Editing a manuscript yesterday, I found the author using "insure" and "ensure." Most style guides make the distinction between the two words. For example, The New York Times Manual of Style and Usage says,
Insure means buy or issue insurance: She insured her camera against theft.
Ensure means guarantee or make safe: The hit ensured a Yankee victory.
Knowing this distinction, I was going to change "insure" to "ensure" in the following sentence: "If you [a flower] produce a scent that attracts only the males of one particular species of bee, you can insure that your pollen will end up precisely where you want it." It's not as though the flower is taking out an insurance policy on its pollen.
Then, two paragraphs below, the author used "ensure": "Yet their small numbers ensure their survival." The author clearly was making a distinction between the two words. Did he know something I didn't?
Webster's Collegiate Dictionary defines ensure as "to make sure, certain, or safe"; and insure as "1: to provide or obtain insurance on or for; 2: to make certain especially by taking necessary measures and precautions." For usage, it says:
ensure and insure are interchangeable in many contexts where they indicate the making certain or inevitable of an outcome, but ensure may imply a virtual guarantee <
ensured the safety of the refugees>, while insure sometimes stresses the taking of necessary measures beforehand < insure the success of the party>
The author seemed to be following the usage in Webster's, and not the NYT guide. I decided to keep his original language, even though I prefer to make the insure-as-insurance distinction. It blurs the line, but then there are so few clear lines in word usage.