Global aspects of biodiversity loss are widespread. Tropical habitats in Africa, South America and South-East Asia have been decimated by deforestation and encroachment by increasing numbers of people. International organisations including the United Nations (UN) have considered population issues, but the list of sustainable goals propounded by the UN makes no more than peripheral mentions of population pressures. There have been international conferences on major environmental issues including climate change and biodiversity declines, but again with few mentions of the impact of the human population. However, there has been a series of international meetings concerned with population growth. Unfortunately, these have proved disappointing, frequently sidetracked into areas that have not resulted in any consensus on the development of population policies.

Wildlife in Britain has been a source of delight for many generations of people. It has also been the inspiration for writers and poets, and has become the subject of serious scientific study. Media coverage has increased steadily, promoting wide interest in animals and plants in the UK. However, there are both costs and benefits arising from wildlife. Damage, including to crops and trees, by a range of ‘pest’ species can be extensive, and conservation measures such as maintenance of nature reserves are also costly. Nevertheless, these downsides are more than outweighed by the benefits to people and the economy. A wide range of ecosystem services make use of wildlife and healthy ecosystems, and access to these outdoor delights is highly valuable in the promotion of mental health. Unfortunately, more than half of Britain’s wildlife has been in continuous decline for several decades. Proximal reasons for this sad fact are summarised before considering them, and the primary issue of human numbers, in subsequent chapters.

The history of recent wildlife extinctions and the widespread role of humans in these events are described. Conflicting views about the role of human numbers in wildlife declines are cited. There follows an outline of the book structure, highlighting the major proximal causes of declines in Britain , including persecution, urbanisation, agricultural intensification, climate change and disease, as detailed in subsequent chapters. There are also chapters on population growth, people's perceptions of population size and the implications for conservation A comparison of population densities in a range of countries reveals that the UK, especially England, has one of the highest densities in the world. The UK therefore provides an ideal study to investigate the impacts of human numbers on wildlife declines.

Humans have a long history of destroying wildlife in a variety of ways. Initially, this was mostly manifest as hunting for food and removal of competing predators. Some species were exterminated by these activities. Later came collection, especially of rare species, for museums and private collections. Plants and bird eggs were especially vulnerable, as were attractive invertebrates such as butterflies. Despite protective legislation, collection continues around the world, and the wildlife trade is thriving. Persecution of ‘pest’ species also continues apace, where the definition of pest is highly contentious. Badgers are killed regularly, as are animals likely to predate game on shooting estates. Predation of wildlife for human consumption is in most cases now only carried out on a small scale, but marine fish are an exception. They continue to be harvested around UK shores, and elsewhere, in unsustainable numbers. Pressure on fish stocks is exacerbated by increasing numbers of human consumers.

Opinion polls indicate that many people in the UK are concerned about wildlife declines and about overpopulation. These feelings are widely shared by naturalists, scientists, artists and many religious groups, as well as by the general public. Unfortunately, such views are uncommon among economists and rarely feature at all in politics. Discussion of population pressure has remained largely taboo, even in wildlife circles, presumably because of fear of causing offence. However, there are adverse consequences for society from a high human population that go far beyond problems for wildlife and countryside. Traffic jams are health hazards, both physically and mentally,and infrastructure expansions generate stress for those affected by them, while public services including healthcare and education are increasingly overwhelmed by people needing to use them.

Wildlife conservation in the UK is based on three main actions: landscape-scale protection of habitats, species protection, especially of rarities, and site management to maintain viable ecosystems. None of these addresses human population issues, and policies to deal with population growth vary around the world. Disturbingly, in many developed countries, there is concern about declining human numbers, and attempts are underway to promote larger families. With greater foresight, the developing world is in many cases trying to curb growth, even in countries with lower population densities than in western Europe. At the time of writing, Britain has no policy on population, and the possible components of such a policy are discussed. It will be important to maintain current small average family sizes, but most critical will be a humane but effective control of immigration. In the long term, this will be key to reversing wildlife declines in the UK.

Farming has experienced a major revolution in post-war Britain. The advent of artificial fertilisers and powerful pesticides transformed agriculture, which, combined with financial subsidies, greatly increased food production starting in the 1950s. These changes proved devastating for farmland wildlife. Hedges were removed to increase field sizes, ponds were discarded, autumn ploughing and use of silage devastated wildflower meadows. Many species of plants, invertebrates and birds declined dramatically as a result of these ‘improvements’. Drainage, water abstraction and pollution from fertilisers virtually exterminated many freshwater organisms, including amphibians, over much of the countryside. The marine environment has not been unscathed, with ongoing damage from offshore fish farms and bottom trawling. Dense conifer plantations, initiated after the First World War, have wrecked precious habitats such as heathlands and have precipitated declines of rare species dependent on them. The primary objective of agricultural intensification, a move towards self-sufficiency in food production, has not been met and has actually decreased as the human population has expanded.

The global human population has increased hugely since the mid-nineteenth century and stands at almost 8 billion at the time of writing. This trend is mirrored in Britain, especially in England, with a total UK population of almost 68 million in 2020. Predictions imply that global increases will slow down, perhaps peaking at around 10 billion by 2100. Three factors contribute to changes in population size. In Britain, the reproductive rate has been below the replacement level of around 2.1 children per couple for several decades. The ongoing increase in human numbers has been dictated primarily by the other two factors. Longevity has increased steadily; people are living longer. However, the most significant driver by far in recent decades has been the high level of net immigration into Britain. Wildlife declines are statistically related to human population density across Western Europe at least with respect to two well-studied taxonomic groups, amphibians and birds.

Three factors also related to impacts on wildlife are climate change, disease and disturbance. The UK has warmed significantly in recent decades, particularly in winter. This is having two main consequences for wildlife. Many plants and animals that reproduce in spring are doing so ever earlier. This phenological change has the potential to disturb food webs, although this does not yet seem to have happened to any significant extent. However, a second consequence, changes in species distributions, has certainly occurred. Many plants and animals have expanded northwards in Britain, and some newcomers have arrived from mainland Europe. On the downside, high alpine plants have declined and some seabirds have suffered from warming-related faunal changes in surrounding seas. Disease is a second factor that has caused some major species declines, including viral infections of rabbits and amphibians, as well as fungal mortality in trees. Finally, excessive disturbance by human footfall has generated environmental damage and at least local declines of sensitive species. This factor clearly relates to human numbers, but climate change is also influenced by the number of people on the planet as a whole.

Urban sprawl has consumed large areas of countryside in Britain, and continues apace in conjunction with associated transport infrastructure. Statutory protection measures aimed at restricting the impacts of urbanisation are in place but are frequently overruled by development decisions. Important wildlife sites with rare species have been lost despite nominal protection. New roads have fragmented wildlife populations and large numbers of some animal species fall victim to traffic death. In some cases, local extinctions have been the result of road deaths. Another consequence of increased urbanisation is a rise in pollution levels, manifest on land, in the air and in the water. Plants, wild animals and humans have all suffered from increased pollution. Air pollutants kill many people annually, and the amount of plastic in marine environments has reached unparalleled proportions. All of these factors clearly relate to the numbers of people living in the UK.