Using empirical tests, a pioneering study shows how plant species, such as the prickly pear, invade Mediterranean ecosystems, and can either rob the native plants of pollinating insects, or, surprisingly, can attract them, thus benefiting the whole plant community, such as in the case of balsam. The research contradicts the hypothesis of the “floral market” whereby only the invasive flowers are seen to benefit and the native flowers are no longer visited by pollinating insects.
Biological invasions (species transported by humans outside their region of origin to other regions where these species become established and expand) are one of the major causes of the loss of biodiversity. The plants fight for nutrients, space and light, and for pollinating insects.
An article about this subject has been published in the “Oecología” [Oecology] journal by scientists at the Universidad Autónoma de Barcelona (UAB), [the Autonomous University, Barcelona], the Biological Station, Donana, CSIC [the Spanish National Research Council] and the Instituto Mediterráneo de Estudios Avanzados (IMEDEA) [Mediterranean Institute for Advanced Studies]. According to the research, the existence of invasive plants in invaded sites can increase visits from insects to the majority of native plants. In this way the “floral market” hypothesis in which only the invasive flowers are seen to benefit and the native flowers are no longer visited by insects is contradicted.
Ignasi Bartomeus, a researcher at the UAB, and the main author of the study, points out some important details to SINC: “the invasions do not follow a single pattern: for this reason it is necessary to understand the mechanisms and structure whereby the native species compete”.
The two invasive plants under study, Opuntia stricata – a type of prickly pear – and Carpobrotus affine acinaciformis – also called Sally-my-handsome or balsam – have more eye-catching flowers and are richer in pollen that the rest of the native plants and receive many more insect visits that the latter.
The study reveals that the invasive plants play a central role in the plant pollination network. This is because during the period of the study, Opuntia stricta received 30.9% of insect visits, compared to 43.4% for Carpobrotus affine acinaciformis.
The scientists discovered that the more resources there are in the plant community, the more pollinators will be attracted towards all of the plants, although it is still not known whether the impact on the seeds of the native plant is positive or not. This is the case for Carpobrotus, which can impact upon the pollination of the native plants.
The researchers observed 23 pollinating insects for Carpobrotus and 17 for Opuntia. Compared to the native plants, the two invasive plants have a different impact. In the first case, there was no insect that was an exclusive pollinator, whereas in the second case, the carpenter bee (Xylocopa violacea) was an exclusive pollinator. “The Opuntia flowers monopolise the market, attracting all the pollinating insects in the area to their flowers, whereas the Carpobrotus attracts more pollinating insects to the area, but all the plants are seen to benefit”, Bartomeus points out.
The study concludes that Carpobrotus can improve the reproduction of the native plants whereas Opuntia reduces it. Bartomeus confirms to SINC that “the presence of the invasive plants can alter the structure of the plant community, and it is difficult to predict the long-term effects of this.”
- Bartomeus I., Vila M., Santamaría L. Contrasting effects of invasive plants in plant-pollinator networks. Oecologia 155(4): 761-770 ABR 2008. doi: 10.1007/s00442-007-0946-1 [link]
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