The biodiversity of the planet is rapidly being depleted, largely as a direct and indirect consequence of human activities (the ‘sixth extinction’: Leakey and Lewin, 1995). IUCN, the World conservation Union, recognizes the need to conserve biodiversity at three levels: genetic, species and ecosystem diversity (McNeely et al, 1990). Genetics is known to be involved in the first and second of these, but has not previously been recognized as important in ecosystem conservation (Frankham et al, 2002). A recent paper in Proc Natl Acad Sci USA by Reusch et al (2005) provides evidence that genotypic diversity enhances ecosystem recovery following an unprecedented heat wave.
Species diversity generally enhances ecosystem function (reviewed by Schlapfer and Schmid, 1999; Loreau et al, 2001). Reusch's group hypothesized that genotypic diversity would act in a similar manner to species diversity to enhance ecosystem function and resilience, especially in ecosystems with low species diversity.
To test this hypothesis, in 2003 they set up replicated seagrass (Zostera marina) plots in shallow seas in the southwest Baltic Sea with one, three or six clonal genotypes. An unprecedented heat wave occurred during the experiment. Recovery following the heat wave was much better in the plots with multiple genotypes. The number of shoots was 34% higher in the six-genotype plots than the monocultures, and the biomass was 26% greater. In addition, there were benefits in terms of higher abundance (but not diversity) for several eipfaunal groups associated with seagrass. Benefits were found for juvenile bivalves and grazers (snails and isopods), but no benefits accrued for detritivorous crustaceans.
The mechanism generating the enhanced ecosystem function for the seagrass was genotypic complementarity, rather than selection. There were large interactions in performance of clones when measured singly versus in mixtures. For example, the clone that performed best singly was ranked 5 out of 6 in six clone mixtures and the worst clone singly was ranked third in six clone mixtures.
This work implicates genotypic diversity as important for ecosystem function, in addition to its previously known roles in species survival. Clearly, it is important to establish how widespread such effects are across geographic regions and different ecosystems. Hughes and Stachowicz (2004) reported that genotypic diversity enhanced community resistance of the same species of seagrass to disturbance by grazing geese in the USA. The effect is not restricted to seagrass. Madritch and Hunter (2003) demonstrated that intraspecific diversity in turkey oak (Quercus laevis) leaves was positively related to soil respiration rate due to microorganisms at high soil nitrogen levels. It is unclear whether the effect described by Reusch et al (2005) is restricted to low diversity ecosystems like seagrass, but it is unlikely, given the generally beneficial effects of species diversity to ecosystem function.
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