SUBJECT: Consequences of biological invasions for plant-animal mutualisms in island ecosystems.
AUTHOR: Anna Traveset, Institute of Mediterranean Advanced Studies, Spanish Research Council-University of the Balearic Islands, Mallorca, Spain.

SUMMARY: Biological invasions are a growing problem that threatens global biodiversity by dramatically altering both the structure and functioning of ecosystems. Most studies have focused on the impacts of invaders on species diversity and community composition, but only a small proportion have explored the mechanisms underlying such impacts. Moreover, invasive species have often been assumed to cause widespread extinctions, although good quality data supporting this are still scarce (Gurevitch and Padilla 2004). An impact that is increasingly receiving more attention is that of alien species on native mutualistic interactions between animals and plants, such as pollination or seed dispersal. Much information on the ecological consequences of pollination or dispersal disruptions due to biological invasions come from insular ecosystems, although an increasing number of studies also provide data from continental areas.

KEYWORDS: Invasions, pollination, seed dispersal.

The introduction of an alien pollinator, in particular, may increase the rate of pollen exchange and, thus, positively influence plant reproductive success. However, introduced pollinators can also disrupt the plant pollination system and have a negative effect on plant fitness through different mechanisms: causing a lower seed set, decreasing pollen removal by efficient native pollinators (and thus paternal fitness), or simply promoting a lower gene flux. Evidence of such disruptions is found in islands such as New Zealand, Japan, Canary Islands, Hawaii, and Santa Cruz (California), where honeybees and/or bumblebees were introduced decades ago. Recent studies show that an introduced herbivore can also, directly or indirectly (by influencing the rate of pollinator visits to flowers), affect plant pollination success. Likewise, an introduced predator on the pollinators may also dramatically disrupt a native mutualism, as appears to have happened in New Zealand, where the introduction of rats, stoats, and opossums has led some species of pollinators (for instance the terrestrial bat Mystacina tuberculata (Webb and Kelly 1993)) almost to extinction. Finally, an invasive plant can disrupt a pollination mutualism if for instance pollinators have a preference for the alien and/or through a mechanism of interference between conspecific and heterospecific pollen on native stigmas. Unfortunately, we still have very little information on this, either from the continent (e.g. Brown et al. 2002) or from islands (Moragues and Traveset 2005).

Studies that examine the impact of invasive species on seed dispersal systems are still much scarcer. Alien animals that displace or prey upon native dispersers can also have an indirect influence on plant dispersal success. Documented examples come from the Canary Islands (Nogales and Medina 1996), New Zealand (Ladley and Kelly 1995; (Spurr and Anderson 2004) and the Balearic Islands (Traveset and Riera 2005). Very little information still exists on how and how often alien plants compete for dispersers with native ones and on their effect on dispersal success of the native plants.

Further research in these specific areas of invasion biology is urgently needed to improve our ability to explain and therefore predict impacts of invasive species on biodiversity. Mutualistic interactions such as animal-mediated pollination and seed dispersal are among the most important processes that maintain and generate biodiversity. A major aim of conservation efforts should be to ensure the continued functioning of these processes. Part of the conservation strategies should therefore be to mitigate disruptions of such process brought about by invasive alien species.