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For example , rising temperature may cause spatial shifts of arthropod species , particularly sedentary ones , that may threaten their survival ( e.g ., specialized species of the upper canopy may move down , to cooler levels where their resources may be less abundant ) . Because of the diverse growing conditions in the United States , all plant lists in the book are organized according to region , and every recommended species or cultivar has been chosen by an experienced landscaper and tested in that region . Catching too many fish has left species that were once common , such as cod in the North Sea and bluefin tuna in the Mediterranean , struggling to survive . While hickory , beech , persimmon , serviceberry , blackgum , American holly , hawthorne , and dogwood are not highly valued species for wood products , they produce food that is important for wildlife . Growing conditions during 2004 were generally good and all of the surveyed species except beech showed some improvement in condition .. Some key conservation strategies propose to " maintain the full range of successional and climax communities in an appropriate balance and distribution pattern " , and to protect highly specialized species as well as their habitat ( Cox , 1993 ) . Selective cutting of the woodlands in his sample area meant that the three most frequently used fruit trees remained relatively constant in cover over the two different areas , while other , less highly valued species had disappeared in the deforested area .