Biodiversity and the threat of urbanisation

Economic opportunities, arts and culture, and the broad acceptance of different religions and ethnicities found in cities all contribute to the increasing urbanisation of our world. Today, roughly 56% of the world’s population live in urban areas; by 2050, that share is expected to increase to 70% of the total global population (1). In many ways, living in a city is considered the more environmentally-friendly option: city-dwellers tend to walk, cycle or take public transport, whereas those in the countryside often need to drive to get around. Urban accommodation is smaller per capita than rural housing, requiring less energy to heat in winter and cool in summer. As the population grows—which it is projected to do until at least 2080—and the development needs of many around the world increase, it is considered more ecologically sound to build up rather than out, to develop on land that has already been converted from primary vegetation, rather than exploiting wild habitat anew (2). In this way, urban development and expansion could be seen as less destructive for wildlife than development alternatives in rural areas.

Despite this, through industry, transport and the concentration of wealth that accumulates in urban areas, cities account for 70% of the total emissions of greenhouse gases (3), which, by changing our climate, intensifies weather patterns, modifies habitats, and warms the planet and seas, disrupting diverse biological functions (4). It is estimated that 24% of the species on the International Union for the Conservation of Nature (IUCN) Red List are threatened by commercial and residential infrastructure expansion (5), which is unavoidable as millions of people look to settle in urban areas. Other indicators of urbanisation—such as light pollution, impervious surfaces and habitat fragmentation—have been directly linked to wildlife and biodiversity loss (6).

Due to urbanisation and humans’ unrelenting pressure on the environment, the world’s biodiversity is under threat. There are more than 157,000 species on the IUCN Red List, of which over 45,000 are species threatened with extinction. This includes 12% of birds, 26% of mammals, 34% of conifer trees and 36% of reef corals (7), as shown in Figure 1.

Figure 1. Percentages of species threatened with extinction. Source: IUCN

Though difficult to quantify, it is estimated that the current rate of species extinction is at least tens—and up to hundreds—of times higher than the average rate over the past ten million years. Furthermore, the rate of extinction is accelerating (8). One class of species particularly vulnerable to our changing climate and urbanising world are insects. Insects make up 75% of all plant and animal species globally (9), play vitally important roles in our lives, but are under severe threat, disappearing at alarming rates around the world (9–11). The IUCN has named ‘housing and urban areas’ as the number one threat to insects (7), and it’s estimated that roughly one third of all insect species are threatened, with as many as 41% suffering some level of decline (10).

Figure 2: Percentage declines in selected insect populations. Source: Statista

Public awareness of both biodiversity in general, and biodiversity in cities specifically, has grown in recent years, and there has been increasing recognition of the importance of urban spaces for wildlife ‘as cities become viewed as novel ecosystems rather than anthropogenic sinks devoid of nature’ (12). This recognition is vital so that public and private entities may work together in the important task of building the kinds of places that benefit people as well as wildlife. Cities could provide important habitat in a world becoming increasingly urbanised, but as yet there is a knowledge gap concerning the role of urban systems to protect and conserve biodiversity. City planners and government need to know how biodiversity responds to different forms of urbanisation, but more research is needed to make recommendations to protect biodiversity in urban settings.

This study aims to fill that gap by identifying and better understanding the relationship between insect biodiversity and urbanisation. Using a database of butterfly species richness, I analyse the impact of key urbanisation metrics and land use on butterfly biodiversity at single-city level. Datasets for artificial light at night, built-up surface area, land cover and green space serve as proxies for urbanisation and human pressures on the environment. I use London as a case study to measure the impacts of urbanisation drivers on butterfly biodiversity. Through my research, I seek to deepen our understanding regarding the extent to which cities can act as habitats for wildlife and contribute positively to biodiversity. My research will add to the emerging field of urban ecology and the growing body of work assessing the impact of urbanisation on insect biodiversity.

Based on the findings of previous studies, overall biodiversity is expected to decline with increasing levels of urbanisation (13). However, some studies—specifically on Aves and Arthropods—showed an increase in overall abundance, but a decrease in species richness, due to an increase in urban generalist or synanthropic species, but a decrease in specialist species. This study will explore these findings, and assess the hypothesis that overall biodiversity declines with urbanisation. Further, it seeks to either confirm or challenge some of the existing thinking that landscapes heavily dominated by humans are depleted of wildlife (14).

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