“The switches weren’t turned off, but there has been a real decrease in quality” of communications, said one United States official, who like others quoted spoke on the condition of anonymity to discuss intelligence programs.

The drop in message traffic after the communication intercepts contrasts with what analysts describe as a far more muted impact on counterterrorism efforts from the disclosures by Mr. Snowden of the broad capabilities of N.S.A. surveillance programs. Instead of terrorists moving away from electronic communications after those disclosures, analysts have detected terrorists mainly talking about the information that Mr. Snowden has disclosed…

American counterterrorism officials say they believe the disclosure about the Qaeda plot has had a significant impact because it was a specific event that signaled to terrorists that a main communication network that the group’s leaders were using was being monitored. The sharpest decline in messaging has been among the Qaeda operatives in Yemen, officials said. The disclosures from Mr. Snowden have not had such specificity about terrorist communications networks that the government is monitoring, they said.

“It was something that was immediate, direct and involved specific people on specific communications about specific events,” one senior American official said of the exchange between the Qaeda leaders. “The Snowden stuff is layered and layered, and it will take a lot of time to understand it. There wasn’t a sudden drop-off from it. A lot of these guys think that they are not impacted by it, and it is difficult stuff for them to understand.”