Witness

In 1975, San Diego Zoo began placing tissue samples of rare animals in cryogenic storage for the benefit of future generations. Called the Frozen Zoo, the refrigeration system now contains the cells of more than 1000 species ranging from the white rhinoceros to the black-footed ferret. Scientists are now using the collection to try to save species threatened by extinction. Simon Watts talks to Dr Oliver Ryder, who has worked at the Frozen Zoo from the very beginning. PHOTO: Northern White Rhino cells in the Frozen Zoo (San Diego Zoo Institute For Conservation Research)

Whales were being hunted to extinction, when in 1967, a biologist called Dr Roger Payne realised they could sing. It changed the perception of whales and helped found the modern conservation movement. Claire Bowes spoke to Dr Payne about his discovery in 2017. This programme is a rebroadcast. (Photo: Humpback Whale, courtesy of Christian Miller of Ocean Alliance)

Silent Spring, written by marine biologist Rachel Carson, looked at the effect that synthetic pesticides were having on the environment. Within years of its publication in 1962, the widespread use of DDT had been outlawed in the USA. Louise Hidalgo has been speaking to Carson's adopted son Roger Christie about the author and her work. Image: A copy of Silent Spring (Credit: Science Photo Library)

A flightless bird, the dodo became extinct just decades after being discovered on the uninhabited island of Mauritius by European sailors. Because dodos couldn't fly they, and their eggs, were eaten by explorers and the cats and rats that came with them on board their ships. By the late 1600s there were none left. Simon Watts charts the demise and subsequent popularisation of the dodo. Image: An engraving of a dodo. Credit: Science Photo Library.

How scientists discovered that a deadly fungus was killing off amphibians around the world. The chytrid fungus has caused the greatest loss of biodiversity in our time. Alejandra Martins spoke to biologist Dr. Karen Lips, one of the key scientists who unravelled the mystery of the extinctions. Photo: dead frog infected with chytrid fungus. Credit: Forrest Brem