New Zealand was once a paradise for birds. Then humans arrived, bringing mammalian invaders like rats and later possums. Within 700 years half the weird and wonderful feathered creatures that had dominated the ecosystem had fallen off their perches. It’s well known it takes a lot longer to fix damage like this than it does to cause it, but how much longer? A new study gives a shockingly long answer.

No amount of time will bring back the exact species we have lost, such as the giant moa, let alone the just discovered giant ex-parrot Heracles extinct long before humans arrived. We shall not see their like again. However, that wasn’t the question Dr Luis Valente of Berlin’s Museum fur Naturkunde sought to answer. Instead, he wanted to know how long it would take before New Zealand had as many native bird species as it did the day before the first humans arrived. The specifics of the species would certainly be different, and impossible to predict, but their numbers can be calculated by the rate at which evolution took place in the past.

In Current Biology Valente reports the answer is 50 million years, three-quarters of the time since the last mass extinction. If all the birds currently considered threatened or endangered were to die out, it would take 6 million years just to restore current levels of diversity.

“Some people believe that if you leave nature alone it will quickly recuperate, but the reality is that, at least in New Zealand, nature would need several million years to recover from human actions – and perhaps will never really recover," Valente said in a statement.

The figure is reached using a new method Valente has helped design to measure biological diversity’s recovery time. He considered New Zealand the ideal location on which to run the analysis as the last large landmass for occupied by humans, and the site of extraordinary avian richness in the absence of mammals aside from bats.

Moreover, Valente said, "The anthropogenic wave of extinction in New Zealand is very well documented, due to decades of paleontological and archaeological research. Also, previous studies have produced dozens of DNA sequences for extinct New Zealand birds, which were essential to build datasets needed to apply our method."

Using these sequences Valente estimated the time it takes for new species to evolve in each different family of birds, as well as the rate of natural extinction and colonization from other places.

Valente noted that while New Zealand’s past is devastating, it has recently become a world leader in bird preservation. He hopes applying his methodology to other islands losing their special character will bring home the depth of damage, and inspire these places to adopt similar conservation programs.