Effects Of Urbanisation On Animals And Birds Environmental Sciences Essay

The effects of urbanization on mammalian and avian communities: why are the usual suspects just so usual?

Urbanization is increasing across colonised regions. I will investigate what this process does to the ecosystems it replaces. I will ask what habitat remains after urbanization and which mammal and bird species are capable of living in an urban setting. I will also ask which species are absent, and what ecological mechanisms lead to the presence or absence of species.

Introduction

An old proverb would have many young people believe that the world is their oyster. Whilst this optimistic thought may not be true for individuals, for mankind as a whole, its resonance is strong. Humans have made planet Earth their own, impacting on every environment imaginable. One thing that we as a species do particularly well is colonise areas, because we have the ability to radically alter the landscape if it does not suit us perfectly (Meyer and Turner, 1992 and Houghton, 1994). One of the most significant processes that changes huge areas of land is urbanization. Urbanization is defined as “the process by which human settlement increases in: 1) population density and 2) intensity of land use in an area” (Marzluff, 2001). While urbanization benefits humanity by providing commodities such as housing and employment opportunities, what happens to the natural community that existed before the introduction of brick and mortar? It would be short sighted to expect no change to the community, but what does happen? Are some species at a better disposition than others to succeed in an urban enivornment, or is it a level playing field? What adaptations might some species have to allow them to succeed whilst others fail? To answer these questions, we must start at the beginning, and establish what type of habitat an urbanized area represents for wildlife: how are habitats changed and how are resources such as food affected? This discussion will focus on mammals and birds for two reasons: 1) the wealth of studies on these two groups and 2) the dispersing abilities of both groups. Compared to other vertebrate groups, mammals and birds have much better abilities to both exit and colonize urbanized areas. Therefore they are an excellent indicator of an urban areas effect, be it positive or negative on wildlife.

What is urbanization?

We, Homo sapiens, are the most dominant species ever to have lived on Earth. Like all other animals, we have basic needs. One that affects the environment we share with animals is the need for shelter. Humans have built homes and settlements for centuries, but recent trends show how human populations are increasingly moving together into densely populated urban areas. Indeed, the United Nations cite urbanization as one of the fast growing uses of land (UN, 2008). The same organization provided data that shows the trend for people to move into urban areas. At the beginning of the twentieth century, only 10% of people lived in cities. This increased to 50% over the hundred years until 2000. Predictions estimate that around 70% of humans will live in cities in 2050 (UN, 1996). In ‘actual’ numbers, this equates to around 6.5 billion people living in cities by 2050 (Brown et al., 1998), which is close to as many people as there are on Earth now (6.9 billion, US census bureau, 2011).

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The change in urban dweller numbers is reflected in the number of large conurbations that now exist. The first city to have a population over 1 million is thought to have been Beijing, China around 1800 (Adams, 1994). Since then, the number of 1 million plus cities has increased with 16 by 1900, and 235 by 1980. Predictions suggest that cities with well over 4 million inhabitants will be common by 2025, with over 200 predicted to exist (Goode, 1991).

Cities are growing both in overall size and abundance. The effect that urbanization has on land use is vast. Forests have decline by 19% and grasslands by 8% over the last three centuries (Matthews, 1983 and Marzluff and Hamel, 2001). What is perhaps more worrying is that rather than being directly linked; population growth is disproportionate to urban land use growth. In USA, urban areas grow faster than their population (O’Meara, 1999). This process is known as urban sprawl, and has devastating effects for natural land uses. From 1982-1992, more than 2.1 million hectares of forest were lost in the United States as a result of urban sprawl (WRI, 1996).

Effects of urbanization on animals.

Marzluff (1997) summarises the effects that urbanization may have on wildlife. Whilst he had birds in mind, the processes in force would elicit responses from any phyla. Urbanization changes ecosystem process, habitat, food, predators and competitors and disease (Marzluff, 1997). These wide ranging effects have been taken as the starting point of many studies and investigated in greater detail, to asses if they are key in shaping urban biological assemblages.

Evans et al. (2011) provide further insight into how urbanization may affect biodiversity. Three aspects are highlighted, and again, while primarily designed with birds in mind, like Marzluff’s processes, they are applicable to other taxa. Evans et al. (2011) suggest that: firstly, urbanization occurs in places where human density is already high. This is certainly true for urban sprawl, but equally so of the growth of settlements as described in Figure 1. Biodiversity is affected by urbanization since “at broad spatial scales human population density is positively correlated with species richness” (Evans et al., 2011). Luck (2007) provides evidence for this, and points to available energy (discussed later) and elevation (both man and other species are predominantly found at lower altitudes) as the reasons why species richness is actually higher where human population density is also high. Furthermore and secondly, the areas that urbanization takes over are “often more ecologically valuable than undeveloped areas” (Evans et al., 2011 and Gonzalez-Abraham et al., 2007). These two points highlight how both mankind and other animals have shared requirements when it comes to habitat. Unfortunately, it is hard for animals to share an area with humans when we alter so much in order to live there. Thirdly, “highly developed urban areas support fewer native species than the rural habitats which they replace” (Evans et al., 2011). This seemingly negative impact can be linked to Marzluff’s (1997) processes. However, any organism that can beneficially exploit the unnatural changes brought into a natural system by urbanization will succeed in that environment. As we shall discover, it is a minority group that is capable of such exploitation which has consequences for a region’s biodiversity. Nonetheless, those individuals that survive, do very well in the urban setting.

Figure 1. “Model of the progression of human settlement (grey boxes) and resulting effects on organism habitat (while boxes)” (Marzluff, 2001 from Berry, 1990).Progression of human settlement.jpg

WHAT IS THE URBAN HABITAT?

To comprehend how certain species succeed in an urban environment, we must better understand the habitat that species colonise. In other words, we must know what makes the urban environment unique. What does building do to the area, and subsequently what habitats are left for organisms to colonise and inhabit?

Figure 1 shows how an urban area grows and the effects it has on habitats of any variety that exist in the area that is transformed by human development. The process of how urban areas expand (the grey boxes) is for geographers to study. The white boxes present two predominant trends, which are of interest to zoologists. For these to occur, all that has to happen is for urban areas to expand, and we know that this is happening on a global scale.

Key features of the urban habitat

The first theme of habitat effects is that of perforation and with increased urbanization sees habitats become more isolated and ultimately degraded into much less favourable habitats for many native species. However, this isolation may benefit some species. Marzluff (2001) explains that species which are naturally adapted to habitat fragmentation will do well in this system. Those species that need large areas of stable habitat will not succeed in an urban area, as this habitat of climax community ecosystems is eradicated. It is often these species that disappear with urbanization, and so biodiversity drops. Urbanization produces the opposite effect in wildlife to humans. As new homes are created for people, the homes of many species are destroyed and the organisms are forced away or the population faces extinction. The latter is the most likely option.

The second theme of urbanization is the creation of exotic habitats. Habitats are changed away from the natural succession that existed pre-urbanization to those that suit humans. The change from natural to exotic habitats is known as the ecological footprint of a city (Wackernagel and Rees, 1996). The exotic habitats are another reason why the number of species that can exploit urbanized areas is relatively low. Johnston (2001) studied urban bird communities and found that as cities became more and more established, the species that became most numerous were synanthropic ones. Synanthropy is a symbiosis between humans and other organisms. Synanthropic species are currently represented by a small number of species, hence the diversity of a region is reduced if only species with a relationship with man can persist. Clearly as urbanization continues to increases, both synanthropic species and the phenomenon of synanthropy will become more common.

As well as habitat fragmentation and the creation of exotic habitats, humans influence ecosystems in a way that is a fundamental principle of ecology. The most basic energy for life comes directly from the sun, and via photosynthesis is stored in plants. This energy is then passed through food chains via herbivorous and carnivorous species. As energy is lost at each exchange between organisms at different levels, there is less energy available at higher levels of the ecosystem (Hutchinson, 1959). The problem for wildlife is that humans use a vast amount of energy too. Wright (1990) estimated that as a species, humans use 20-30% of all energy available to ecosystems. Furthermore, this figure is predicted to rise to nearly 40%. The simple fact is that there will be less energy available for other species as the human population continues to grow. In order to survive, the source of energy for a species (i.e. food) must still be available, if it is not then the species cannot persist. Humans put pressure on plants and this effect is exacerbated higher up the food chain. With reduced primary producers, there is little hope for secondary and tertiary consumers. By these three ways, species richness in an urban area is most likely reduced.

A wider ranging impact of urbanization is that as different as two cities may seem to us, to wildlife they bear a striking resemblance. To an animal, a city is very much a concrete jungle, a vast expanse of impermeable surfaces and vertical brick faces, with small pockets of greenery. Blair (2001) explains the power of mankind as “the unique ability to shape the landscape according to (our) vision…if it doesn’t look like ‘home’, we alter the flora to make it look more familiar. If it doesn’t sound like ‘home’, we introduce songbirds with which we are familiar”. Put simply, urban areas are designed to be homogeneous in terms of habitats (Clay, 1994). The fauna of cities follow this trend. Blair’s (2001) study of urban bird assemblages showed that in cities, similarity in ecosystem responses has lead to a large overlap in species types. A 19% overlap of species occurred in cities, compared to only 7% in rural areas of North America. The sharing of species in cities separated by large distances shows very elegantly the effect the urbanization has in homogenising the land it takes over.

Blair (2001) provides further insight into the effects of urbanization on avian community composition. Urbanization affects birds on different environmental scales (Alberti et al., 2001): at the territory, population and landscape level. In terms of territories, urbanization can divide up resources in much more drastic ways via perforation and isolation than animals would be accustomed to. Indeed, the majority of resources are simply removed by urbanization and so the carrying capacity of the area is reduced. With regards to the population of a species, by proliferating and isolating habitats, the longevity of populations is also reduced (Bolger et al., 1991). The type of effects that urbanization can have at the landscape level include changing the amount of different types of habitat in terms of size, connections to other patches and the proportion of a habitat that is represented by edges (Miller et al., 2001, see Fig. 1). This third feature, as with other impacts, is not necessarily a negative one. Are there some instances when urbanization leads to better habitats for some edge species? Whilst urbanization changes the landscape, it does not destroy it. Animals have been adapting to change for millions of years, so although urbanization is happening at a rate well above that of natural processes, animals are showing that they can keep up. Many animals are common sights in cities. Later I will ask which species, but firstly, I will investigate if there are patterns in the types and numbers of animals that can inhabit urban areas.

WHAT TRAITS DO ANIMALS POSSESS WHICH ALLOW THEM TO SUCCEED IN URBAN AREAS?

More so than any other habitat, the urban space is under the control of humans. Areas of vegetation can become buildings in a matter of days in some cases. This state of flux is vastly different from the changes seen in natural habitats. Instead of changes occurring in a predictable and stable succession, land use changes in an urban setting are sudden. For wildlife, this puts species under extreme pressure to cope with such transformations. Consequently, we would expect urban areas to favour occupation by adaptable, generalist species. Commonly, adaptable species are seen as “R” strategists by biologists. First coined by MacArthur and Wilson (1967), “R” strategists are favoured by urbanization as they typically exhibit early reproduction with rapid development for the offspring. Reproduction often only occurs once in the organism’s life time, but with the emphasis being to produce the maximum number of young. “R” strategists also are often small-bodied. The opposite of “R” strategists, “K” strategists show delayed reproduction with slow development of the offspring, but reproduction occurs more than once during the adult’s life span. “K” strategists are often large bodied. It is not surprising that “R” strategists do well in urban environments. “R” strategists have evolved to cope with naturally unstable environments, where resources are not guaranteed to be readily present (Shirley, 1996). “R” species evolved to be able to take advantage of favourable conditions whenever they intermittently occur. This strategy should work well in modern cities. An animal that is classified as an “R” strategist is a generalist. It can exploit almost any habitat and a wide range of food sources. In contrast, “K” strategists evolved in natural habitats that remained stable in the absence of unnatural causes of disturbances (i.e. humans). Sometimes known as “equilibrium” species (Shirley, 1996), “K” strategists often have populations that expand until they reach the maximum carrying capacity of an area. This approach is in response to “K” strategists co-evolution with the stable environments with reliable resource supplies. Urban areas have very few stable resources, and consequently do not readily support “K” strategists.

In the following sections I consider the life history strategies of avian and mammalian species and how they translate into either success or failure when put to the test of the urban environment.

Avian Species

In accord with the idea that “R” strategists do well in urbanized areas, we would expect that it is adaptable, generalist species rather than specialist species which have co-evolved with specific habitats such as grassland or woodland, that are better disposed to survive in the urban habitat. The effect of being a generalist or specialist has been investigated empirically. Evans et al. (2011) in asking “what makes an urban bird?” investigated several features, one of which was the prevalence of generalist species, as they were believed to be “less vulnerable to habitat deterioration and loss” (Evans et al., 2011). The findings were conclusive as specialists (as defined by niche position) were seen to have “lower urban densities and ratios of urban to rural densities” (Evans et al., 2011) than generalists. It is safe to say that, for urban birds, being an ecological generalist predisposes a species to better survival in urban areas.

Many ideas as to why some avian species are apparently better predisposed to survive in urban environments have been suggested, so many so that Evans et al. (2011) set out to establish which traits played the largest role. Seven traits were investigated which were: whether a species was a habitat generalist, a species relative brain size as an indicator of behavioural flexibility, whether a species migrated or not, species dispersal ability, species fecundity, diet and finally nesting habits. Evans et al. (2011) found limited evidence that non-migratory species had lower urban densities and no evidence linking dispersal abilities and a high fecundity to urban inhabitation. Perhaps surprisingly, there was “no evidence that relative brain size is associated with a species ability to adapt to urban environments” (Evans et al., 2011). Whilst this finding receives support from Kark et al. (2007), it flies in the face of evidence provided by Maklakov et al. (2011) who show that “species of passerine birds that breed in at least one city centre have relatively larger brains and are more likely to belong to large-brained families”. Whichever finding is true, urban birds are known to exhibit novel behaviours that are useful in an urban setting. Magpies (Pica pica) take advantage of human garbage when they inhabit cities and have been seen to take food straight from bins (Harmata, 1985). The blue tit (Parus caeruleus) takes advantage of an already evolved skill, peeling bark from trees and has transferred this to enable it to remove the foil lids from milk bottles in order to access the rich cream (Shirley, 1996 and Gould, 2004). Evans et al. (2011) found evidence to support the hypothesis that generalist species do well in urban centres (see earlier). In terms of diet, there was “no evidence that species which included invertebrates in their adult diet had lower urban densities” (Evans et al., 2011), but there were signs that suggested those species that included plant matter fared better, since they could take advantage of supplementary food sources. Lastly it was found that “those species that did not nest on or close to the ground tended to have higher urban densities than ground nesting species” (Evans et al., 2011). It was concluded that this was due primarily to the reduction of suitable ground nesting habitat, but also slightly to alterations in predator numbers, mainly via the large numbers of domestic cats. Predation by cats has received some support as a cause of avian mortality in cities, and Balogh et al. (2011) suggest that in Washington D.C., 79% of chicks of the grey catbird (Dumatella carolinensis) are predated, and around half of this figure are lost to cats. This species nests close to the ground, and is experiencing increased predation pressure as a result of elevated domestic cat numbers.

Urban birds are species which can exploit a range of habitats, feed on a wide range of sources and breed away from potential predators. Species are sorted into those that fit these categories and those that don’t, leaving a smaller proportion of bird species which inhabit cities.

Mammalian Species

A similar pattern to birds is seen when urban mammals are investigated. VanDruff and Rowse (1986) found that cities support fewer species of mammals than rural areas and that once again, the species are typically habitat generalists rather than specialists.

The group mammals can be divided up into different ecological groups, depending on their food source. Unsurprisingly considering both pyramids of resources and life strategies, small bodied herbivores are the most abundant in cities (Adams, 1994). Specifically, the most numerous are mice, rats and squirrels. Indeed, in the case of the grey squirrel (Sciurus carolinensis), urban densities can be many times higher than that of rural areas (Hathaway, 1973). Conversely, whilst generalists such as the squirrel do well, specialist small herbivores can and do suffer at the hands of urbanization. In America, the salt marsh harvest mouse (Reithrodontomys raviventris) has become endangered due to habitat loss (Shellhammer, 1989). This and many species like it, show how despite fitting the bill as a potential urban survivor (small bodied, “R” strategist), the salt marsh harvest mouse is in actual fact not suited to cope with human intervention as a result of its specialist habitat requirements. This trend for specialised species to do poorly in urban areas is common across taxonomic classes.

Body size is also an excellent indicator of a species potential to succeed in the urban set up. Whilst small animals of any group tend to do well, what becomes of large bodied species? Surprisingly, some species seem to deal with urbanization well, and there is evidence both from Britain and USA. In North America, both white tailed deer (Odocoileus virginianus) and mule deer (Odocoileus hemionus) occupy habitats that occur within urbanized areas, as long as they are suitable. It is often the case that urban areas may benefit deer, as predators and hunting are no longer a factor (Adams, 1994). This is the case in Chicago, Illinois. Both predators and hunting have been removed, there is plentiful cover and food and accordingly, deer populations flourish, with 26 deer per square kilometre known to exist (Witham and Jones, 1990), in comparison to 10 deer per square kilometre in the forests of Michigan. Of course, urban deer do not occur in every city across America’s north-west, but they are becoming increasingly common in those cities that provide the correct conditions. One crucial factor is the presences of travel corridors that the deer can use to move around cities. Chicago boasts corridors of forest that cover 8.7% of the entire cityscape (Adams, 1994). In Britain, the roe deer (Capreolus capreolus) is increasingly encroaching into peri-urban sites (Dandy et al., 2009). This species is able to do so in the same way that the American species have colonized cities, via travel corridors. Angold et al. (2006) studied populations of many organisms in Birmingham, UK and found that medium sized mammals (such as the roe deer) would most likely rely on corridors to disperse across the urban habitat.

Predatory mammals encompass many body sizes and all feed on other living and motile creatures, covering a wide range of prey. As such, predators occupy a wide range of positions within a food chain, and may not necessarily be the top predator in a system. The most represented group of predators in an urban system is insectivores. This group includes small bodied mammals such as shrews, moles and their aerial relatives, the bats (Adams, 1994). Of these three groups, bats may be the most welcome as they are capable of consuming vast amounts of insects which may negatively affect humans. In fact, a bat may feed on 1,000 insects in a single night (Greenhall, 1982). Bats may utilize the fact that moths, a large proportion of their prey, are drawn to street lights, and therefore prey is easier to find in the urban setting (Gehrt and Chelsvig, 2003).

As a mammalian predator’s body size increases, in order to survive in urban areas, they must be able to exploit as many resources as possible. In the UK, the medium sized carnivorous mammals that do this best are the badger (Meles meles) and the fox (Vulpes vulpes). The badger is interesting in that in response to urbanization, their main food source has changed away from earthworms towards more omnivorous sources, such as fruit when it is available (Harris, 1984). Much like birds that are generalists, mammals rely on a wider range of food sources in response to living in an urban area. A similar response is seen in the urban red fox. Contesse et al. (2004) found that urban populations in Switzerland fed largely on food sources provided by humans, and that berries, fruit and cultivated crops also played a large role in completing an urban fox’s diet. Most interestingly, the prevalence of anthropogenic food sources (more than half of all stomach contents) is believed to be the driving force for urban fox population increases in Zurich, Switzerland (Contesse et al., 2004) and several British towns (Harris and Rayner, 1986).

The trend for taking advantage of exotic food sources is mirrored in America. Raccoons (Procyon lotor), opossums (Didelphis virginiana) and coyote (Canis latrans) all utilize anthropogenic food sources. All three species share some food sources with their rural brethren, but take advantage of human garbage and direct handouts of pet food to boost urban populations (Manski and Hadidian, 1987, Adams, 1994, Gill and Bonnet, 1973). These species receive such increased food sources that their populations can reach higher densities than in rural areas. Individuals may have smaller home ranges and move less as a result of their ranges containing more food than in a rural setting. In addition, individuals may have larger bodies and have higher reproductive rates (Adams, 1994).The true predators that would occupy the top of a food chain are forced out of urban areas both via human ecosystem impacts and directly by man’s actions. In Europe this includes wolves (Canis lupus lupus), lynx (Lynx lynx) and bears (Ursus arctos arctos). All three species were once present in the UK, but are now extinct. America shows a similar trend with the mountain lion absent Favored or Reduced.dib

Figure 2. Mechanisms by which bird species are favoured or reduced by urbanization. There are more causes for species declines then increases. Only a few species fit the criteria of “species favoured”, which is why biodiversity if often reduced in cities. From Marzluff, 2001.

from the most built up cities, although in recent times it is becoming increasingly common in urban outskirts. This has lead to an increased occurrence in direct conflict between animal and man (McKee, 2003). It is for this reason that when urban areas are forming, predators are eliminated by mankind, who are simply looking out for themselves and also, in the case of early settlers, livestock. In the USA, urbanization and eradication of top predators has occurred on such a large scale that wolves inhabit only 5% of their range from 1700, and mountain lions now exist on only 30% of their past range (Robinson and Bolen, 1989). Whilst these losses are dramatic, it is not realistic for cities to include top predators, even if the resources were present for them to persist.

Urban avian and mammalian assemblages are dominated by a group of species that are, for several reasons able to exploit the habitat that mankind creates. Figure 2 (Marzluff, 2001) shows the possible mechanisms underlying the ability of some bird species that do well in cities, whilst others fail. Each mechanism can be looked at in more detail, and I have done so earlier. The most obvious trait possessed by successful urban colonists is being a generalist. This can mean both in terms of habitat and in dietary requirements. Both of these resources will be limited in an urbanized area, so for a species to do well it must take full advantage of those resources that are available to it. By exploiting a range of resources, generalists are able to occupy urban areas as other factors, such as those investigated by Evans et al. (2011) are less important in determining a species’ urban success.

Although Marzluff’s model is based on studies of birds, with only very minor tweaks I believe that the same mechanisms are just as important in determining urban mammal populations. There is enough evidence to suggest that generality is just as important to mammals as it is to birds (see previous section). Figure 3 (appendix) shows several examples of how species that are in some aspects ecologically similar react very differently to urbanization. It shows very simply how some species are suited to cities and others are not.

The next section provides specific evidence of species that can truly be called urban animals. They are some of the most common species seen in cities across much of the world and provide unequivocal evidence that being a generalist is advantageous in an urbanized landscape.

SUCCESFUL URBAN SPECIES

The Red Fox (Vulpes vulpes).

If you were to ask a member of the general public in the UK to name an urban mammal, the chances are they would say “fox”. Foxes are without a doubt the most visible of mammalian urban inhabitants. As Shirley (1996) puts it: “they gatecrashed our suburban garden party – and stayed”. Urban foxes were first observed in Britain as early as 1930s (Teagle, 1967) and have been steadily increasing in prevalence ever since. Densities of up to 12 adult foxes per square kilometre have been recorded (Harris, 1981), compared to rural highs of 7 animals (Lloyd, 1980). During the 1980s, foxes were only known to reach such remarkably high densities in the urban environment in Britain, but since then their range has grown to include such cities as Boston, New York, Montreal, Brisbane, Paris, Stockholm, Copenhagen and Essen (Shirley, 1996).

Foxes live in almost every habitat found in the UK (Shirley, 1996) and so the move from rural to urban was always likely to happen. In fact, it may have been man that forced foxes into the newly suburbanized areas. Persecution from farmers, gamekeepers and of course hunting, was adversely affecting the rural populations (Shirley, 1996). The urban zones offered a much safer place for foxes to live.

So fox populations in the suburbs of Britain rocketed from the early 1900s onwards. This expansion was helped by the creation of suitable habitat in the back gardens of housing and also a plentiful food supply. Foxes are true omnivores, eating whatever they find, whether it is meat, fruit or the scraps thrown away by humans. Doncaster et al. (1990) provide evidence that for urban foxes in Oxford, scavenged food constitutes the largest proportion of the fox’s diet. Scavenged food is of such an importance that foxes may commute from the surrounding rural area into a city to take advantage of the rich pickings available (Shirley, 1996). Furthermore, there is so much food available that the high level of fox density can be supported. Individuals have smaller territories in cities because they do not have to travel so far in search of food. A rural fox may have to include 15 square miles in its home range. By contrast, an urban fox may only need 0.1 square mile (Shirley, 1996).

It would seem then that the fox, rather than being damaged by urbanization, has benefited. The environment that humans create for themselves is almost tailor-made to suit the fox. Places to live and breed and things to eat are all provided accidentally. For the fox, there were few obstacles to overcome when moving into the urban area, so it is not surprising that it thrives in our cities.

The Magpie (Pica pica).

As well as being a very common site in cities, the raucous cackle of the magpie may be the soundtrack by which urban birds are represented. What is perhaps most impressive about the magpie’s domin