Halfway House

The conventional thinking is that as people move into an area, biodiversity drops. But a wide-ranging study in Canada has revealed that land with a moderate amount of development contains the most plant species.

The so-called “intermediate disturbance hypothesis” is actually an old idea. Researchers have proposed in the past that a moderately-disturbed area will have a richer species community than completely undisturbed or highly-disturbed areas. But evidence for the idea has been sparse, and large-scale studies usually focused on the effects of natural disturbances, not human development.

To settle the question, the study authors surveyed vascular plants at 242 sites spread over 381,000 square kilometers in Alberta, Canada. The plots included a pristine national park, farms, towns, and areas criss-crossed by roads and pipelines. Using satellite images and aerial photos, the team assigned a value of 0 to 100% disturbance to each site.

Species richness rose as development increased, peaking when disturbance reached 47.7 percent. After that point, the number of species dropped off again. The pattern held even when the researchers accounted for environmental differences, such as climate and soil, and was “consistently supported across trees, shrubs, forbs and grasses,” the authors write in Nature Communications.

When the team studied native and exotic plants separately, the data told a slightly different story. Disturbance boosted the number of exotic species, even in highly-developed areas, and lowered the proportion of native plants. So managers will need to consider how development affects not just the number but the types of species. Roberta Kwok | 18 October 2012

Source: Mayor, S.J. et al. 2012. Regional boreal biodiversity peaks at intermediate human disturbance. Nature Communications doi: 10.1038/ncomms2145.

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