Discussion

Because of their economic importance, pine species have been introduced out of their natural range of distribution, in some cases spreading from the areas where planted to other natural areas. Pinus canariensis and P. pinea are both invasive species in South Africa (Richardson et al. 1994) and also both of them are phylogenetically very close (Liston et al. 1999; Grotkopp et al. 2004). Therefore, the analysis of the regeneration of both species in an area where P. canariensis is native can reveal competitive process among similar species of pine as well as indicate the invasive character of P. pinea.

One of the main differences among both species of pine is the seed, Pinus canariensis has small seeds and a little wind can provide long distance dispersion, but in the case of P. pinea the seeds are large disperse mainly through barochory (dispersion by gravity providing short distances from the parent individual) on the Gran Canaria Island (Arevalo et al. 2005). There is no information about any dispersal by animals, although in other areas of the world it is dispersed by squirrels and jays (Grotkopp et al. 2004) becoming very invasive as in South Africa (Richardson 1989; Rouget et al. 2001). However, a recent invasion of a species of African squirrel, the barbary ground squirrel (Atlantoxerus getulus) in Fuerteventura Island, the most eastern island of the archipelago, may be able to alter this stable situation of the population of P. pinea. The island of Gran Canaria has been determined through several models to be one of the most suitable islands to be invaded by this species, which in Fuerteventura has been estimated to have a density between 100-200 millions of individuals (Lopez-Darias et al., 2008).

The invader characteristics of Pinus canariensis can be considered stronger than the invader characteristics of P. pinea, as it has been demonstrated in studies based on the structure of the seed and the dispersion abilities of the species (Grotkopp et al. 2002; Rejmanek and Richardson 1996). Our results have demonstrated, that even in areas where the canopy of P. pinea was dominant, the regeneration were dominated by P. canariensis and we did not find a significant relationship among the regeneration composition and the canopy composition.

However, once the plant established, we did not find different in species survivorship in a period of 2 years (Fig. 3a), indicating that once established, and the competitive ability become more similar. We also were unable to determine a significant effect of small herbivores in P. canariensis, although this effect has been demonstrated important for survivorship in other species of pines (Bergman et al. 2005).

Figure 3. a) Mean percentage of survivorship for Pinus canariensis and P. pinea in the five plots and b) Mean percentage of survivorship for P. canariensis excluded from grazing vs. non exclude.

In previous studies were suggested that the status of P. pinea cannot be considered at present time dangerous from an invasive point of view. However, we suggest the necessity to control de population of the exotic pine favoring the establishment of the native one (Arévalo et al. 2005) to prevent future changes, specially knowing that the main control of the species is through the limited dispersion, and because of the broad ecological capacity of the Barbary ground squirrel and the presence of large favorable climatic areas in Gran Canaria. We also agree that in addition of the control of the pine, a management strategy is urgently needed to prevent the expansion of A. getulus through Gran Canaria.

Control of this species will require ecological restoration in order to favor the establishment of native vegetation that has been largely disturbed by past management practices. These recommendations, based on the ecology and biology of the species, are also in accordance with the laws of the Canarian Government, which require exotic species to be controlled and if possible, eliminated. As Walters (1986) indicated, experimentation and quantification through monitoring provide the best adaptive management approach.

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