Three species thought to be extinct have been found again, to the delight of conservationists.

In the UK, the rare ghost orchid, declared extinct in this country just last year, has been found in England, and a caddisfly – a small flying insect – last seen more than a century ago has been discovered again in Scotland. On the global stage the yellow-spotted bell frog, presumed "possibly extinct" by the International Union for the Conservation of Nature, has been seen on a creek-bed in Australia.

The good news stories follow a warning by a leading IUCN expert that humans are now driving plants and animals to extinction faster than new species can evolve.

Simon Stuart, the IUCN expert who chairs the Species Survival Commission which declares species endangered or extinct, said that although roughly one "possibly extinct" species each year was re-discovered, many more plants and animals were added to the list.

There will also be continuing concern for species that are re-discovered in very small numbers. For example only a single 5cm high ghost orchid was found by the botanists who revealed it is still alive in the UK. The sighting of the caddisfly by a PhD student beside a river in north-west Scotland – 350 miles north of the previous record, according to the conservation charity Buglife – could further suggest the influence of climate change in driving species out of their traditional habitats, something some plants and animals will be able to adapt to better than others.

"The whole point of the 'possibly extinct' list is they can come in and out," said Stuart. "But we're adding species on to the 'possibly extinct' list much faster than we're taking them off it."

The IUCN has much stricter rules about declaring a species fully extinct, including that it must have been actively searched for by teams of experts in the field. However in 2008 the Switzerland-based organisation did have to move the Miles' robber frog (Craugastor milesi) from the extinct to critically endangered list after a single specimen was found in Honduras.

Among the reasons conservationists dislike a species being declared extinct are that it is no longer possible to get money to research and preserve its habitat. The locations of the orchid and the Australian frog are both being kept secret to protect them, however one of the bell frogs and a tadpole have been taken to Taronga zoo in Sydney for a captive breeding programme.