The second important contributing factor, the reduction of diversity in large crop monocultures, has long been associated with reasons for pest outbreaks. The reasons for this are that a monocrop is thought to provide a highly suitable habitat for a pest, but a highly unfavourable one for the pest's natural enemies, thus creating conditions appropriate for outbreaks. However more recently it has generally been recognized that outbreaks are not an inevitability of such trophic simplicity, an idea long recognized in forest entomology.Monocultures do occur in natural ecosystems,e.g. bracken,heather or natural forest.