The extent of urban areas is increasing around the world, and most humans now live in cities. Urbanization results in dramatic environmental change, including increased temperatures, more impervious surface cover, altered hydrology, and elevated pollution. Urban areas also host more non-native species and reduced abundance and diversity of many native species. These environmental changes brought by global urbanization are creating novel ecosystems with unknown consequences for the evolution of life. Anthropogenic activities in general are known to affect evolutionary processes across a wide range of organisms, but these results from nonurban environments do not necessarily pertain to urban environments because cities are a unique anthropogenic disturbance. Urbanization causes simultaneous and often largescale changes in numerous abiotic and biotic environmental factors, so that cities represent novel ecosystems with no natural analog.
Because of their dynamic socio–ecological nature, urban areas are human modified environments that encompass multiple levels, scales, processes and systems. These areas are increasingly drivers of change and novel ecosystems, able to create challenges and offer opportunities for humankind’s current and future resiliency and sustainability. Life interplays with the physical environment. These myriad interactions have made Earth habitable for billions of years.
In a more philosophical way: it represents the knowledge learned by evolving species over millions of years about how to survive through the vastly varying environmental conditions Earth has experienced.
THE CENTRALITY OF URBAN CHALLENGES AND OPPORTUNITIES integrated with, driving, and responding to planetary change forces to rethink the roles of cities in ecology. Questions about changes in morphology, distribution and abundance of ecosystems and organisms in and around cities, and on how these are affected by and have an effect on the human populations and the structure of cities, have driven discourses on human–ecological systems interactions.
In this perspective cities are dynamic, integrated and multi–scalar systems, urban areas as human–driven ecosystems or socio– ecological systems, focusing on the biophysical features of cities including the complex socio– ecological relationships of cities. The ecology of cities, builds upon the city as a mosaic of land uses and management practices, subjected to biophysical (i.e., nutrients cycles, climate pattern, diversity distribution) and human (i.e., households, municipalities, agencies) driven influences and feedback loops. This concept emphasizes that urbanization alters the biophysical landscape and its functioning and quality, as much as it changes human behavior, community structures and social organization, altering abiotic and biotic environments over time and space.
Cities develop gradually, and their impact on the environment can depend on their age, density, size, geographical context, socioeconomics, and governmental policies, among other factors. Some of the clearest changes to the physical environment caused by urbanization involve:
- increased impervious surface cover (such as buildings and roads);
- urban climates (higher temperatures, urban heat islands, atmospheric accumulation, hydrology);
- urban soils (i.e., changes in soil composition, soil moisture, contamination, soil carbon, nitrogen dynamics);
- urban vegetation (i.e., species richness, heterogeneity of vegetation)
- urban biodiversity (i.e., composition, colonization, species richness, trophic dynamics) and urban biogeochemistry (i.e., nitrogen and carbon budgets, urban footprints, lawns and soil carbon, invasive species, biogeochemistry)
- changes to the biotic environment include increased habitat fragmentation, more invasive species, lower diversity, and abundance of some native species, and a loss of phylogenetic diversity within communities.
- elevated air, noise, and light pollution.
- effecting the evolution of microbes, plants, and animals that inhabit cities. Rapid adaptation has facilitated the success of some native species in urban areas, but it has also allowed human pests and disease to spread more rapidly. Urban areas also host (more) non-native species and reduced abundance and diversity of many native species.
These environmental changes brought by global urbanization are creating novel ecosystems with unknown consequences for the evolution of life. Despite advances in urban ecology, we do not adequately understand how urbanization affects the evolution of organisms, nor how this evolution may affect ecosystems and human health.
A major goal in urban ecology is to create cities that are more resilient to environmental perturbation (as aforementioned) and sustainable for humans and nonhuman urban dwellers alike.
UNDERSTANDING AND FACILITATING EVOLUTION IN URBAN ENVIRONMENTS is an important consideration in this regard. For example, facilitating the maintenance of genetic diversity within populations promotes not only adaptation but has immediate positive effects on the diversity and stability of communities and ecosystems. Increased genetic diversity frequently leads to greater species diversity at multiple trophic levels and promotes beneficial ecosystem services such as resistance to invasive species and greater primary productivity.
The clearest results of urban evolution shows that cities elevate the strength of random genetic drift (stochastic changes in allele frequencies) and restrict gene flow (the movement of alleles between populations due to dispersal and mating).
Fragmentation and urban infrastructure also create barriers to dispersal, and consequently gene flow is often reduced among city populations, which further contributes to genetic differentiation between populations. The influence of urbanization on mutation and adaptive evolution is less clear. It is very possible that industrial pollution can elevate mutation rates, but the pervasiveness of this effect is unknown. A better studied phenomenon is the effects of urbanization on evolution by natural selection. A growing number of studies shows that plant and animal populations experience divergent selection between urban and nonurban environments. Urbanization affects adaptive and nonadaptive evolutionary processes that shape the genetic diversity within and between populations.
THIS DIVERGENT SELECTION HAS LED TO ADAPTIVE EVOLUTION IN LIFES HISTORY, morphology, physiology, behavior, and reproductive traits. Evolutionary analyses of genetic variation may also guide choices about lineages that could be used to construct ecological communities in and around cities. Of particular interest will be lineages with a high potential for local adaptation in cities or high capacity for phenotypically plastic responses promoting acclimation or tolerance of urban conditions. More drastic interventions such as assisted migration or rewilding of habitats in and around cities would particularly benefit from analyses of evolutionary potential.
AN INTEGRATED URBAN SYSTEM, stresses the importance of using integrated social– bio–geophysical frameworks, considering the heterogeneity of systems and recommends drawing insights from hierarchy theory to organize and structure integrated approaches.
A perpetually dynamic, complex system with continuous adaptation with people as the dominant species, other organisms, and abiotic elements, as well as the social and ecological contexts for these components, demands a coherent system of biophysical and social factors that regularly interact in a resilient, sustained manner. A system that is defined at several spatial, temporal, social and organizational scales, which may be hierarchically linked. A system with a set of critical resources (natural, socioeconomic, and cultural) whose flow and use is regulated by a combination of ecological and social systems.