According to the United Nations, the second biggest problem for humanity after global warming is disorganized urbanization—urbanization without planning and integration of natural environments. Since 2008, for the first time in history, the majority of people live in urban areas, and this pattern is expected to keep increasing in the oncoming decades, until 5 billion people are living in cities in 2030. In general, urbanization occurs at fast rates in tropical countries relative to temperate countries and it is expected that the biggest increase of urbanization will occur in Asian and African countries.
Urbanization reduces natural habitats (such as forest, grasslands, and wetlands), creates new habitats (such as parks, lagoons, and gardens), increases the levels of contamination and species introduction, favors disease dissemination, and produces changes in temperature, wind currents, and the water cycle. Aside from contamination, fragmentation is probably the most influential issue for preserving natural habitat and the species that inhabit it; because fragmentation separates populations, it prevents species movement between patches, reduces the area available to inhabit, changes wind currents and temperature levels, increases temperature and noise levels, and allows alien species to colonize the patches, just to mention a few examples.
Each of the previous effects of habitat fragmentation (and others) may act alone or as a part of a set of effects to cause that native species to go locally extinct. For example, if a fragment starts to be noisy due to the occurrence of more people and traffic, animals that use acoustical signals to attract pairs, defend territories, or announce predator presence—such as crickets, frogs, or birds—must change the structure of their vocalizations (e.g., frequency and duration) by singing at higher volume or changing the periods of singing (e.g., diurnal species singing at night), to allow the message in its vocalizations to arrive at the receiver without degradation caused by the additional, new noise. If individuals of the species cannot change the acoustical properties of their vocalizations or their behaviors, the species is likely to be driven locally extinct from the habitat fragment. Likewise, if the remaining fragments are more open than the original, continuous habitat, the fragment is likely to be brighter and drier because light and heat will arrive more directly to the understory and ground. This will cause species adapted to dense and dark habitats, such as fungi, mosses, understory plants, some insects, insectivorous birds, and newts, to locally disappear or to be reduced in abundance.
These two examples of habitat fragmentation and modification illustrate how changes in the structure of the surrounding environment affect the occurrence of native species in remaining natural habitat fragments. But, these are not the only problems that face the remaining native species in fragmented habitats, because when the native species cannot survive in the new fragment conditions, other species more adapted to these conditions—such as pioneer plants, open-area animals, and introduced species—arrive and colonize the empty space. Those species are named alien species and could have two possible origins: (1) species that naturally inhabit the country, state, or region but inhabit other habitats and ecosystems different from the fragmented one, or (2) could be introduced species from other countries or continents by humans. Under this scenario, native species start to compete and interact with the alien species in a war that determines which species will survive and flourish in the fragmented habitats.
In the Neotropics, fragmentation of natural habitats allows dry forest species to colonize what were once more humid habitats. In this case, although dry forest species are native to several countries, they are alien to humid forests. For example, Hoffmann’s Woodpecker (Melanerpes hoffmannii), White-winged Dove (Zenaida asiatica), and Rufous-naped Wren (Campylorhynchus rufinucha), which are Mesoamerican dry forest species, have colonized more humid habitats after fragmentation in the Caribbean rain forest or Pacific rain forest. Hoffmann’s Woodpecker started to increase its distribution along the Caribbean coast of Costa Rica in approximately the last 10 years (Sandoval & Vargas 2007). It was able to compete for nesting substrates with the native Black-cheeked Woodpecker (M. pucherani) because both species use the same type of substrates for nesting and are territorial against other woodpeckers. Based on its actual occurrence on the Caribbean coast (using data from the citizen science-based app, eBird), Hoffmann’s Woodpecker is pushing Black-cheeked Woodpecker out from urban places. This pattern is apparently occurring with other dry forest species, which are pushing out the native species of humid fragmented habitats (Biamonte et al. 2011). For example, in Mexico, after cloud forest fragmentation, the most common species in the remaining fragments are Great-tailed Grackle (Quiscalus mexicanus) and Brown Jay (Psilorhinus morio), which are alien species associated with disturbed habitats. Insectivores and large frugivores are now less common or have disappeared from small fragments (Rueda-Hérnandez et al. 2015).
Introducing alien species into fragmented habitats, where they compete, parasitize, or predate native species, occurs more commonly by human intervention. For example, house cats (Felis catus), both pets and feral, occur in all urban environments and are one of the biggest predators of wild birds in fragmented habitats around urban areas. Researchers estimate that between 1 and 3 billion birds are predated each year by house cats in the United States (Dauphine and Cooper 2009, Loss et al. 2013), between 350,000 and 1 million in Canada (Blancher 2013), and approximately 27 million in the United Kingdom (Woods et al. 2003). A more detailed study showed that house cats are responsible for 47 percent of nest predation in Gray Catbird (Dumetella carolinensis); house cats are directly affecting the recruitment and population growth of the species (Balogh et al. 2011). However, house cats also predate rodents, lizards, and large insects, locally reducing or extinguishing a large set of native species. In this case, it is necessary for humans to intervene to avoid local extinctions and to control the species dynamics inside natural habitat fragments.
These few examples represent scenarios in temperate and tropical habitats where native animal species are losing the war against alien species in fragmented habitats inside urban areas. How native plant species respond to the occurrence of alien species is less well-understood because, in some cases, a single individual or small population can persist several years in a place completely isolated from other individuals of its own species, giving a false perception of a lack of harm by alien species to the occurrence of native species. It is important to remember that fragmentation affects the occurrence of native species at several levels by changing the structure of remaining habitats and allowing the occurrence of other, better-adapted alien species in the fragmented habitats. I recommend the development of studies that evaluate how native and alien species interact in fragmented habitats inside urban areas throughout the world, if we want to understand whether those fragmented habitats are working as a reservoir of native species or as a black hole where natives will be replaced by alien species in the near future.
Good management of residual natural habitats inside cities will aim for the preservation of native species because native species (e.g., animals, plants, mushrooms, and bacteria) interact to keep populations and their surrounding ecosystems balanced. For example, a bird species that eat insects (e.g., flycatchers or wrens) keep insect populations under control, preventing them from becoming a plague. Similarly, native plants provide nectar for nectarivorous animals; these animals pollinate flowers, allowing fruit production.
However, to help native species win the war against alien species, it is necessary to consider whether the natural habitat fragment inside the urban area is recent, old, or intermediate in age. The age of the fragment will influence how possible it is to manage and preserve the native species within it, as well as avoiding or reducing the occurrence of alien species. Recently-created fragments will have more native species and less alien species; in this case, with the correct management and relatively low costs, it is possible to preserve the habitat and maintain a large diversity of native species. On the other hand, older fragments that have gone unmanaged probably will have more alien species than native species. In those cases, the cost to help return and maintain the native species could be very expensive and require a lot of management. Therefore, it is very important to analyze the age of fragment creation and the proportion of each type of species (native and aliens) before deciding whether it is prudent to actively manage the fragment.
San José, Costa Rica
Balogh, A. L., T. B. Ryder, P. P. Marra. 2011. Population demography of Gray Catbirds in the suburban matrix:sources, sinks and domestic cats. J Ornithol (2011) 152:717–726.
Biamonte, E., L Sandoval, E Chacón & G. Barrantes. 2011. Effect of urbanization on the avifauna in a tropical metropolitan area. Landscape Ecology 26: 183-194.
Blancher, P. 2013. Estimated Number of Birds Killed by House Cats (Felis catus) in Canada. Avian Conservation and Ecology. 8(2): 3. http://dx.doi.org/10.5751/ACE-00557-080203
Dauphine, N., and R. J. Cooper. 2009. Impacts of free-ranging domestic cats (Felis catus) on birds in the United States: a review of recent research with conservation and management recommendations. Proceedings of the Fourth International Partners in Flight Conference: Tundra to Tropics 205-219. [online] URL: http://www.pwrc.usgs.gov/pif/pubs/McAllenProc/articles/PIF09_Anthropogenic%20Impacts/Dauphine_1_PIF09.pdf
Loss, S. R., T. Will, and P. P. Marra. 2013. The impact of freeranging domestic cats on wildlife of the United States. Nature Communications 4:1396. http://dx.doi.org/10.1038/ncomms2380
Rueda-Hénandez, R., I. MacGregor-Fors, & K. Renton. 2015. Shifts in resident bird communities associated with cloud forest patch size in CEntral Veracruz, Mexico. Avian Conservation and Ecology 10 (2): http://dx.doi.org/10.5751/ACE-00751-100202
Sandoval, L. & E. Vargas. 2007. Melanerpes hoffmannii (Aves: Picidae) en el Caribe central de Costa Rica. Brenesia 68: 87-88.
Woods, M., R. A. McDonald, and S. Harris. 2003. Predation of wildlife by domestic cats (Felis catus) in Great Britain. Mammal Review 33:174-186. http://dx.doi.org/10.1046/j.1365-2907.2003.00017.x