Species are going extinct at 100-1,000 times the natural rate, key habitat is disappearing, and ever more water and land is being used to support people.
Some economists say this is already damaging human prosperity.
The Convention on Biological Diversity (CBD) meeting will discuss why governments failed to curb these trends by 2010, as they pledged in 2002.
Delegates will also try to finalise a long-delayed agreement on exploiting natural resources in a fair and equitable way.
Before the start of the two-week meeting, Achim Steiner, executive director of the UN Environment Programme (Unep), said it was a crucial point in attempts to stem the loss of biodiversity.
"There are moments when issues mature in terms of public perception and political attention, and become key times for action," he told the BBC.
"And this is a moment when the recognition that biodiversity and ecosystems need preservation urgently is high, when people are concerned by it, and are demanding more action from the global community."
A UN-sponsored team of economists has calculated that loss of biodiversity and ecosystems is costing the human race $2 trillion to $5 trillion a year.
Governments first agreed back in 1992, at the Rio Earth Summit, that the ongoing loss of biodiversity needed attention. The CBD was born there, alongside the UN climate convention.
It aims to preserve the diversity of life on Earth, facilitate the sustainable use of plants and animals, and allow fair and equitable exploitation of natural genetic resources.
The convention acquired teeth 10 years later, at the Johannesburg Summit on Sustainable Development.
Noting that nature's diversity is "the foundation upon which human civilisation has been built", governments pledged "to achieve by 2010 a significant reduction of the current rate of biodiversity loss at the global, regional and national level as a contribution to poverty alleviation and to the benefit of all life on Earth".
Since 2002, most measures of the health of the natural world have gone downhill rather than up.
The majority of species studied over the period are moving closer to extinction rather than further away, while important natural habitat such as forests, wetlands, rivers and coral reefs continue to shrink or be disturbed.
"Since the 1960s we've doubled our food consumption, our water consumption," said Jonathan Baillie, director of conservation programmes at the Zoological Society of London (ZSL).
"The world's population has doubled, and the economy has grown sixfold; in 2050 there will be 9.2 billion people on the planet."
There are signs of change in some regions. The forest area is growing in Europe and China, while deforestation is slowing in Brazil.
About 12% of the world's land is now under some form of protection.
But in other areas, countries - particularly in the tropics - have made little progress towards the 2010 target.
Government delegates here will consider adopting a new set of targets for 2020 that aim to tackle the causes of biodiversity loss - the expansion of agriculture, pollution, climate change, the spread of alien invasive species, the increasing use of natural resources - which conservationists believe might be a more effective option than setting targets on nature itself.
Delegates will also be negotiating a draft agreement on exploiting the genetic resources of the natural world fairly and sustainably.
The protocol, named Access and Benefit Sharing (ABS), aims to prevent "biopiracy" while enabling societies with abundant plant and animal life to profit from any drugs or other products that might be made from them.
Agreement on ABS has been pursued since 1992 without producing a result. But after four years of preparatory talks, officials believe the remaining differences can be hammered out here.
"We are confident that on 29 October, we'll celebrate the birth of another baby, with the support of all parties, and we'll have a protocol on access and benefit sharing," said Ahmed Djoghlaf, CBD executive secretary.
"This protocol will be a future investment for the human family as a whole."
However, the bitter politicking that has soured the atmosphere in a number of UN environment processes - most notably at the Copenhagen climate summit - threatens some aspects of the Nagoya meeting.
Some developing nations are insisting that the ABS protocol be signed off here before they will agree to the establishment of an international scientific panel to assess biodiversity issues.
The Intergovernmental science-policy Platform on Biodiversity and Ecosystem Services (IPBES) is due to be signed off during the current UN General Assembly session in New York.
Many experts believe it is necessary if scientific evidence on the importance of biodiversity loss is to be transmitted effectively to governments, in the same way that the Intergovernmental Panel on Climate Change (IPCC) assembles evidence that governments can use when deciding whether to tackle climate change.