These responses are not automatic but are the result of purposeful activity in response to signals of increased scarcity. Successful outcomes are not guaranteed. Consequently there has been a persistent tension between impending scarcity and technological progress. Recent decades have seen increasing concern about the environmental impact of population and economic growth. Because environmental resources-ecosystem services and "resource amenities"—are not generally traded on markets, scarcity signals for these resources may be inadequate, and appropriate policy responses are difficult to implement and manage.