Habitat fragmentation in densely populated landscapes is a major ecological threat over a large fraction of the Earth. Globally there has been an extraordinary surge in urban populations in the past several decades, and the associated urban sprawl has created a disproportionate impact upon habitat loss and habitat fragmentation by this burgeoning human population. During the last 40 years, the human population of Europe living in urban areas has increased twice as much as the population in rural areas. Today almost 80% of Europeans live in cities and their fringes. The spatial extent of urban areas is increasing even faster than urban population growth. While the urban population increased by approximately six percent during the period of 1980 to 2000, urban land areas expanded by 20%. In most cities of Central Europe, residential areas have particularly expanded during the last 50 years, indicating both a drop in residential density and a decentralisation of urban land-use.
On a coarse spatial scale, human presence is sometimes positively related to biodiversity, suggesting that people contribute to biodiversity by species introductions and habitat diversification; however, much of this alteration is at the expense of alien species introduction and destabilsation of ecosystems. People also tend to settle preferentially in areas of high biodiversity. The latter process poses a threat to global biodiversity and stresses the importance of human demographic and socioeconomic dynamics in biodiversity conservation. On a smaller spatial scale, however, urbanisation destroys, alters and dissects natural and semi-natural habitats, and at the same time, also creates new habitats. Patch size of semi-natural areas decreases with increasing urbanisation, and densely populated areas are characterised by small geographic patches isolated by roads, settlements or intensively managed agricultural land.
Effects on biodiversity
The concept of habitat fragmentation is a complex geometric and functional issue. A major problem is that, in the process of habitat fragmentation, changes in the spatial configuration of habitat and resources are inevitably confounded with a reduction in total habitat area. Fragmentation studies may not distinguish between the reduction of habitat area and fragmentation per se, i.e. the breaking apart of a habitat and the spatial separation of habitat fragments. Furthermore, there can be a mismatch between theoretical and empirical studies. Theoretical studies emphasise the importance of dispersal, while empirical studies indicate that fragmentation is rather a matter of habitat degradation and edge effects than of decreased migration among populations.
Densely populated, especially urban areas may exert robust and abrupt edge effects. There are both physical and biological edge effects. Physical edge effects are mainly due to microenvironmental changes (such as abiotic factors), while biological edge effects result from changes in species interactions, e.g. increased predation. Often physical and biological edge effects interact. Fragmented habitats harbour a larger amount of edges and a smaller amount of core areas, only the latter being unaffected by changes in surrounding land-use. For densely populated areas, several studies show that agricultural lands and settlements result in negative edge effects that change species composition and jeopardise habitat specialists. Disturbances from human leisure activities change the bird species composition of edges in urban forests. At the edges, the breeding density of bird species foraging on the ground and of species nesting in tree cavities is reduced. Disturbance by pedestrians and traffic noise impede some bird species from breeding near streets and footpaths. However, other bird species may have higher breeding densities at the edges of urban forests; these species likely exploit anthropogenic resources such as garbage and are not significantly disturbed by human activities.
Effects of small patch size
Fragmentation also increases the number of habitat patches and decreases patch size. At some point, each habitat patch will be too small to sustain a local population. Species that are unable to cross the matrix will then be restricted to patches below viable size to support a population, jeopardizing the entire metapopulation and the probability of its persistence. In densely populated regions of Europe many types of semi-natural areas have become too small to permanently maintain viable populations of habitat specialists. Semi-natural vegetation like extensively used calcareous grasslands and fens were abundant in the former, extensively managed agricultural landscape. These habitat types strongly declined during the last century due to land-use changes. Studies indicated that the population size of plants, especially those of habitat specialists, is reduced in the remaining patches. In some cases dominant vegetative species endure through considerable severe habitat fragementation, while at the same time, major loss of biodiversity is seen among arthropods and other lower life forms.
Fragmentation effects of urban land-use
Land-use between habitat patches affects the communities within patches as well as the effective isolation of animal and plant populations. For instance high road and traffic density augments migration mortality of animal species and enhances the isolation of populations. There are only few studies explicitly examining the isolating effect of urban land-use on plant and animal populations. Populations of common toad (Bufo bufo) and common frog (Rana temporaria) in urban regions are smaller than populations in rural areas and the urban matrix isolates local populations genetically, leading to inbreeding. Thus, urban land-use acts as a barrier to gene flow in animal populations, even in relatively abundant and widespread species. In contrast, species richness and total abundance of leaf-mining moths on oaks (such as Quercus agrifolia) were not affected by urban land-use. These small insects can maintain large populations in relatively small patches of one to several oak trees. In summary, urban land-use seems to effectively separate populations of vagile animals, but may not necessarily affect small and sedentary animals.
Fragmentation effects of roads and traffic
See main article: Roadless space
Transport infrastructure is a prominent component of urban land-use, and road and traffic densities are generally high in human-dominated landscapes. The fragmentation effect of roads and traffic on populations of animal species is based on three main effects: (1) reduced accessibility to resources, (2) increased road mortality due to collisions with vehicles and (3) subdivision of metapopulations by roads.
(1) As roads can act as barriers to terrestrial animal movement, resources like food and breeding sites may not be accessible for individuals that cannot cross roads. Reduced access to resources can lead to lower reproductive and survival rates, which, in term, may reduce population persistence. This separating effect of roads especially affects species avoiding roads, species requiring multiple habitats and species migrating over long distances to access different resources at different stages of their life-cycle.
(2) Each year, vast numbers of animals are killed on roads due to collisions with vehicles. However, the number of road kills is not necessarily a meaningful indicator for population decline, because low numbers may indicate reduced population densities near roads or road avoidance. Nevertheless, there are several species suffering population declines due to traffic mortality. For badgers (Meles meles) traffic mortality is a major threat. In Holland, yearly traffic mortality of badgers is estimated at up to 25% of the species total population size and is largely responsible for the decline of the species during the last decades. Similarly, road traffic is the largest single cause of deaths in badgers in Great Britain and other European countries. Many amphibian populations are compromised due to traffic mortality, as their seasonal migration between aquatic breeding sites and terrestrial habitats makes them particularly vulnerable to roads and traffic. The species most imperiled by roads are vagile species and species using multiple habitats separated by roads.
(3) Population subdivision occurs when populations become separated into smaller, isolated subpopulations. Populations living in habitats surrounded by roads are less likely to receive immigrants from other habitats and thus suffer a lack of gene flow potentially leading to inbreeding. Several studies proved the isolating effect of roads on animal populations: Highways disrupt gene flow among populations of bank voles (Clethrionomys glareolus), common frogs (R. temporaria) and a Carabid beetle (Carabus violaceus), both reducing the genetic diversity of subpopulations and enhancing their genetic differentiation.
The effects of a reduction of habitat area on species richness and species composition are better established and more thoroughly studied than the effects of fragmentation per se. Hence, the amount of available habitat and its quality is sometimes superficially judged to be more important for the persistence of animal and plant populations than the spatial configuration of habitat patches. Environmental management and planning should therefore pursue strategies to enhance habitat quantity and quality; however, when restoring habitat, the spatial configuration of habitat patches is important so that plants and animals can recolonise new patches rapidly. In densely populated landscapes, patches of semi-natural areas are often isolated by urban land-uses, especially by traffic infrastructure. The latter strongly enhance the effective isolation of animal and plant populations by impeding individual and gene exchange among populations.
Due to high road and traffic densities in densely populated regions, it is assumed that the isolating effect of roads on populations is especially strong here. In addition to a strong focus on habitats, urban planning should therefore compose measures that can reduce the separating effects of traffic infrastructure. When planning mitigation measures, an integrated strategy considering regional networks of major and minor roads is needed. The regional planning approach is especially appropriate for highly fragmented landscapes by coordinating biological corridors, such as overpasses, at neighbouring infrastructure barriers.
A specific strategy of minimizing fragmentation in urban areas is to design urban infrastructure elements to provide habitat continuity. An example of such urban design is the use of bioswales. A bioswale is a drainage-way for urban stormwater, which uses natural materials such as a grass-lined trough or vegetated streambed to transport stormwater runoff. Such a bioswale has two ecological advantages: (1) It creates a biological corridor allowing more readily crossed terrain than a conventional concrete man-made stormwater structure; and (2) It absorbs much of the water pollutant load normally carried by stormwater, thus reducing the pollutant discharge load to receiving waters.
^ Brigitte Michele Braschler. 2005. Effects of experimental small-scale grassland fragmentation on the population dynamics of invertebrates. Doctoral Dissertation. Universitat Basel
^ K.R.Crooks & M.Sanjayan. 2006. Evolution and Systematics 37: 317-342. Connectivity Conservation (1st ed.) Cambridge University Press.
^Robert Lawrence France. 2002. Handbook of Water Sensitive Planning and Design, CRC Press LTD ISBN 1-56670-562-2
^C.Michael Hogan. 1998. Hydrology and biology studies for Carneros Business Park, Lumina Technologies. Prepared for the William A. Saks Company and the County of Sonoma, California.