1.3.1 Introduction

The Earth’s biosphere describes the sum of all global ecosystems. It forms a key part of the Earth system, driving the many biogeochemical cycles that  maintain the climate system and keep Earth habitable (Kump, Kasting, and Crane, 1999). Ecosystems are the complex systems composed of assemblages of living organisms and their physical environment at the local scale (e.g. an area of rainforest in the Brazilian state of Amazonas). 

At a larger scale, they form regional groupings (e.g. Madeira-Tapajós moist forest ecoregion in Dinerstein et al., 2017), ecosystem functional groups (e.g. tropical/subtropical lowland rainforests), biomes (e.g. tropical-subtropical forests), and ultimately the whole biosphere (Keith et al., 2022). Humans are also an integral part of the biosphere, with social systems being so closely intertwined with ecosystems that they can be seen as joint ‘social-ecological systems’ in which the dynamics of both interact as a single complex adaptive system (Folke et al.,2016; 2021; Ellis et al., 2021).

Ecosystems are being globally degraded by multiple human-driven pressures. At the species level, one million animals and plants face extinction (IPBES, 2019). Extinctions are happening at up to 100 times natural background rates averaged over the last century, leading some to assess that the Earth has now entered the sixth mass extinction event in the nearly 4 billion years of life’s history (Barnosky et al., 2011; Ceballos et al., 2015). The Living Planet Index indicates that populations are declining in around half of vertebrate species, with an average decline across all species of 69 per cent since 1970 (WWF, 2022). The key drivers of biodiversity loss in order of importance are land and sea use change, direct exploitation, climate change, pollution, and invasive alien species (IPBES, 2019; Maxwell et al., 2016). Climate change is not currently the leading driver, but will become a substantial threat with further warming (IPBES, 2019). Global warming moving from 1.5 to 2^oC increases the number of species facing the loss of most of their ranges from 4 to 8 per cent for vertebrates (e.g. mammals), 8 to 16 per cent for plants, and 6 to 18 per cent for insects, while 3.2^oC of warming would increase these to 26, 44, and 49 per cent respectively (Warren et al., 2018). Together these losses are harming many ecosystems’ ability to function and so threatening the critical ecosystem services that humanity relies upon, including providing food, clean water, and removing ~31 per cent of human-emitted CO₂ (Friedlingstein et al., 2022).. 

As with many other complex systems, ecosystems have been proposed to feature nonlinear changes such as tipping points, beyond which dramatic shifts to a different ecological state are expected, further threatening biodiversity and bio-abundance (Scheffer et al.,2001, 2009). Ecosystems are also subject to many co-stressors with complex interactions, with changing disturbance regimes eroding resilience (e.g. Nystrom et al., 2000; Folke et al., 2004) and making tipping points easier to reach (Willcock et al., 2023). However, complex ecological and social-ecological dynamics crossing multiple scales can make it hard to discern tipping thresholds in observations (Schröder et al., 2005; Hillebrand et al,, 2020; Spake et al., 2022). Organisms have agency that enables complex network and spatial dynamics to emerge – with human agency making social-ecological systems particularly complex – making ecosystem tipping dynamics often more difficult to detect and project relative to more physical systems (Kéfi et al., 2022; Rietkerk et al., 2021; Bastiaansen et al., 2022). Furthermore, while ecosystem functions or composition can have threshold responses to biodiversity loss or environmental change, in many cases responses remain relatively linear (Cardinale et al., 2011; Meyer et al., 2017; Hodapp et al., 2018; Strack et al., 2022). 

Tipping at the global biosphere scale has been discussed (Barnosky et al., 2012; Hughes et al., 2013; Lenton and Williams, 2013) but is deemed unlikely, with local ecosystem shifts globally aggregating to relatively linear changes in response to human-driven pressures (Brook et al., 2013; Montoya et al., 2017; Rockström et al., 2018). Empirical evidence for tipping has, though, been found in multiple ecosystems from the local to regional scale – for example, in lakes, coastal zones, marine food webs, rangelands and forests (Scheffer et al., 2001, 2009; Folke et al., 2004; Walker and Meyer, 2004; Brook et al., 2013; Rocha et al., 2015; regimeshifts.org), and model evidence suggests tipping is possible in some biomes across sub-continental scales (Armstrong McKay et al., 2022; Wang et al., 2023). As such, ecological tipping points remain a useful concept (alongside gradual and nonlinear change) in understanding and managing ecosystems, despite being sometimes hard to observe in practice (Lade et al., 2021; Spake et al., 2022; Norberg et al., 2022).

In this chapter we follow the wider section’s tipping point definition to categorise proposed tipping systems (see Box 1.1). In ecology, the terms ‘regime shift’ and ‘critical transition’ have been used interchangeably with ‘tipping points’, despite differences in meaning (Dakos, 2019). A regime shift refers to a shift in the current state of an ecological or social-ecological system from one partially stable state to another that is often large, relatively sudden (depending on system size and feedback timescales) and long-lasting, and entails a reorganisation in the structure and functioning of the system (Biggs et al., 2009; Maciejewski et al., 2019; Cooper et al., 2020). A critical transition refers to an abrupt shift in a system that occurs at a specific critical threshold in external conditions (Scheffer et al., 2009). In this chapter, we use tipping event to describe the crossing of a tipping point (which is equivalent to critical threshold), and regime shifts to describe the resulting changes that unfold (equivalent to critical transition above). Resilience – the ability of ecosystems to maintain functioning in response to change and regenerate in the face of shocks, sometimes adapting and transforming in the process – is also a key concept, with declining resilience being a potential precursor to tipping (see Chapter 1.6) (Folke et al., 2004, 2016).