For the birds

Published June 1, 2000

Most people think of the Industrial Revolution as a turning point in human efficiency. But it is also an important milestone in natural efficiency.

After all, our increased production has caused atmospheric CO2 to rise, allowing plants to become more water-efficient and more biomass to spring up–even in dryland areas that, when the concentration of CO2 is lower, cannot support a variety of woody plants.

A change in flora makes possible an expansion of fauna to areas they previously could not inhabit. Mesquite trees, for example, invaded the desert southwest throughout the last century, changing the landscape and the animals within it.

At the Buenos Aires Wildlife Refuge in southeastern Arizona, Lloyd and colleagues examined a variety of vegetation variables and found that the density and distribution of mesquite trees had the greatest impact on bird populations.

Not only did the number of birds increase with increasing mesquite density, but “greater bird species richness [was] found on plots with higher mesquite densities.”

Also interested in what greening up means for animal life, and vice versa, Brown and Archer conducted a field experiment to determine the cause of the recent (200-year) expansion of honey mesquite in the southwestern United States with particular interest in the role of livestock grazing.

They found that the density and level of grazing of grasses did not affect the emergence or survival of honey mesquite seedlings, regardless of soil moisture provided for the plants.

The emergence and establishment of honey mesquite occurs “regardless of resource availability and livestock grazing pressure on grasses.”

Nor could the authors attribute the recent expansion of the honey mesquite into southern Texas to any unusual patterns in regional precipitation.

Though Brown and Archer failed to remark on the fact, their study obviously provides still more evidence that elevated CO2 is at least partially responsible for the increase in woody biomass in many of the world’s marginal drylands.

Forest cover has indeed increased since the Industrial Revolution began influencing the Earth’s greenhouse.

The Knight research team used aerial photographs taken between 1939 and 1985 to analyze the spatial extent of gallery forest on the Konza Prairie Research Natural Area in Kansas. They found the total gallery forest area increased by more than 50 percent during the 46-year period.

Furthermore, they examined relevant Original Land Office Surveys and concluded the region’s total forest area increased by nearly 100 percent between 1859 and 1939. Indeed, forest growth there increased substantially over the last century, and elevated atmospheric CO2 likely is playing a role in this trend.

When the atmospheric CO2 concentration rises, woody species expand into semi-arid grassland areas, increasing the diversity of plants in these marginal areas, and by association, animal diversity as well–providing a better life for animals, vegetables, and humans. A revolution indeed!

Robert C. Balling Jr., Ph.D. is director of the Laboratory of Climatology at Arizona State University and coauthor of The Satanic Gases.


Brown, J.R., and S. Archer, 1999. Shrub invasion of grassland: Recruitment is continuous and not regulated by herbaceous biomass or density. Ecology, 80, 2385-2396.

Knight, C.L., et al., 1994. Expansion of gallery forest on Konza Prairie Research Natural Area, Kansas, USA. Landscape Ecology, 9, 117-125.

Lloyd, J., et al., 1998. The effects of mesquite invasion on a southeastern Arizona grassland