Advertisers have started moving their spending patterns in the months since Apple Inc. began requiring apps to gain iPhone and iPad users’ permission to track them.

After the tracking change took effect in April, many users of Apple’s iOS operating system have received a high volume of prompts from apps asking permission to track them-requests that most have declined. Less than 33% of iOS users opt into the track, according to ad-measurement firm Branch Metrics Inc.

As a result, the price for mobile ads directed at iOS users has dropped, while ad prices have increased for advertisers seeking to target Android users. Those shifts come after many in the digital-ad industry warned that Apple’s changes, which the tech behemoth framed as part of a wider user-privacy crackdown, would limit advertisers’ access to data regarding consumers and harm their business.

Digital advertisers state that they have lost much of the granular data that made mobile ads on iOS devices effective and justified their prices. In recent months, ad-buyers have deployed their iOS ad spending in much less targeted ways than were previously possible, marketers and ad-tech companies say. The shortage of user data to empower Facebook Inc’s powerful ad-targeting tools minimizes their effectiveness and appeal among a few advertisers.

Apple, for its part, sells ads only in a handful of its own apps and doesn’t take a cut of ad revenue in third-party iOS apps. While advertisers have changed their habits of spending across the ad products of Apple’s big competitors like Facebook and google-which depend much more on ad revenue-it isn’t clear yet how the change has affected overall spending across digital-ad giants.