Seed germination percentage and rate of Rubia fruticosa, an endemic Macaronesian shrub was assessed with seed lots collected at three levels of an altitudinal gradient (200–800 m asl) located in El Hierro Island (Canarian Archipelago). Both seeds collected from control plants and those found in lizard droppings were studied. A significant decrease in the germination percentage was observed with control seeds and those seeds which are previously eaten by lizards if collected at 800 m asl. This indicates that R. fruticosa is a plant whose optimal germination environment is located between 200 and 500 m asl, occurring in higher altitudes only under sub-optimal conditions. The effect of lizard gut on R. fruticosa seeds was variable in the two study years: while germination percentage of seeds having passed through the lizards was not significantly different to that of control seeds in the first year, an increased germination was found for the animal-treated seeds in all three altitudinal zones in the second year. In general, germination rate of control seeds was rather variable between years. However, improvement of R. fruticosa seed germination caused by lizards may be important for its fitness; and a co-evolution may have occurred, since lizards and seeds of this endemic plant have been intensively interacting for millions of years in the lower zones of the Canary Islands.

Benigno Padrón, David P. Padilla and Patricia Marrero received grants financed by the Ministerio de Educación y Ciencia of Spain, Gobierno de Canarias and Consejo Superior de Investigaciones Científicas (Proyectos Intramurales Especiales, 2004 3 0E 169), respectively. This contribution was financed by the projects CGL2004-04884-C02-01/BOS (Ministerio de Educación y Ciencia) and PI042004/037 (Consejería de Educación, Cultura y Deportes, Gobierno de Canarias). The two projects have been partially supported by Feder funds from the European Union.

6 pages, 2 figures, 1 table.-- Article in press.