Oxfordshire County Council voted to acknowledge an ecological emergency across the county during a full meeting on Tuesday, November 3.
The proposal was tabled by the Green Group.
The council had backed the Climate and Ecological Emergency Bill in 2020.
Its latest resolution also requires the ecological crisis to shape future policy and decision-making.
Oxford City Council, South Oxfordshire District Council and West Oxfordshire District Council have previously acknowledged ecological emergencies.
Cherwell District Council and Vale of White Horse District Council have instead declared climate emergencies.
Green councillor Emma Markham, who represents a rural farming community, told councillors that productive agriculture relies on good soil, clean water, pollinators and functioning natural habitats.
She said those assets were facing growing pressure and that the UK ranks among the countries with the greatest loss of nature.
Falling insect numbers could damage crop yields, raise production costs and undermine food security.
Markham said the proposal did not place conservation and economic growth in direct competition.
Instead, she argued that long-term prosperity relies on restoring natural systems.
Making nature recovery a strategic priority could strengthen farming and food security, reduce flood risk and support a healthier economy, she added.
The resolution also instructed the council to back the Nature and National Security Bill as it progresses through Parliament.
The bill has backing from numerous security and military specialists.
It treats the natural environment as nationally important infrastructure, alongside roads, energy networks and water supplies.
A majority of councillors voted for the motion, while Conservative and independent members abstained.
Green councillor James Barlow said ecological considerations should be central to all Oxfordshire County Council work, but noted that they currently were not.
He called for a practical approach based on the need for a healthy environment if the local economy is to prosper.
Oxfordshire is experiencing rapid population and economic expansion, with significant housing and employment development around Oxford, Bicester, Didcot and the Harwell Campus.
Plans for major solar farms are also under way at Botley West and Cearn Solar Farm, among the largest proposed sites in the UK and Europe.
Nationally protected landscapes account for around 27 per cent of Oxfordshire.
They include sections of the Chilterns National Landscape and the Cotswolds National Landscape.
Nevertheless, important habitats are divided from one another by roads, settlements and intensively farmed land.
Nationally, species abundance has declined by approximately 19 per cent since 1970, while nearly one in six species is threatened with extinction.