The flow of people across the United States’ southern border has long depended on many factors, including the economy and violence in their home countries, and even the weather. But chief among them is America’s own immigration policy, and experts said they were not surprised to see a slowdown even before Mr. Trump’s executive orders on immigration have taken full effect or his signature wall has been built.

“Deterrence through perception is central to these executive orders,” said Faye Hipsman, a policy analyst at the Migration Policy Institute. “Even floating the possibility of expanding detention at the border makes somebody less likely to come.”

Since taking office, Mr. Trump has made clear that undocumented immigrants will have a tougher time getting into and staying in the United States. Beyond planning to build a border wall, a rallying cry during his campaign, he has expanded the authority of immigration enforcement officers and pledged to hire more of them; raised the threshold for entry to the United States based on claims of persecution back home; and called for more detentions and quicker deportations of those already in the country illegally.

Mark Krikorian, executive director of the Center for Immigration Studies, a Washington-based group that favors more limits on immigration, said all signs pointed to a decline in migration similar to that seen after the Reagan administration’s sweeping immigration reform law of 1986. It gave amnesty to many immigrants who had entered the United States before 1982, while promising tighter security at the Mexican border and tough penalties for companies hiring undocumented workers.