It takes just one pregnant guppy for an alien invasion!

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The well-known ability of female guppies (Poecilia reticulata) to store sperm for up to six months and give birth long after being inseminated is responsible for their runaway success as one of the world's worst invasive fish species.

In research published in a recent issue of the online journal PLoS ONE,  
Amy Deacon, Indar Ramnarine and Anne Magurran document the current global distribution of the guppy, producing the most complete picture of the current distribution of this invasive species to date, and show that it is contributing to the homogenisation of fish communities on a global scale.

At the same time, they conducted experiments to test the prediction that a single pregnant female guppy is capable of routinely establishing viable populations when introduced to a new habitat.

Despite the small native range of the guppy, the authors found that its introduced range now spans every continent with the exception of Antarctica, as well as numerous oceanic islands.

Their data show that the worldwide distribution of the guppy is considerably more extensive than previously described in the literature or in any database.

For the second part of their study, the authors carried out an experiment in which 30 plastic mesocosms were established and a single pregnant female wild-caught guppy was added. The experiment was allowed to run for two years, during which the fish were counted, measured and their performance (in activity and evasion) were recorded.

The authors found that the single female guppies were capable of establishing viable populations within the mesocosms during the study period in a large majority of the cases: 91% of mesocosm populations persisted at the end of the first year and 86% at the end of the second.

Furthermore, their studies showed that the behavioural performance of the offspring from such introductions was not impaired. Although inbreeding is very likely to take place as a result of using a single founder individual, the guppies may be employing a number of pre- and post-copulatory strategies to minimise inbreeding, surmise the authors.

The authors conclude, "Our results demonstrate how introductions consisting of a few animals, or even a single individual, can lead to thriving populations of invasive species. A highly specialised reproductive system, coupled with a remarkable adaptability is likely to have led to the phenomenal success of the guppy outside of its native range.

"These findings reinforce the need for caution when releasing exotic species, and show that seemingly innocuous or beneficial activities such as a child freeing a few pet fish or a concerned householder using guppies to control mosquitoes can result in a thriving population of invasive poeciliids that may then go on to compete with the indigenous freshwater fauna.

"They also illustrate how many small actions replicated across the globe, in the form of the accidental or deliberate release of a few fish, combined with natural adaptations in these fish for life in ephemeral habitats, can contribute to the reduction of diversity in freshwater fish assemblages worldwide."

For more information, see the paper: Deacon AE, IW Ramnarine and AE Magurran (2011) How reproductive ecology contributes to the spread of a globally invasive fish. PLoS ONE 6, e24416. doi:10.1371/journal.pone.0024416

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