanismo por el que las plantas invasivas aumentan la presión sobre las autóctonas proporcionando un refugio para los consumidores generalistas. En el Reino Unido, la especie invasora Rhododendron ponticum no proporciona alimento a los consumidores generalistas de semi-llas, pero el dosel perenne ofrece refugio frente a roedores depredadores, y la depredación y el robo son los principales riesgos que afectan al comportamiento de alimentación y almacenamiento de los roedores. En el presente estudio, llevamos a cabo un experimento sobre la retirada y el destino de las semillas con objeto de entender cómo puede afectar la invasión de un arbusto perenne a la dispersión y el destino de las semillas y al reclutamiento temprano de árboles autóctonos. Utilizamos semillas de cuatro especies: semillas pequeñas y anemócoras (arce blanco Acer pseudoplatanus y fresno común Fraxinus excelsior) y semillas grandes y zoócoras (roble común Fraxinus excelsior y avellano común Corylus avellana) e hicimos el seguimiento de la predación y el almacenamiento de semillas en bosques abiertos, en hábitats de transición y debajo de Rhododendron. En los bosques abiertos, las semillas anemócoras tuvieron una mayor probabilidad de ser consumidas in situ que almacenadas, mientras que en las semillas zoócoras ocurrió lo contrario.