You should know first which boost will have positive and negative effect to the elements of your dragons. For example, you have fire boost, which will have positive effect on all your dragons with fire element, but will have negative effect on dragons with cold elements, so should not place the cold element dragons in an island which you have fire boost.
Here's a picture to help you understand clearly:
<img src="http://www.ipadforums.net/attachment.php?attachmentid=17555"/>
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