ReviewAmphibian ecology and conservation in the urbanising world: A review
Introduction
Urbanisation is currently affecting many of the earth’s ecosystems and is expected to continue to increase rapidly well into the future (United Nations Human Settlements Programme, 2004). It is estimated that the world’s urban population will reach almost 5 billion by 2030 with the bulk of the increases occurring in undeveloped countries and in cities with less than half a million inhabitants (UNFPA, 2007). Urbanisation is a complex process driven by an increase in human density that generates significant changes in the chemical, physical, and ecological conditions in areas of human development, and specifically results in the creation of new land cover and new biotic assemblages of plants and animals, and it can alter the types and frequency of disturbance regimes (McDonnell and Pickett, 1993, Kinzig and Grove, 2001). The process of urbanisation occurs as large cities grow, but also occurs in city-rural fringes and small towns and villages. Census surveys of countries around the world commonly use a density of 400 human individuals/km2 to define an urban area (Demographia, 2008), but there are many other factors such as density of buildings, roads and other types of infrastructure that contribute to creating urban environments (McDonnell and Pickett, 1993). McIntyre et al., 2000, Hahs and McDonnell, 2006 have contributed toward developing a standard set of measures to characterise urbanisation, but as of yet there is no one universal definition.
Urbanisation can cause habitat loss and fragmentation (McKinney, 2002, McKinney, 2006), change hydrology via the construction of impervious surfaces, thus increasing runoff, increase sedimentation and pollution of streams and wetlands (Paul and Meyer, 2001, Miltner et al., 2004), and modify soils (Effland and Pouyat, 1997). Urbanisation may also result in an increase in the establishment of exotic and domesticated plants and animals (Pickett et al., 2001, McKinney, 2006), and in climatic differentials between urban and less-populated surrounding rural areas (McDonnell et al., 1993, Grimm et al., 2008). Urbanisation is therefore currently one of the most pervasive causes of natural ecosystem disturbance and change worldwide, and thus presents a major threat to biota (Czech et al., 2000, Miller and Hobbs, 2002).
Amphibians have the highest proportion of species on the verge of extinction among the world’s vertebrates (Stuart et al., 2004), currently estimated as one in three species under the 2004 IUCN Red List (Baillie et al., 2004). Globally, amphibians have suffered massive, widespread, often unexplained, and probably irreversible, declines over the last several decades (see Collins and Storfer, 2003, Beebee and Griffiths, 2005 for recent reviews on amphibian declines). In total, 21% of amphibian species are critically endangered or endangered, whereas the proportions for mammals and birds are only 10% and 5%, respectively, and this high level of threat might be an underestimate, as 23% of amphibians could not be assessed because of insufficient data (Baillie et al., 2004). Habitat loss, fragmentation and degradation, which often result from urbanisation, currently impact 88% of threatened amphibians (Baillie et al., 2004), and are therefore among the greatest threats to amphibian populations (Stuart et al., 2004, Beebee and Griffiths, 2005, Cushman, 2006). For example, the global amphibian assessment (GAA) lists 2197 amphibians of a total 5918 as being threatened by urbanisation through infrastructure development (IUCN, Conservation International, and NatureServe, 2006). It is clear that urbanisation will remain a dominant threat to amphibians worldwide, as human transformation of the landscape continues to expand and intensify.
Amphibians often comprise a significant proportion of the vertebrate biomass in forest and wetland ecosystems, as well as being important carnivores and prey species (Burton and Likens, 1975, Gibbons et al., 2006). Despite their importance to ecosystem function and the widespread declines observed for many species, however, amphibians are among the least studied taxonomic groups in urban and suburban areas (Pickett et al., 2001, McDonnell and Hahs, in press).
While many of the direct and indirect impacts of human development on amphibians have been addressed within recent reviews (e.g. Cushman, 2006), there have been no explicit reviews to date of the impact of urbanisation on amphibians or on the ecology and conservation of amphibians in urban and suburban areas (but see Windmiller and Calhoun, 2007, for a discussion on conserving amphibians in urban landscapes of the northeastern USA). The aim of this paper is to review the current literature on the ecology and conservation of amphibians in urban and suburban landscapes. Due to the complexity of potential impacts on amphibians in human settlements and the variety of potential responses to these impacts by different taxa, we have presented the results of our review in the context of a hierarchical framework that recognises four critical components that affect the persistence of amphibian populations in urban and suburban landscapes: (a) habitat availability; (b) habitat quality; (c) species availability; and (d) species response (Fig. 1). This framework provides a tool to assess the current knowledge base on the ecology of amphibians and their response to urbanisation. In addition, this framework assists in identifying gaps in our knowledge and future research opportunities, and it serves to inform the development of conservation strategies for amphibians in urban and suburban areas. This framework is a tool to identify broad generalisations regarding the response of amphibians to urbanisation. There will of course be species with unique or specific life-history traits and behaviour that are exceptional, and therefore, difficult to include in any generalisations. This paper has been separated into two sections. The first half of the paper presents the results of our literature review on the ecology and conservation of amphibians in urban and suburban landscapes, while the second half discusses future opportunities and challenges regarding the study of amphibians in these areas.
Section snippets
Habitat availability
The maintenance and survival of amphibian populations in urban and suburban landscapes requires the availability of suitable aquatic habitats, such as a waterbody (pond, dam or lake), wetland (swamp, marsh) or stream, and terrestrial habitats (Wells, 2007). The amount and type of amphibian habitat available in a landscape is affected by several processes that occur in urban and suburban environments including: (1) habitat loss; (2) habitat fragmentation and isolation; and (3) habitat creation
The study of amphibians in urban and suburban landscapes: future opportunities and challenges
Several critical issues need to be addressed to advance the study of amphibians in urban and suburban landscapes, including: (1) the limited scope of our knowledge base because most amphibian studies are conducted in temperate environments, with relatively few in tropical and sub-tropical environments; (2) a lack of standardisation regarding how researchers define “urban”; and (3) the need to define and standardise an appropriate landscape scale for the study of amphibians. Appreciation of
Conclusion
In order to maintain amphibian diversity in urban and suburban landscapes we need to: (1) prevent further habitat loss and degradation of habitat quality, including aquatic and terrestrial habitat; (2) ensure the availability of targeted species of amphibians to maintain viable metapopulations and regional communities, and/or individuals for reintroduction into restored or newly-created habitats; and (3) develop strategies to reconnect the landscape and allow amphibians to disperse between
Acknowledgements
We thank Briony Norton, Kirsten Parris, Rodney van der Ree, Josh Schwartz and Julia Stammers for improving an earlier version of the paper. We would like to thank two anonymous reviewers for their constructive comments which helped improve the paper. The Baker Foundation provided generous support for this research.
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