With a million species at risk of extinction, Sir David Attenborough explores how this crisis of biodiversity has consequences for us all, threatening food and water security, undermining our ability to control our climate and even putting us at greater risk of pandemic diseases.
Everything in the natural world is connected in networks that support the whole of life on earth, and we are losing many of the benefits that nature provides to us. The loss of insects is threatening the pollination of crops, while the loss of biodiversity in the soil also threatens plants growth.
Last year, a UN report identified the key drivers of biodiversity loss, including overfishing, climate change and pollution. But the single biggest driver of biodiversity loss is the destruction of natural habitats. Seventy-five per cent of Earth's land surface (where not covered by ice) has been changed by humans, much of it for agriculture, and as consumers we may unwittingly be contributing towards the loss of species through what we buy in the supermarket. Human activities like the trade in animals and the destruction of habitats drive the emergence of diseases. Disease ecologists believe that if we continue on this pathway, this year’s pandemic will not be a one-off event.

The next instalment focuses on dietary needs and how different species have evolved beaks to suit their individual requirements. The latter come in a multitude of forms. Blue tits and goldfinches have beaks akin to tweezers, with which to extract seeds, while the hawfinch's razor-like bill can deal with a cherry-stone. However, the crossbill is the only finch that can twist its mandibles in opposite directions. Jays store acorns for winter by burying them in the ground, whereas woodpeckers can keep up to 60,000 of them in one tree trunk. Sap is also desirable, and there are a variety of methods used to obtain it. The hoatzin is the only specialised leaf-eater, and accordingly has a digestive system more akin to that of cattle. Plants recruit birds to aid pollination, and offer nectar as a reward. Hummingbirds eat little else, and the sword-bill's beak is the longest of any bird in relation to its body. Insects are also highly prized, and Galapagos finches are shown to possess some ingenuity as they not only strip bark, but also use 'tools' to reach their prey. Crows are hailed as being among the most intelligent birds, and one is shown using a twig to spear a grub within a fallen log. The robin is an opportunist, and Sir Attenborough observes one seizing morsels as he digs a patch of earth. In South America, a cattle tyrant sits atop an obliging capybara and uses its vantage point to spot passing food that may be dislodged by its grazing partner.

The third episode is devoted to the ways in which plants reproduce. Pollen and a stigma are the two components needed for fertilisation. Most plants carry both these within their flowers and rely on animals to transport the pollen from one to the stigma of another. To do this, they attract their couriers with colour, scent and nectar. It isn't just birds that help pollination: some mammals and reptiles also do so. However, it is mostly insects that are recruited to carry out the task. To ensure that pollen is not wasted by being delivered to the wrong flower, some species of plant have developed exclusive relationships with their visitors, and the gentian and its attendant carpenter bees is one example. Since pollen can be expensive to produce in terms of calories, some plants, such as orchids, ration it by means of pollinia and a strategically placed landing platform. Other orchids offer no reward for pollination, but instead mislead their guests by mimicking their markings and aroma, thus enticing males to 'mate' with them (Pseudocopulation). The most extreme fertilisation method is one of imprisonment, and one plant that uses it is the dead horse arum. It is often found near gull colonies, and mimics the appearance and smell of rotting flesh. Blow-flies are attracted to it, and are forced to stay the night before being allowed to depart in the morning, laden with pollen. Finally, Attenborough introduces the world's largest inflorescence: that of the titan arum.