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The burden for Nigeria, Africa’s most populous country, is about $1 trillion per year, researchers said. They placed losses at $317 billion for the Democratic Republic of Congo, $248 billion for Niger and $229 billion for South Africa.
Data on invasive alien species, or I.A.S., in Africa is scarce, and getting the material for the study was “challenging,” Dr. Eschen said, so researchers relied on estimates in some cases.
Weeding costs, for example, were based on farmed areas in each country and average wages for farm hands. The authors used openly available data from organizations like the International Union for Conservation of Nature and the United Nations Food and Agriculture Organization. That was supplemented with a survey of 110 agricultural experts from 30 African countries.
For countries with insufficient data, they used numbers from areas with similar climates. Western Sahara, Djibouti and Equatorial Guinea were omitted altogether.
That prompted some experts to question the scope of the study’s conclusions. “A major drawback of this study is the collection of data on the monetary cost of I.A.S. to mainland Africa and information on I.A.S. from literature and online surveys,” said Ikechukwu Kelikume, professor of economics and agribusiness at the Lagos Business School. “The implication is that data collected from the literature may not be a true reflection of the dimension and intensity of I.A.S. in the African Continent.”
The study’s estimates for labor represent opportunity costs more than actual wages, the authors said. Small-scale farming and weeding, for example, are often carried out by women and children, and that labor is generally unpaid. “If people didn’t need to weed I.A.S., they could do something else, such as going to school or undertaking an income generating economic activity,” Dr. Eschen said. “Even though the estimate doesn’t reflect paid salaries, it is an indication of the effort needed to deal with these species.”
To address the problem, he said, governments must be proactive.
“Investment to find more efficient ways to tackle I.A.S. — including prevention of new species establishing and established species from spreading further — as well as cost-efficient management of widespread species using, for example, biological control, could reduce management costs and yield losses,” Dr. Eschen said.
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