It can loosely be described as travel that takes place at regular intervals-often in an annual cycle-that may involve many members of a species, and is rewarded only after a long journey.
这句话破折号那一截是插入语吗，修饰intervals？起强调作用？
另后面的that定语从句修饰的也是travel吧？是不是有两个定语从句修饰同一个先行词？
这种用法的话两个从句隔开的用法有哪些？intervals没查字典时我觉得后面that从句可能是修饰他的，怎么判断定语从句修饰哪个先行词，特别是从句没有紧跟先行词的情况？
