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When Development Causes Disease: Linking Ecosystems and Human Health

Ecosystems provide a wealth of services to human populations, among them, disease regulation. But narrowly-focused development projects can threaten these ecosystems and put entire populations at risk. Here are three cases where development has compromised nature's way of keeping us healthy.

Forty years ago, malaria was almost eradicated in Peru, and only 1,500 cases were reported. Today, however, the number has climbed to over 50,000. One activity being blamed for this rise is logging in the Amazon forest.

Why? Tree removal and new logging roads leave behind pools of standing water, and increased sunlight allowed by the destruction of forest canopy warms the pools, leading to algae growth. Tree removal also reduces water acidity in the pools. These three factors, all linked to deforestation, create the ideal growing environment for the Anopheles mosquito. This insect---the only genus which carries malaria---is then able to out-compete other “benign” mosquitoes, and malaria rates rise.

Development projects---intended to increase people’s well-being---can do just the opposite when they don't consider the ecosystem services they affect. For example, the Diama Dam in Senegal was built to facilitate irrigation of rice fields, and its construction did help double the region’s rice production. However, the dam also brought snail fever to the region, and certain villages experienced infection rates of 60% in less than three years.

Again, we follow the chain: the dam blocked the intrusion of salt water into the Senegal River, allowing freshwater snails to out-compete the native saltwater snails. These freshwater snails act as hosts for schistosomes---small larvae that burrow into human skin and cause snail fever. As the freshwater snail population grew, so did the disease rate. As in Peru, the “benign” species---in this case the saltwater snail---was overwhelmed by the disease carrier.

These trends are not limited to developing countries. A 2003 study by Rutgers University, Bard College and the Cary Institute of Ecosystem Studies showed suburban sprawl in the eastern U.S. may contribute to higher incidences of Lyme disease. Humans get Lyme disease from black-legged tick bites, and the ticks live in the fur of white-footed mice. As suburbs expand further and fragment surrounding forests, predator populations decrease, and mice multiply---and so do the ticks.

Such cases are not isolated. Every environment provides many ecosystem services, some of which---in these cases disease mitigation---are often ignored. While it is complex to try to predict what will happen to an ecosystem when we interact with it, we do need to learn from past examples in order to ensure that our health and nature’s health can continue to coexist.

These case studies were adapted from a new book, Sustaining Life: How Human Health Depends on Biodiversity, edited by Eric Chivian and Aaron Bernstein, Chapter 7. This book contains numerous examples linking ecosystem services with human health.

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