BioEd Online

Communities of kissing cousins may be at a disadvantage in the plant world, according to a study in this week's issue of Science¹.

It is well known that having a number of different plant species in a field can help to promote insect diversity, boost the plants' productivity and improve the overall ecological health of an area. Now it seems that genetic diversity within a species has similar effects. The findings could lead to better habitat restoration and agriculture.

Gregory Crutsinger, a graduate student at the University of Tennessee in Knoxville, studied fields of goldenrod a weedy perennial that can grow taller than 3 metres and produces clusters of yellow flowers. He first gathered a selection of genetically distinct plants, picking them from patches at least 100 metres apart. He then planted 63 plots of goldenrods in the foothills of the Appalachian Mountains in Tennessee. In some plots he planted only one genetic type, in others he grew a range of types.

Crutsinger returned to his plots five times during the growing season to count and identify every insect on every plant. The final product: a one-by-one count of 36,997 individual insects, representing over 130 species.

Miracle grow

Crutsinger found that plants from plots that had the most genetic diversity were bigger and contained not only the most insects but also the most species of insects. Plots with 12 genotypes of goldenrod had 27% more arthropod species crawling about than plots with a single plant genotype.

But why? Crutsinger reasoned that there could be two ways that host plant diversity had influenced the insects: either by increasing the quantity of resources available or by improving their quality. But when he analyzed his data, Crutsinger found that, in the plots of goldenrods that were genetically more diverse, insect diversity had increased regardless of the sheer quantity of plant material. So it wasn't just a case of more food leading to more insects.

That suggested that the goldenrods in the genetically diverse plots had provided better quality resources to the insects, with a resulting positive impact on the plants too.

It is not clear just how this works, however. Tiny differences in plant characteristics, from leaf shape to stem thickness, might have promoted a wider diversity of pests and pollinators and spread out their effects over the plot. "Some are really nailed by insects and some are barely touched at all," says Crutsinger. The plants might also have picked nutrients out of the soil differently from each other, helping them make best use of the resources available.

Mix it up

Anurag Agrawal, an ecologist at Cornell University in Ithaca, New York, says that ecologists have largely thought of species as being homogenous units. "They've paid relatively little attention to genetic diversity within the species," he says. But recently that has been changing. "The notion that genetic diversity is important in a community context has been gaining ground," says Agrawal.

In 2000, for example, a paper published in Nature reported similar results after mixing two varieties of the same rice species: a disease-susceptible variety and a disease-resistant variety². Growing the two together boosted yields.

The results may be important when considering agricultural strategies for relatively homogeneous crops. Agrawal points to Hass avocados as an extreme example of monoculture: thousands of hectares of Hass avocados are derived from cuttings of a single tree, meaning that genetic diversity within the crop is virtually nil.

Crutsinger adds that the findings are significant for conservation biology as well. "If you're thinking about habitat restoration projects," says Crutsinger, "you want to use a multitude of genotypes from a local area."

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