The impact of urbanization on wildlife
Over the last few of centuries, the human population has grown at an exponential rate. With this growth, urban areas across the globe have spread faster than any other habitat type, with 3% of the total surface area of land now covered in cities and around 50% of the global population currently residing in urban areas.
The issue with this is that up to 80% of these urban areas are covered in either pavement or buildings, so that only around 20% remains as some form of natural or semi-natural vegetation. The effects that this can have on wildlife can be immense; in fact, urbanization contributes to species extinction more than any other human activity. As the density of infrastructure and housing increases, natural habitats are degraded or lost completely, pollutants are added to the air, water, and earth, and disturbance is increased to a level beyond what’s typically found in a natural environment. It becomes difficult for plants and animals to survive, as many species are either intentionally removed, indirectly driven away, or pushed to local extinction. To make matters worse, the modifications made are usually long-term, so that there is no chance for succession to occur and the changes become permanent.
An interest of many ecologists is to look at the ways in which urbanization affects wildlife diversity and abundance. In what is called the “species-area effect”, the high percentage of impervious area in cities leaves little room for the wildlife that used to inhabit the area, reducing the odds that you will find as many species or individuals as before land conversion occurred.
However, some species actually benefit from cities because they can make use of the small patches of remnant vegetation and the increased habitat heterogeneity that urban areas provide. Cities also provide many species with food they may otherwise not have access to; a good example of this is raccoons looting through garbage bins, or birds using feeders in peoples' yards.
For this piece, I reviewed a number of studies conducted on various taxa to determine how animals are affected by urbanization. For the sake of keeping this post a reasonable length, I will focus on the biodiversity and abundance of mammals (in a future post, I will explore birds, insects, and fish). Based on the fact that different species have different lifestyles and needs both within and between different taxonomic groups, I hypothesized that urbanization benefits some species while having a largely negative effect on others, and took social behaviour, diet, physical adaptations, habitat requirements, competition, predation, and non-native species into consideration.
Urban areas cause fragmentation of natural environments, leaving only patches of remnant vegetation for wildlife to use. It’s been suggested that since small mammals don't require a large amount of space and have a relatively small range, they should be able to make pretty good use of these small patches; for example, parks or gardens.
The reality is that small mammal abundance and diversity increases when there is a diversity of habitats, but is still higher in more pervious areas (such as forests) when compared to the impervious urban jungle; in other words, small mammals are generally negatively affected by urbanization. However, some small species (including rodents, like squirrels) are well adapted to survival in the urban environment, as long as there is connectivity between habitat patches.
Large carnivores are quite sensitive to urbanization, because they typically have extensive home ranges, occur in small densities due to their solitary lifestyles, have slow rates of reproduction and population growth, and may occasionally be removed from urban areas by humans. This can make for a slippery slope, because due to their position at the top of the food chain, the loss of key predators can cause a cascade that impacts entire ecosystems. While some carnivores, like grizzly bears, generally avoid humans and urban centres completely, others seem to be quite comfortable living in cities, so clearly there is a lot of variation in this taxon's response to urbanization.
Studies on carnivores have looked at species such as coyotes, opossums, foxes, skunks, raccoons, weasels, badgers, bobcats, and mountain lions. The size of habitat fragments as well as population isolation are the two most important variables in determining the distribution and abundance of carnivores, particularly for larger carnivores whose presence increases with fragment size. It appears that there’s threshold beyond which an animal will be too big to exist in the small fragments created by urbanization. It has been suggested that a body mass of 20 kg is the threshold, and indeed, all the carnivorous species that are successful in cities are below this mass. Based on this, it seems then that medium-sized predators are in the best position to survive in urban areas, as the success of raccoons, foxes, opossums, and skunks confirms.
However, another key reason for this has to do with predator dominance. For example, when larger predators (bears or cougars) are displaced or excluded, smaller ones undergo what is referred to as “mesopredator release”, where their numbers grow to fill the void. What this suggests it that small and medium-sized predators may do well in urban areas, not only because of their size and adaptability, but also because these areas protect them from larger, more dominant predators. In fact, the survival rates of opossums, raccoons, foxes, skunks, badgers, and coyotes are high in urban areas because they are protected against their natural predators and competitors.
So how do these mesopredators exploit urban areas? One way that species can succeed in urban areas is by making use of man-made structures for shelter. For example, raccoons may use parts of houses such as chimneys for their dens and skunks often utilize crawl spaces. Opossums are also known to take advantage of a variety of anthropogenic structures.
Another way to exploit the urban environment is to make use of the supplementary food sources that are associated with human activity, such as garbage, domestic pets and livestock, crops and vegetable gardens, roadkill, and food that’s intentionally left out for wildlife. These food sources may not only be higher in nutrition and caloric value than what may be found in a more rural or natural region, but they also do not fluctuate as much seasonally, providing animals with more reliable resources.
Overall, it appears body size, diet, habitat are the factors that will determine a carnivorous mammal'‘s success in exploiting an urban environment. However, a sweeping generalization cannot be made as to how urbanization affects this taxon, as some seem to have quite a positive relationship with the modified environment, while others do not.
Ultimately, it is overreaching to make any sort of generalization about the response of mammals to urbanization, because there is so much variation between species. An animal's diet, habitat requirements, physical qualities, social behaviour, competitive interactions, and natural home range are all factors that will play a role in determining its ability to adapt to an urban setting and it’s high levels of human disturbance.
Not only that, but it is important to consider an animal's response to urbanization in the context of the region of the world it lives in, because the natural environment surrounding the city being examined (and factors of that environment, such as soil quality, climate, water quality, tree cover, etc.) provide relevant information. For example, if a natural environment has extreme temperatures, seasonal fluctuations in food availability, and low structural heterogeneity (such as Alberta’s prairies), then an urban environment with a “heat island” effect, human-provisioned food sources, and high habitat heterogeneity might actually be more enticing to some species. I studied the little brown bat (Myotis lucifugus) for my undergraduate thesis, and the southern Albertan populations are a great example of a group of animals that have actually benefitted from nearby cities, due to the fact that man-made structures provide vertical roosting opportunities that the flat prairies do not.
It’s important to keep in mind that because a select few species do adjust to cities particularly well, they often end up dominating the local ecosystem. Sometimes these are introduced species, such as feral cats. The number of non-native species typically increases the closer that you get to the urban core, while the number of native species decreases. Although non-native species can add to biodiversity locally, they decrease biodiversity globally, as native species are driven to extinction.
In conclusion, some species will benefit from urbanization, while others are negatively impacted, often to the point of extirpation (local extinction). As we continue to disturb natural environments, fragment and degrade habitat, and introduce exotic species into sensitive environments, many native species will be replaced, reducing biodiversity in a process known as “biotic homogenization”. This means that preserving biodiversity will require us to protect both native species and their habitats, while being conscious of the impact that animals such as our pets have on local wildlife. (I plan to tackle the latter issue in a blog post in the near future).
With the human population continuing to grow and urban land-use expanding further out into natural areas, it’s more important than ever to get educated about biodiversity conservation, protect remaining habitat, restore modified habitat, plan our land-use very carefully, and learn to coexist with the fauna around us.
References
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