Cities, Roads, & the Sixth Extinction Event

Earth’s biosphere entered the stage of large, complex, multicellular life (following an extended – approximately three billion year – period of dominance by unicellular life) starting approximately 650 million years ago.  This ‘metazoan’ stage saw the tree of life proliferate and complexify, and has expanded into a huge variety of niches across the planet through to the present day.  A key phenomenon over this period has been extinction events; this describes periods of rapidly changing environmental conditions which have resulted in species die-offs and restructuring of ecosystems at different scales.  In particular, five very large mass extinctions have resulted from cataclysmic events of global magnitude; these generated drastically altered environmental conditions over short timeframes, which in turn led to ‘great dyings’ (large reductions in total biomass and biodiversity across diverse environments).  These have caused the demise of many venerable parts of the biosphere (the non-avian dinosaurs being the most famous), but have also served as ‘resets’ that have spurred evolution of new branches of life over deep time.

Geological and paleontological evidence indicates that the underlying drivers of these events are varied, and likely include large scale volcanic activity, asteroid impacts, and potentially even radiation bursts originating in deep space.  What is common between these phenomena is that they produced rapid and large-scale perturbations of fundamental Earth System processes and cycles, which resulted in environmental changes occurring at rates faster than species and ecosystems could adapt.  These were rare events (five in approximately half a billion years) but the sixth mass extinction (described as the Holocene extinction) commenced just a few thousand years ago and is unlike any before in that it is being driven by the actions of one part of the biosphere; us.  The ‘explosion’ of human activity which has emerged since we first started manipulating our environment (and which has dramatically accelerated in just the last two centuries) is generating perturbations every bit as large as the phenomena that caused the previous events, and may be even more damaging due to its scale and reach, the rapidity (in geological terms) of its onset, and the novel mechanisms by which it operates.

Although the magnitude of anthropogenic impacts is growing in lockstep with the relentless growth of our overall numbers, energy and material use, and waste production, there is a subset of impacts which have been identified as being particularly significant in driving extinctions.  Namely, homogenisation of species (i.e., due to hunting, agriculture and introduced/invasive species); commandeering of net primary productivity and tapping stocks of ancient stored energy (i.e., due to very widespread agriculture, hunting and fishing, and use of fossil fuels); directing of the evolution of other species (i.e., through selection and management of species); and the creation of a planet-spanning technosphere (defined as the global emergent system that includes humans, technological artefacts, and associated social and technological networks) which interacts directly with the biosphere.

It is the final one of these influences which is potentially the most pernicious and expansive in terms of effects on other life forms, and this is largely because Earth and its biosphere have not encountered anything like our technological activity previously (despite some hypotheses to the contrary).  One phenomenon which is particularly significant in the expansion, development and influence of the technosphere is the increasing urbanisation of the human species; a grand migration of humanity from rural to urban living started during the Industrial Revolution, and continued throughout the 19th-20th centuries as the global population grew.  The ‘tipping point’ at which humanity became a majority-urban species finally occurred in 2008; this trend has continued (55% by 2018), and may be 70% by the middle of the 21st century.

Accommodating this huge translocation of humanity has required an equally huge reconfiguration of infrastructure worldwide; in the last ~45 years alone approximately half of the estimated 10,000 cities globally have either been newly founded or physically expanded (by mechanisms which appear invasive).  Although urban and built-up land (encompassing cities, smaller settlements and urban sprawl) occupies only between approximately 1-2% of the total global dry land area, during the period 1990-2014 the aggregate global built-up area grew by almost 28 km2/day (equating to approximately 1.2 km2/hour) and may expand by an additional million square kilometres by 2050.  Infrastructure vital to the functioning of cities has also grown commensurately, particularly transport corridors (primarily roads and railways) which thread cities, and link them to each other and surrounding hinterlands.  Although approximately 80% of the Earth’s land area contains no or very few roads (e.g., Siberia, Greenland) in populated regions of the world there is an estimated aggregated total road length of approximately 20 million km, which may increase by another five million km by 2050.

Many aspects of this expanding global network of cities and roads may be significant in terms of ecosystem pressure, as impacts on their host environments are manifold.  For cities, the most direct impacts are the physical destruction of habitats and ecosystems (and farmland) by replacement with anthropogenic materials and structures.  These structures then alter hydrology/hydrogeology (by sealing the ground surface) and evaporation/evapotranspiration (by removing water and vegetation) at different scales, which contributes to temperature changes (the ‘urban heat island effect’) and in turn alters downstream watersheds.  Ecosystems within and adjacent to cities are affected through mechanisms such as the creation of physical hazards, zones of artificial light at night (ALAN, which now affects almost a quarter of the Earth’s land surface) which disrupt the light-dark cycle and therefore many ecosystem functions (particularly for insects), and anthropogenic noise which also interferes with many aspects of normal ecosystem function.  In addition to these direct effects, cities also contribute to general systemic impacts by collectively consuming vast quantities of materials and energy, and comprising concentrated ‘nodes’ of carbon emissions, waste production and chemical pollution.

Transport corridors also generate a range of impacts, some of which are analogous to those of cities (e.g., contribution to ALAN and chemical pollution), but others are unique to these types of infrastructure.  The road effect zone (REZ) describes the spatial zone within and adjacent to transport corridors (up to ~hundreds of metres in all directions) in which impacts such as air and water pollution, noise generation and direct organism mortality occur.  A wider consequence is that transport corridors (and their intersecting REZs) may cause significant fragmentation of habitats and ecosystems, meaning that large, cohesive habitats are broken into smaller, discontinuous areas leading to edge effects and ecological decay via the ‘barrier and corridor’ effect. In populated regions of the world with high road densities, land has been fragmented into approximately 600,000 ‘patches’ or ‘islands’, of which more than half are <1 km2, and only 7% are >100 km2.  A final impact is synergistic with cities; expanding transport infrastructure has underpinned the spatial spread of human settlements; and may therefore be akin to the spatial spread of invasive species.

The collective impacts of the effects described above (along with their complex interplays) tends to support the idea that cities and roads may be contributing significant pressure to species, habitats and ecosystems in widespread environments and settings at planetary scale.  Once in place, human settlements and roads usually become permanent features, and typically undergo ongoing expansion and intensification of use.  Therefore, their impacts are relentless and pernicious, which combined with their novel natures (e.g., for billions of years life evolved with a steady and regular light-dark cycle but the appearance of street lighting in recent centuries has introduced a widely-distributed, generalised environment change which has no natural analogue) comprise environmental perturbations which are many species are struggling to adapt to.  It is therefore likely that the ongoing urbanisation of humanity is generating egregious effects which are increasingly operating in conjunction with many other expansive effects of our civilisation (e.g., ‘agricultural sprawl’ and ubiquitous pollutants) to drive the sixth extinction event.

Barring major future changes in the operation and organisation of human societies, the huge momentum of urbanisation will likely contribute ever more acute pressure on global ecosystems in the years and decades to come.  Improving understanding of these impacts, and how they might be effectively mitigated as far as possible, could become a crucial area of investigation.  A first step may be to consider cities and transport corridors collectively as discrete form of human influence (perhaps described as anthropic environments) acting at global scale, potentially by applying the planetary boundaries model.  This would require the definition and quantification of a single metric of influence, along with Earth System thresholds describing a safe zone, a zone of increasing risk, and critical thresholds for dangerous perturbation.  Due to the variation in the nature and extent of anthropic environments, their host environments, and the interactions between these, this could present a significant challenge.  However,  one thing is certain; few areas of Earth remain undisturbed by human influence, and unless efforts are directed at slowing and reversing this trend, all life on Earth (us included) remains at risk.

Above photo: The Third Ring Road in the area of the Moscow International Business Center. stroi.mos.ru.