The poo of large animals that are now extinct was key to the Earth's nutrient cycle, new research has shown.
In the past, the whole world looked like an African savannah with large, free roaming, animals.
These so-called megafauna evolved with the current ecosystem and probably would still be here had human hunting practices and climate change not led to their extinction.
By eating nutrient rich food, travelling large distances and then depositing their waste in a different location, these animals kept the nutrient content on land well distributed.
Many of these megafauna have gone extinct, and without them, we are left with small areas of land with high nutrient concentrations separated by areas with very low nutrient content.
A similar nutrient cycle disruption is happening out at sea, describes the publication in PNAS this week.
"Normally nutrients flow from mountain tops to ocean bottoms," explains lead researcher Chris Doughty.
This process is reversed by the help of marine mammals, seabirds and migrating fish.
Large marine mammals, such as whales, feed in deep water, collecting nutrients and then depositing them at the surface when they come up to breathe.
The decline in whale population has led to a reduction of vertical transport of the nutrient phosphorus by 77%.
They are then picked up by seabirds, and transferred back to the land. Seabird populations have also decreased, reducing this movement of phosphorus by 96%.
Phosphorus is required for plant growth, and so impacts both plant and animal life. It also affects humans, as we use it for fertiliser.
Mining phosphorus to use in fertiliser makes the problem worse by adding to the flow of nutrients out to sea.
"Easily accessible mined phosphorus may run out in as little as 50 years," explains Doughty.
To prevent phosphorus reserves being depleted, "we could try and restore this natural system of recycling phosphorous that existed in the past," he says. To restore marine animal populations, "we just need to stop hunting them."
On land, the situation is more complex, as the planet becomes ever more crowded.
"Biodiverse, fenceless pasture systems" might help to increase the flow of nutrients on land, says Doughty.