MONTREAL, Quebec — Roughly 190 countries early on Monday approved a sweeping United Nations agreement to protect 30 percent of the planet’s land and oceans by 2030 and to take a slew of other measures against biodiversity loss, a mounting under-the-radar crisis that, if left unchecked, jeopardizes the planet’s food and water supplies as well as the existence of untold species around the world.

The agreement comes as biodiversity is declining worldwide at rates never seen before in human history. Researchers have projected that a million plants and animals are at risk of extinction, many within decades. The last extinction event of that magnitude was the one that killed off the dinosaurs 65 million years ago.

While many scientists and advocates had pushed for even stronger measures, the deal, which includes monitoring mechanisms that previous agreements had lacked, clearly signals increasing momentum around the issue.

“This is a huge moment for nature,” Brian O’Donnell, director of the Campaign for Nature, a coalition of groups pushing for protections, said about the agreement. “This is a scale of conservation that we haven’t seen ever attempted before.”

Overall, the deal lays out a suite of 23 environmental targets. The most prominent, known as 30×30, would place 30 percent of land and sea under protection. Currently, about 17 percent of the planet’s land and roughly 8 percent of its oceans are protected, with restrictions on activities like fishing, farming and mining.

The United States is just one of two countries in the world that are not party to the Convention on Biological Diversity, largely because Republicans, who are typically opposed to joining treaties, have blocked United States membership. That means the American delegation was required to participate from the sidelines. (The only other country that has not joined the treaty is the Holy See.)

President Biden has signed an executive order that would similarly place 30 percent of United States land and waters under protection, but any legislative efforts to support that goal are expected to face strong opposition when Republicans take control of the House in January.

Countries also agreed to manage the remaining 70 percent of the planet to avoid losing areas of high importance to biodiversity and to ensure that big businesses disclose biodiversity risks and impacts from their operations.

Now, the question is whether the deal’s lofty targets will be realized.

A previous 10-year agreement failed to fully achieve a single target at the global level, according to the body that oversees the Convention on Biological Diversity, the United Nations treaty that underpins the old agreement and the new one reached here on Monday. But negotiators said they had learned from their mistakes, and the new pact includes provisions to make targets measurable and to monitor countries’ progress.

“Now you can have a report card,” said Basile van Havre, a Canadian who was a co-chairman of the negotiations. “Money, monitoring and targets” would make the difference this time, he said.

While there are multiple causes of biodiversity loss, humans are behind each one. On land, the biggest driver is agriculture. At sea, it’s overfishing. Other factors include hunting, mining, logging, climate change, pollution and invasive species.

The agreement aims to address these drivers. Target 17, for example, commits to reducing the overall risk from pesticides and highly toxic chemicals by at least half, while also addressing fertilizer runoff.

Conservation groups had pushed for stronger measures related to extinctions and wildlife populations.

Anne Larigauderie, an ecologist and the executive secretary of the intergovernmental scientific platform on biodiversity, known as IPBES, regretted that omission but praised the overall agreement as ambitious and quantified.

“It’s a compromise, but it’s not a bad one,” Dr. Larigauderie said.

Questions over how to balance the deal’s ambition with the ability of countries to pay for it generated sharp disagreements at the talks, along with demands to create a new global biodiversity fund. China, which led the talks, and Canada, which hosted, worked to strike a delicate middle ground.

The European Union had sought more forceful conservation targets. Indonesia wanted more leeway on how it used nature.

An outsize amount of the world’s biodiversity lives in countries of the global south. But these nations often lack the hefty financial resources needed to restore ecosystems, to reform harmful agriculture, aquaculture, fisheries and forestry practices; and to conserve threatened species.

Developing countries pushed hard for more funding, with representatives of dozens of countries from Latin American, Africa and Southeast Asia walking out of meetings on Wednesday in protest that they weren’t being heard.

The Democratic Republic of Congo expressed fierce opposition and held up final approval into the early hours of Monday morning. When the president of the talks proceeded over the Congolese objections, delegates from several African countries spoke out in protest.

The deal reached on Monday would roughly double overall biodiversity financing to $200 billion a year from all sources: governments, the private sector and philanthropy. It earmarks up to $30 billion per year to flow to poor countries from wealthy nations. The financial commitments are not legally binding.

Representatives of developing countries said that money should not be seen as charity.

Joseph Onoja, a biologist who directs the Nigerian Conservation Foundation, noted that the former colonial powers had grown rich by exploiting natural resources all over the world. “They came around and plundered our resources to develop themselves,” he said.

Now that developing countries are trying to use natural resources for their own growth, he said, they’re being told they must preserve them in the name of global conservation.

Dr. Onoja, a conservation biologist, said he believed in protecting nature but wanted industrialized countries to be held accountable for past actions.

A study by the Paulson Institute, a research organization, found that reversing biodiversity decline by 2030 would require closing a financing gap of about $700 billion per year.

A major source of funding could come from reallocating the hundreds of billions or more per year currently spent on subsidies that harm nature, such as certain agricultural practices and fossil fuels. Target 18 has the world reducing those by at least $500 billion per year by 2030.

Indigenous rights have been a point of contention around the 30×30 idea. Some feared that the measure could cause communities to be displaced, while others championed the target as a means to secure Indigenous land rights and called for an even higher percentage of land to be placed under protection.

Maisa Rojas Corradi, Chile’s environment minister and a climate scientist, said she hadn’t grasped the depth of the biodiversity crisis until a major intergovernmental report on the topic in 2019. Returning home, she said, her plan is to bring other ministers on board. While conceding that agriculture issues are especially tricky right now because of food security issues triggered by the Russian invasion of Ukraine, she said it was important to press ahead.

“We have to understand that there will be no food on the planet with no biodiversity.”