Human impacts on natural systems are unprecedented, intensifying, and widespread. While many of the consequences of human actions are plainly evident or at least well publicised, the full extent of anthropogenic causes of environmental decline are more pervasive and extensive than many realise. Now that the majority of the worldâs population lives in cities, it is perhaps still possible to think there are areas, away from human population centres, that are untouched by humans. It is tempting (and comforting) to believe there is a wilderness âout thereâ that remains pristine, providing a refuge for nature. There are certainly refuges, but to say they are pristine would be an overstatement. Estimates of the extent of human impacts vary, but a recent study found up to 95% of the Earth has been modified by humans in some way, primarily through human settlement, agriculture, transportation, natural resource extraction, and energy production (Kennedy et al. 2019). Humans are a relative newcomer to the planet, appearing 5â7 million years agoâjust a few seconds before midnight if all of geological time were presented as a 24-hour clock. It was much more recent still that we began to have truly large-scale impacts on the planet and its natural environment, specifically when the first crops were cultivated and cities constructed 10,000 years ago. During this brief time in command of the planet, humans have become a powerful force of change globally, and perhaps even the worldâs most powerful pressure in accelerating evolutionary change in other species (Palumbi and Mu 2007). When considered on a geographical scale, our âsuccessâ in modifying our environment over such a short period is almost impressive.
It is less impressive when considered through the lens of ecological sustainability. Despite many modern metrics of human progress showing improvement, the extent of human impacts jeopardises the functionality of the very systems we rely on for human health and economic prosperity. Although not always evident in the short term, the steady expansion of human footprints across the globe has, inevitably, risked the long-term health of many of the Earthâs systems. With respect to ecosystems, changes can be the result of large-scale activities such as land clearing, but often they are the cumulative result of less-dramatic actions. Known as âdeath by a thousand cuts,â even with environmental legislation in place, many smaller-scale changes over time have caused a gradual but steady loss of species, habitats, and ecosystem function1 (Dales 2011). The cumulative impacts are substantial, with persistent and pervasive changes leading to degradation of ecosystems at a planetary scale. Most of these changes are gradual, but some have argued that they will also lead to more abrupt changes called âtipping pointsâ. Although this idea is being contested and difficult to evidence (Brook et al. 2013; Hillebrand et al. 2020), the idea that we have already exceeded several planetary boundaries that would constitute a âsafe operating spacesâ for the planet has achieved purchase, particularly for climate change and biodiversity (Rockström et al. 2009a, b; Mace et al. 2014; Newbold et al. 2016). The concern is that these steady changes will lead to much more dramatic, abrupt changes. Although this has proven difficult to evidence and anticipate, the concern is that drivers of change can be synergistic and reinforce each other (e.g. through feedback loops). This concept of a âsafe operating spaceâ reflects concerns that there are thresholds that should not be crossed, and that the gradual changes we observe will ultimately lead to changes so significant that we cannot reverse them, leading to even greater challenges for humanity.
Although environmental change is often studied by focusing on particular systems (e.g. the hydrosphere, geosphere, biosphere, and atmosphere), a more human-focused study reminds us that the planet functions as a system (âthe Earth Systemâ2), with social dimensions truly embedded into these dynamics. The latter includes economic drivers, which are social in origin and key influence on human behaviours. Although drivers of environmental change can be natural in origin, the vast majority of environmental change is now caused, either directly or indirectly, by humans. Humans are not just sources of change, but they also are dependent on natural systems and vulnerable to changes in those systems. The Earth System and social systems are mutually vulnerable and mutually dependent (Fraser et al. 2003). Environmental change is not just a concern for âenvironmentalistsâ but for all inhabitants of the planet, given this reflexive relationship.
Although globally humans have demonstrated remarkable capacity to adapt to change through technology and other innovations, there are natural limits. It is now possible we are approaching thresholds that, once crossed, could lead to cascading effects and even irreversible changes at continental and even planetary scales, with subsequent impact on life and livelihoods (Rockström et al. 2009a, b; Hughes et al. 2013; Steffen et al. 2015). While the goal of most environmental policies is to maintain the stability of ecosystems, this can be problematic for a number of reasons. First, the intuitive notion of stability as it is commonly understood by lay people and policymakersâthat is, as something that is relatively static or that ecosystems reach a âclimaxâ state and return to this state following disturbanceâis not consistent with modern understandings of ecological stability or ecosystem dynamics more generally. Policy documents and peak environmental bodies also often leave the concept of ecological stability ill-defined and rarely capture the multiple components of stability covered in the ecological literature and observed in the âreal worldâ. In reality, stability is a multidimensional concept that tries to capture the different aspects of the dynamics of the system and its response to perturbations3 (Donohue et al. 2016). Both disturbances and responses to disturbances are multifaceted and occur at multiple levels (e.g. species, communities), and neither this complexity nor the dynamic nature of ecosystems is captured in most current policies.
Even if the goal of stability were to be retained, but with a more modernised approach to understanding and measuring it, achieving that goal is not straightforward, in part because the relationship between human causes and ecological effects is complex. For example, there are delays between cause and effect, which make it challenging to predict and craft responses on political timescales. Disturbances themselves are multidimensional, varying in both type and intensity, and interacting with each other, potentially producing synergistic effects (KĂ©fi et al. 2019). Even though most ecological change is from incremental, persistent human impacts that gradually degrade ecosystems, some research suggests that dramatic âregime shiftsâ can occur, where large, often abrupt and unexpected changes affect biodiversity and ecosystem function (Biggs et al. 2018). There is even some suggestion that we might be living on borrowed time, and even though many places are changing slowly over longer periods of time, there are dramatic changes that eventually will need to be reckoned with (Hughes et al. 2013).
Extinction is a normal feature of the biosphere, but rates are currently 100â1000 times the background extinction rate (Pimm et al. 2014), and there can be large delays between habitat degradation now and extinction of species. There can be substantial delays between losses of habitat and species extinctions, known as extinction debt (Kuussaari et al. 2009). Although it is thought that we still have some time to repay these debts, we are racking up substantial extinction debts. Without faster and more effective responses, those debts will eventually need to be paid. Knowing how to predict and prevent dramatic changes is critical, as it can be costly and difficult, if not impossible in some cases,...