Breakage of mutualisms by exotic species: The case of Cneorum tricoccon L. in the Balearic Islands (Western Mediterranean Sea)

Abstract

Aim: In this study we tested the hypothesis that the dispersal success (estimated here as fruit removal rate) of a native shrub species living in the Balearic Archipelago, Cneorum tricoccon L. (Cneoraceae), has decreased significantly in those islands where endemic lizards of the genus Podarcis have disappeared. These lizards acted as the main seed dispersers of the plant and became extinct after the introduction of carnivores. At least one of these carnivores, the pine marten (Martes martes L.), is also an important frugivore, consuming the fruits and dispersing the seeds of C. tricoccon and thus allowing the comparison of fruit removal rates between the two groups of vertebrates (lizards and mammals). We further tested the possibility that lizards (in particular, P. pityusensis Boscà) could be exerting selection on seed size. Methods: In seven populations from four islands, we monitored fruit removal by either lizards or mammals. The fruits of C. tricoccon do not drop after ripening, remaining attached to the branches for long periods if not removed. In order to test whether lizards might be exerting selection on seed size, we compared seed diameter and weight between defecated and uningested (collected directly from plants) seeds for each of the populations. Results: Fruits were removed in significantly greater proportions in those populations where lizards are still present. Data showed that in two of the examined populations in the Pityusic islands, defecated seeds were lighter and smaller than controls suggesting that lizards selected fruits of smaller size than the average of the population. Main conclusions: The introduction of carnivores in the Balearics has led to important changes in the population dynamics of many native species. In the larger islands (Mallorca and Menorca), carnivores seemed to have caused the extinction of endemic lizards who acted as the only dispersers of some plants such as C. tricoccon. Pine martens, in particular, are in turn frugivorous and thus can 'replace' to some extent the 'lost' seed dispersers. We hypothesize that besides having decreased fruit removal rates in this shrub, these 'new' dispersers have modified considerably the distribution of the plant on Mallorca island, as the fossil record shows that lizards lived at low altitudes (< 500 m a.s.l.) and the plant can be currently found up to 1000 m a.s.l. Furthermore, preliminary data suggest that lizards might be exerting a selective pressure on seed size. If this is true, this pressure might have been released - or counteracted if carnivores select for fruit size as well - after the extinction of lizards from some islands, which would have important ecological consequences for the plant.