Insects are the most abundant and diverse group of animals on Earth. They are critical to ecosystem function in terrestrial and aquatic systems, yet they are one of the most understudied groups of organisms. Only a small proportion of the more than five million insect species have been assessed by the IUCN Red List. For most of these species, there is not enough evidence to know what is happening to their populations. In fact, for most insect species globally, there is very little data available on where they live, how they live and what environmental conditions they need to persist in the long term. A number of threats affect insect biology and life cycles generally, including climate change, habitat clearing, invasive species, use of broad-spectrum pesticides, and pollution of soil and waterways. These threats should be addressed immediately to prevent further declines in insect populations. To understand insects better, greater investment in research and documentation of the world’s insect diversity is urgently needed.

The conservation status of the taxa in this book is measured using the criteria of the Red List of Threatened Species™. The Red List is overseen by the International Union for Conservation of Nature (IUCN), and categorises species according to extinction risk. This chapter summarises the history of the Red List and explains the criteria used to assess species’ extinction risk, as well as the quality control procedures in place today. This chapter also introduces a new part of the Red List, formalised in 2021: The Green Status of Species, a set of metrics which assess species’ progress towards functional recovery across its range and the impact of conservation actions.

In Chapter 1 it was emphasised that the first cellular life forms to evolve were bacteria, and that photosynthetic bacteria called cyanobacteria released oxygen through photosynthesis and thus set in train the conditions which allowed the many forms of aerobic life to evolve.

Worldwide, freshwater biodiversity is in decline and increasingly threatened. Fishes are the best-documented indicators of this decline. General threats to persistence include: (1) competition for water, (2) habitat alteration, (3) pollution, (4) invasions of alien species, (5) commercial exploitation and (6) global climate change. Regional faunas usually face multiple, simultaneous causes of decline. Threatened species belong to all major evolutionary lineages of fishes, although families with the most imperilled species are those with the most species (e.g. Cyprinidae, Cichlidae). Independent evaluation of California’s highly endemic (81%) fish fauna for comparison with IUCN results validates the alarm generated by IUCN evaluations. However, IUCN overall evaluation is conservative, because it does not include many intraspecific taxa for which extinction trends are roughly double those at the species level. Dramatic global loss of freshwater fish species is imminent without immediate and bold actions by multiple countries.

Only four terrestrial invertebrate phyla dealt with in this chapter appear in the IUCN Red List of Threatened Species. Three of these phyla are composed of mostly marine animals, but all the listed species of Nemertea and Platyhelminthes are limnoterrestrial, and of the 224 listed annelids, 222 are limnoterrestrial. Conservation issues related to their marine counterparts are discussed in other chapters of this book.

Numerous non-insect limno/terrestrial arthropods appear in the IUCN Red List of Threatened Species. Nearly all arachnids and myriapods are terrestrial, but within Pancrustacea, many taxa can inhabit marine, limnic and terrestrial environments and it is not possible to easily disentangle the numbers of listed species without sorting them species by species. In some cases, as in Malacostraca, the number provided includes species inhabiting either environment, or even cases of amphidromous species that spend part of their life cycle at sea and part in rivers and streams, as is the case of many shrimp species.

Fishes are the original and most diverse group of vertebrates, including over 35,000 of the estimated 69,000 species with backbones. Most marine fishes have large geographic ranges that may provide some protection from extinction, but there are very important exceptions

Non-marine molluscs stand out as the major animal group under the most severe threat. Among the 8664 mollusc species evaluated for the IUCN Red List (version 2019-1), 300 are considered Extinct out of a total 872 listed Extinct species. However, only ~10% of molluscs have been evaluated and other assessments of the number of extinct species are much higher, 3000 to over 5000, almost exclusively non-marine species. As for most other groups, threats faced by non-marine molluscs are habitat loss, probably the most important, but also impacts of introduced species, exploitation, generally of less concern, and climate change, likely to have serious effects into the future. Oceanic island species, often narrowly endemic, are especially threatened and constitute a high proportion of recorded extinctions. Anthropogenic activities have caused non-marine mollusc extinctions since prehistory, but threats have increased greatly over the last few centuries and will probably continue to increase. Most mollusc species for which a population trend has been evaluated by IUCN are stable or declining; those few that are increasing are primarily introduced and invasive. Most threatened are oceanic island snails, North American and other freshwater bivalves, and the diverse and highly endemic micro-snails of Southeast Asian limestone outcrops.

One species – humans – is ultimately responsible for devastating much of the only planet in the cosmos that is known to support life – Earth. Our population has expanded exponentially since the Industrial Revolution and this, along with the resources required to sustain us, is ultimately driving the decline in the condition of the world’s ecosystems. Examples of species threatened by development (residential/commercial), agriculture, energy production and mining, transportation, biological resource use, natural system modification, invasive species, pollution and climate change are provided. Yet there are solutions to these problems and many species and ecosystems have bounced back from the brink of extinction. Provided the general public and politicians have the will to invest sufficiently in conservation, it can be highly successful. This chapter ends by exploring the variety of ways that humanity can use to improve the status of the world’s ecosystems.

Reptiles, despite being among the largest and most ecologically important vertebrate groups, have until recently received less research attention than other terrestrial vertebrates and their conservation has been hampered by a lack of both data and interest. Around 20% of reptile species are thought to be at risk of extinction (rising to 50% in turtles), but population trends for most species are not known with certainty and the IUCN Red List does not yet have complete coverage for this group. Reptiles are at particular risk from habitat loss and fragmentation, invasive species and overharvesting. They are thought to be especially sensitive to climate change, the effects of which are probably underestimated due to data limitations. The impacts of a recently identified emergent disease in snakes are unclear but may become a significant driver of future declines. Conservation successes among reptiles are scarce, but notable achievements include population recoveries in most crocodilian species since the 1970s and successful control of invasive species that threatened many island reptiles, particularly in New Zealand. There is a pressing need to better understand the ecology and conservation needs of most reptiles, and to increase their representation in conservation planning.

This chapter will attempt to predict what the future will hold for life on Planet Earth. It reviews the changes which have come about and which will continue as a result of factors such as climate change. The chapter also discusses the emergence of the Extinction Rebellion movement, the likely future declines in insects and other species, as discussed in the 2019 State of Nature report, and the factors that are likely to continue to impact survival of wild species on Planet Earth. In closing, the chapter outlines some likely future increases in species adapting to life in large cities.

Spore-bearing land plants are much fewer in number than flowering plants, with around 20,000 bryophytes and 12,000 pteridophytes, but they have a much longer history, with the first recognisable land plant fossil dating from the Silurian. Bryophytes and pteridophytes are not a significant food source for man, nor do they provide essential commodities like timber or cloth, but they have a significant role in maintaining healthy ecosystems and storing carbon, and bryophytes deliver key ecological functions in arctic, boreal and peatland ecosystems. The major threats to bryophytes and pteridophytes are habitat loss and climate change, followed by overexploitation. Global conservation assessments are available for just 1.5 percent of bryophyte species and 5.7 percent of pteridophytes. However, progress towards an accessible worldwide flora is growing through international collaboration and coordination, and molecular studies are increasing understanding of relationships between species, genera and families.

Over the millennia, and across all cultures, people have developed an intimate bond with birds and, for many, birds are their principal connection to the natural world. With so many eyes trained on the planet’s avifauna, birds provide us with a unique insight into the unfolding extinction crisis; the sixth such episode in our planet’s 4.5-billion-year history and the first to be driven by the actions of a single species – our own. Avian extinction risk is comprehensively assessed by BirdLife International using the criteria of the IUCN Red List. The situation is alarming – around the world, birds are in steady decline, with approximately one in eight species now at risk of extinction. Each year, more species slip closer to extinction, whilst even once common birds are now disappearing fast. Yet the universal appeal of birds provides cause for hope. Their plight has been a rallying point around which a large and growing conservation movement has coalesced. A century of global bird conservation has demonstrated that when sufficient effort, resources and political will are brought to bear, bird populations can rebound and their habitats can be restored. Although imminent, mass avian extinction is not (yet) inevitable, and may still be averted if we so choose.

Amphibians are the most threatened vertebrate class on Earth. They play important roles in ecosystems and are often cited as sentinels of environmental health. Around 84% of the 8208 amphibian species have been assessed by The IUCN Red List of Threatened Species, with 41% categorised as threatened with extinction. As is the case with other species, the main threatening process for many amphibians is habitat destruction, disturbance and fragmentation. However, amphibians are also highly vulnerable to emerging infectious diseases, climate change, invasive species and pollution. These threats often interact, resulting in complex impacts on amphibian populations. Fortunately, there are several initiatives (IUCN SSC Amphibian Specialist Group (ASG), Amphibian Survival Alliance (ASA) and the Amphibian Ark(AArk)) focused on understanding and protecting the many threatened species through global coordination, conservation planning, habitat protection, supporting conservation action, fundraising, emergency rescues and captive breeding for conservation. Diverse amphibian lifestyles, coupled with the complexity of threats, means that different species will respond in different ways and in different places. Consequently there are likely to be ‘winners’ and ‘losers’ in a changing world, rather than complete extinction of a class. Amphibian conservation therefore remains one of the greatest challenges of our times.

The importance of plant diversity to humankind is immense. Plants are primary producers defining and supporting ecosystems worldwide and providing a wide range of ecosystem goods and services. Vascular plants are species characterised by a well-developed system of specialised cells that transport water, minerals and photosynthetic products and provide structural support enabling plants to grow on land. Flowering plants, also known botanically as angiosperms, are by far the largest group of vascular plants. They are characterised by their distinctive reproductive structures, the flowers. Designed to be pollinated by wind, insects or other animals, all flowers have ovules, which become seeds after fertilisation, enclosed within an ovary. In contrast, the gymnosperms (see Box 2.1), which include conifers and cycads, have reproductive structures with naked seeds that are not enclosed. The other groups of vascular plants are ferns and fern allies which do not produce seeds.