Species have evolved and disappeared since the very first species (thought to be microorganisms living in hydrothermal vents) made an appearance on Earth. Some species outcompeted others for access to limiting resources; some were driven to extinction by dangerous pathogens; some just found it hard to survive in constantly evolving ecosystems. While many extinction events have been rather limited in scope and, hence, caused only one or a few extinctions at a time, there have been instances where perturbations were so impactful that they drove very large numbers of species to extinction over a short period of time. There have been five such past mass extinction events—periods marked by the sudden and dramatic loss of a large percentage of species (Figure 8.1). But these mass extinctions have also been followed by periods that favoured increased rates of speciation, during which new species evolved to fill the niches left empty by the extinctions.
Nature’s ability to balance extinctions with speciation was greatly disturbed around 300,000 years ago, when Homo sapiens made their appearance on Earth. Since then, humans have gradually increased their dominance on the natural world, leading to large-scale restructuring and destruction of biological communities. Human modifications of Earth’s climatic, biological, and geochemical environments accelerated greatly during the rise of agriculture (12,000–15,000 years ago) and again during the Industrial Revolution (1760–1840), when fossil fuel usage and urbanisation became the norm. Now, many scientists recognize today’s new and distinct human-dominated geological epoch, the Anthropocene (Waters et al., 2015). One notable feature of the Anthropocene is that species extinctions are increasing at such rapid rates that many conservation biologists now recognize that we are also witnessing the beginnings of Earth’s sixth extinction episode (Barnosky et al., 2011; Ceballos et al., 2017). However, unlike previously, this extinction episode is caused by human activities rather than natural events.