Malthus and the Sparrow

When Rev. Thomas Malthus wrote "An Essay on the Principle of Population as it Affects the Future Improvement of Society" in 1798, the population of Britain was about 11 million.

At the time Malthus thought Britain was pretty close to its agricultural production threshold and that a doubling of Great Britain's population to 22 million was "probably a greater increase than could with reason be expected."

To suppose that Britain could feed a population double this size again (or 44 million people), would be "impossible to suppose," said Malthus, and this impossibility "must be evident to those who have the slightest acquaintance with agricultural subjects".

Of course, Malthus was wrong about agriculture.

Rev. Malthus failed to anticipate the degree to which outputs could be increased in Great Britain -- and all over the world -- thanks to machinery, fertilizer, modern farming practices, genetic improvements, and irrigation.

Today Great Britain has a population of over 60 million people and they are not starving. In fact, all over the world food production has risen faster than population growth, and this has been true for most of the last 200 years.

While increased agricultural outputs have been good for humans (less starvation) and generally good for wildlife (less wild lands put under the plow), there are increasing signs that not all is well.

Across the world many species of once-common wild birds are in decline. This is true not only in North America but in Europe as well.

The British are certainly taking a hard look at the situation. The British government monitors wild bird populations as a key measure of environmental health and they have noted fairly precipitous declines in their field and forest birds.

Now comes the latest news: reports from across Europe that populations of common house sparrows are in rapid decline. In fact, the decline in the house sparrow population has been so steep that it has now been put on the "red list" in the U.K.

No one is quite sure why house sparrows are in decline.

An observed decline in the number of eggs in house sparrow nests is an important clue. The size of the clutches of songbirds (passerines) and most raptors rises and falls in direct relationship to how much food is available (yes, birds understood and believed Malthus long before we did). A small clutch of eggs is typically a sign of a decline in one or more critical food components. One theory of house sparrow decline is that the phenomenon is linked to loss of critical insectivorous habitat. Insects are a critical component of house sparrow diets immediately after hatching.

Why are the insects disappearing?

One factor may be the rapid decline in British hedgerows. During a single 10-year period (1984-1993), more than one-third of all of the hedgerows in the United Kingdom were lost -- a whopping 121,875 miles of destruction. At least another 96,000 miles of hedgerow were lost in England from 1945 to 1984.

British hedgerows are fabulously vibrant ecosystems supporting myriad plant and insect species in dense thickets. An analysis of hedgerows has found a close correlation between the age of a hedgerow and its plant diversity, with some British hedgerows estimated to be as much as 1,000 years old. As hedgerows have vanished, so too have seeds and insects that once sprang from these hedgerows.

Why are the hedgerows disappearing so rapidly?

Much of the blame lies with agricultural policy and the desire to boost agricultural outputs by plowing edge-to-edge with ever-larger farm machinery. In addition, as more and more people have moved into the countryside to live on mini-estates, hedgerows have fallen to new housing developments and road widening.

Though a 1997 law was enacted in the U.K. to try to slow hedgerow destruction, the bulldozers continue to do their work. The decline in hedgerows across much of Europe may, in fact, prove to be a critical factor pushing the house sparrow over the edge.

The good news is that the population growth in Europe which first spurred the push to boost agricultural productivity, is finally slowing. The U.K., with a population of 60 million today, is expected to effectively achieve zero population growth around 2025. Much of the rest of Europe is headed the same way.

The bad news is that human population growth has pushed a great deal of the natural landscape of Europe right to the edge and changing land use patterns may be pushing it past the tipping point in some areas.

As more and more people move out of urban cores and into far off suburbs and once-rural areas, less and less land is available for agricultural production. In order to keep food production up, the land must be farmed more and more intensively in order to wrestle the same outputs from a declining number of acres.

The result, as we see in both the U.K, and the U.S., is the loss of hedgerows and shelter belt forests, increased incursions into the few large blocks of forest that remain, and a steady decline in forest and field bird species across much of the temperate-zone regions of the world.
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