A shocking indictment of the effect humans are having on the Earth

The world's populations of animals have dropped by more than half since the 1970s, according to a new survey by the conservation group WWF and the Zoological Society of London (ZSL).

It's a shocking indictment of the effect that the growing numbers of humans are having on the Earth. We are polluting, consuming and degrading ecosystems at an unprecedented rate and many of the worst effects are occuring in some of the world's poorest areas - those least able to deal with the damage.

The core of the WWF's Living Planet Report is the Living Planet Index, a number calculated by ZSL scientists and based on measurements of wild animal populations around the globe.They tracked conservation reports of 10,380 populations of more than 3,000 species (including fishes, amphibians, reptiles, birds and mammals) and found that, on average, their population sizes had dropped by 52 percent between 1970 and 2010.

The decline is the worst in freshwater species, 76 percent of which have declined since 1970. Marine and land-based species have dropped by 39 percent each, though these figures hide tragedies waiting to happen: sea turtle numbers have dropped by 80 percent and forest elephants in Africa could become extinct within a lifetime.

A large part of the problem is increasing human encroachment into the wild parts of the world. Most of us live in cities, which need space and fuel as they grow, and that need chews up ecological habitats, leaving wild animal populations with less space in which to live and breed.

Read: Wildlife population has halved in 40 years, WWF report finds

Pollution also plays its part in further degrading environments. On top of that, animals are being increasingly hunted or fished from their homes.

Waiting in the wings to make things worse for animal populations is something that will make their natural habitats unliveable within a few decades or less: climate change.

Some of that change has already made itself felt in the Arctic, where warming seas and reduced sea ice has led to a decline in polar bear numbers.

The report's authors conclude that we would need, on average, 1.5 Earths to meet the demands that humans are placing on natural ecosystems.

If the entire human population wanted to live at US levels of consumption, we would need four Earths, or 2.5 Earths to live at UK levels of consumption.

"These demands include the renewable resources we consume for food, fuel and fibre, the land we build on, and the forests we need to absorb our carbon emissions," write the report's authors.

"For more than 40 years, humanity’s demand has exceeded the planet’s biocapacity – the amount of biologically productive land and sea area that is available to regenerate these resources. This continuing overshoot is making it more and more difficult to meet the needs of a growing global human population, as well as to leave space for other species."

Forest ecosystems are the direct sources of these things for more than two billion people, a figure that includes most of the world's poorest people, many of whom rely on forests for their direct survival.

Marine areas and fisheries support hundreds of millions of jobs around the world and if they cease to be productive, the economic and social losses to people will be catastrophic.

Populations of vertebrate species, animals that are generally near or at the top of their food chains, are important indicators for the health of our environment. If the numbers of birds, monkeys, elephants, tigers or lions are in freefall, that means their environments are in deep trouble.

"These are the living forms that constitute the fabric of the ecosystems which sustain life on Earth – and the barometer of what we are doing to our own planet, our only home," wrote Marco Lambertini, director general of WWF International, in the introduction to the report.

"We ignore their decline at our peril. We are using nature’s gifts as if we had more than just one Earth at our disposal. By taking more from our ecosystems and natural processes than can be replenished, we are jeopardising our very future."