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Environmental change can also affect entire populations. For example, from 2002 to 2004 the southwestern part of the United States underwent a severe drought. The lack of water caused acres of ponderosa pine and pinyon-juniper trees to die in Arizona and New Mexico. These trees have a natural predator called the bark beetle. Normally, these trees produce sap to help protect themselves against the beetle, but because of the drought conditions, the trees had to conserve water and could not produce the sap. After 15 months of the drought, Arizona, Colorado, and New Mexico had lost more than 90% of their pinon trees. These trees did not have the adaptions needed to survive in this changing environment. Once the pinyon and ponderosa pine trees died, they were replaced by grasses, shrubs, and trees that did have adaptations which enabled them to survive the drought. They had better access to the resources in the area. The surviving species were able to reproduce and populate the ecosystem with more of the organisms that had the advantageous traits. The pictures below show the forest around Los Alamos, New Mexico, before and after the stress and the bark beetle outbreak. Can you see how the population has changed?

Source: Las Alamos Forest, Craig Allen, U.S. Geological Survey