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The introduction bias: when a species changes during the invasion

When a species is transported from its native region to a new environment where it was not previously present, and is released into the wild, many things can happen. The normal thing is that the released individuals do not survive, or if they do they do not reproduce. However, some species not only manage to naturalize successfully, but also begin to colonize new environments and cause various environmental and socio-economic impacts. When this happens, that species, in that place, is said to be invasive .

But in the invasion process there is a particularity, a kind of founder effect, motivated by selective pressure, which makes the invading population significantly different from the wild population that remained in the original native region. This selective pressure is called the introduction bias .

What is the founder effect?

Within population genetics, the formation of a new population from a limited number of individuals from a previous population is known as the founder effect .

As a result of the reduced number of organisms that make up the founding population, a representative and genetically homogeneous sample of the original population is not always obtained. It usually happens that certain alleles or variants of genes present in the original population do not appear in the founding population, or genes or alleles that were rare in that population appear more frequently.

Genetic variability is therefore significantly reduced . This can be a great disadvantage at times; a disease would affect the founding population more severely. But it can also present an advantage, if the final genetic population has not been obtained at random, but through a selection process, be it natural or artificial. In fact, the founder effect is considered a driver of speciation, that is, the appearance of new species.

The introduction of alien species into a new environment can be done deliberately or accidentally. And in both cases, there are selective pressures that drive that founder effect.

deliberate selection pressure

Deliberately obtaining individuals of a species from its native distribution range is not a random action, far from it. Most of the capture and transport of wild animals and plants is carried out for direct exploitation. In the case of animals, the most common reasons are their use as exotic pets or their use as furriers . In plants, the most common use is ornamental , although many other species are transported for timber harvesting or its derived products , such as flowers or fruits.

In the subtraction process, therefore, a first selection is produced. Those individuals are chosen who will best serve the future purpose assigned to them. A bird with bright colors is chosen before one with duller colors; Animals that are less fearful of humans or that have more friendly traits are preferred. The plant that reproduces more easily is selected, the one that gives better wood or provides it faster.

In some cases, these characters do not necessarily have a direct influence on the survival and competition capacity of the selected individuals, but in many other cases they do. In this way, a portion of the population is being preselected that presents a series of specific traits , which appear with a different frequency than that of the original population.

Other selective pressures

Selective pressure does not end there. Upon capture or procurement, the organism must travel to reach its destination . Sometimes it does so in totally deplorable conditions and many individuals die in the process. Furthermore, only those specimens capable of reproducing can perpetuate the population. Storage and distribution can also be stress factors that decimate the population.

This selective pressure is not only suffered by those organisms that are deliberately transported. When accidental transports occur, the specimens also suffer this pressure, and many of them die before reaching their destination.

The result is that the final population that arrives in the new territory, be it an ornamental plant, a parrot sold as an exotic pet, or an algae that arrives in the ballast water, is made up of the surviving individuals most apt to all these stress factors.

This introduction bias causes populations of invasive species to descend from a subset that is preadapted to the invasion process , and that is genetically distinct from native populations of the same species. And it is for this reason that it is not a good idea to try to capture invasive organisms and return them to their native environment: because it is no longer the same population.

References:

Kliber, A. et al. 2005. Interaction between Founder Effect and Selection during Biological Invasion in an Aquatic Plant. Evolution, 59(9), 1900-1913.

Palma, E. et al. 2017. Functional trait changes in the floras of 11 cities across the globe in response to urbanization. Ecography, 40(7), 875-886. DOI: 10.1111/ecog.02516

Palma, E. et al. 2021. Introduction bias: Imbalance in species introductions may obscure the identification of traits associated with invasiveness [Preprint]. Ecology. DOI: 10.1101/2021.03.22.436397 

Schlaepfer, D. R. et al. 2010. A multi-species experiment in their native range indicates pre-adaptation of invasive alien plant species. New Phytologist, 185(4), 1087-1099. DOI: 10.1111/j.1469-8137.2009.03114.x

Templeton, A. R. 1980. THE THEORY OF SPECIATION VIA THE FOUNDER PRINCIPLE. Genetics, 94(4), 1011-1038. DOI: 10.1093/genetics/94.4.1011

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