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Biodiversity: a "Rock-Paper-Scissors" Game Played at Multiple Scales
26 Sep, 2007 11:08 am
Biological diversity is being lost at unprecedented rates around the world, both in the number of different species and the genetic variety within a species. Do these processes happen independently, or can loss of diversity at one level result in losses at the other? In a study recently published in the journal Science, my collaborator and I describe a ?rock-paper-scissors? competitive game played among different plant species and different genetic variants of a single species. In this system, losing genetic diversity within a species would lead to the extinction of the other species, and in turn, extinction of one species would lead to the loss of genetic diversity in the remaining one.
In our study, we asked how diversity at the genetic level might help maintain diversity at the species level, and vice versa. Working with the annual plant black mustard (Brassica nigra), we asked whether genetically based variation in a chemical trait of black mustard might determine whether it could coexist with three competitor species. At the same time, we wanted to know if the presence of multiple competitor species (species diversity) could help maintain genetic diversity in the chemical trait.
Black mustard produces a chemical called sinigrin, or allyl-glucosinolate, which is toxic to other plants and to microorganisms in the soil that many plants need in order to grow well. We used artificial selection to create genetic variants of black mustard that produced either high or low levels of this chemical. We then used these varieties to create experimental communities, which consisted of all high sinigrin black mustard, all low sinigrin black mustard, or a mix of three other plant species. Finally, we planted one individual of each type (a high sinigrin black mustard, a low sinigrin black mustard, or a different plant species) into each type of community. This approach allowed us to determine if each type (genotype or species) could invade at least one community. Like in a game of “rock-paper-scissors”, if each type can beat at least one other type, then all of the types are likely to persist in the community.
We found that the two varieties of black mustard and the other plant species indeed displayed a “rock-paper-scissors” cycle of competition. High sinigrin black mustard could invade diverse communities of other species, but lost out in competition with low sinigrin black mustard variants. Low sinigrin genetic variants of black mustard on the other hand, could invade communities of high sinigrin black mustard variants, but were out-competed by other plant species. Thus no one type could win in all cases, and so each type could be maintained in the larger community. However, if any one genetic variant or species were removed from the system, the cycle would break down, and the community would become dominated by a single species. This is the first study of its kind to show that diversity within a species is necessary to maintain diversity among species, and at the same time, that diversity among species is necessary to maintain diversity within a species.
As we strive to protect the remaining biological diversity, our efforts must be guided by knowledge of the processes that maintain diversity under natural conditions. Our study suggests that we should not focus on one particular aspect of biodiversity in isolation. Preventing the erosion of genetic diversity within species may require maintaining a diversity of species in a community. At the same time, we may need to focus on protecting high levels of genetic diversity within species in order to maintain diverse communities of species.
Lankau, R.A. and S.Y. Strauss, Science 317, 1561-1563 (14 September 2007)