Mr Scheiman blames the loss of open spaces and grasslands for the drop.

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Researchers Russell Ciochon and E Arthur Bettis III, from the University of Iowa, US, believe these climatic cycles may have caused the expansion of open habitats, such as grasslands and steppe.

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The modern freeways and suburban sprawl give way to high grasslands in the Sierra Azul Open Space Preserve and dense redwood forests north of Santa Cruz.

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