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.

Seed dispersal effectiveness in a plant–lizard interaction and its consequences for plant regeneration after disperser loss

“…For instance, such selection has been reported in several studies, either for small (e.g. Jordano 1995;Alcántara et al 1997;Riera et al 2003) or large fruit and/or seeds (e.g. Courtney and Sallabanks 1992;Wheelwright 1993;Stanley et al 2002).…”

Pollination and seed dispersal by lizards: a review

“…Moreover, some of these studies have suggested that herbivorous lizards could affect the reproduction of several species of plants as well as the vegetation structure of the community (Valido & Nogales 1994, Pérez-Mellado & Casas 1997. Lizards also establish tight interactions with some plants, exerting selective forces that affect the evolution of flowers and fruits (van der Pijl 1972, Côrtes et al 1994, Vasconcellos-Neto et al 2000, Riera et al 2002.…”

Indirect Effects of Introduced Predators on Seabird Islands

“…In the Balearic Islands, Spain, the loss of endemic lizards ( Podarcis spp.) to introduced carnivores has resulted in reduced dispersal of Cneorum tricoccon (Cneoraceae; see Riera et al 2002 ). In some cases, introduced species may replace other species in mutualisms, or create new ones (Chapter 4).…”