![]() This may allow tardigrades to take advantage of artificial substrates that undergo periods of desiccation ( Séméria 2002). Tardigrades can also survive periods of desiccation through cryptobiosis ( Jönsson 2005 Boothby et al. Terrestrial tardigrades can be found in substrates that retain a film of water, such as bryophytes, lichen crusts, tree bark, leaf litter and soil ( Glime 2013), and they should be able to colonise similar types of substrates within urban environments. 2011 Meyer, Hinton, and Samletzka 2013) or more abundant ( Morgan 1977 González-Reyes et al. For example, tardigrades in urban areas have been found to be as abundant ( Johansson et al. However, available studies present contradictory results regarding tardigrade densities in urban sites compared to rural sites ( Morgan 1977 Meyer, Hinton, and Samletzka 2013 Rocha et al. 2006 Šatkauskienė 2012), so it is possible that those species exist in low numbers within urban environments ( Meininger, Uetz, and Snider 1985). ![]() The fewer species found in cities may be impacted by pollution and other urban stressors ( Steiner 1994a, b, 1995 Hohl, Miller, and Nelson 2001 Vargha, Otvös, and Tuba 2002 Moly de Peluffo et al. 2011 Meyer, Hinton, and Samletzka 2013 Rocha et al. Despite their ability to survive extreme conditions, tardigrade diversity is consistently lower in urban sites than in rural sites ( Johansson et al. Tardigrades are small invertebrates (200–500 µm length on average) and can survive under extreme conditions, such as extremely low and high temperatures, lack of oxygen, lack of water, exposure to radiation levels that would kill most other organisms and extreme high pressure ( Jönsson et al. They are cosmopolitan and present in all types of ecosystems, including urban environments ( Nelson 2002 Mitchell, Miller, and Davis 2009 Meyer, Hinton, and Samletzka 2013). Tardigrades are a very interesting group for investigating the challenges and opportunities encountered by urban colonisers. 2010 Gaston, Visser, and Hölker 2015), possibly resulting in novel adaptations and usage of alternative urban niches ( McDonnell and Hahs 2015 Johnson and Munshi-South 2017 Winchell et al. Animals colonising urban environments are exposed to a series of novel stressors and ecological challenges, including air pollution, endocrine-disrupting chemicals, light pollution, and complex and changing habitats ( Sattler et al. In fact, cities can sustain a relatively high number of threatened species and can thus be important areas when planning and managing their conservation ( Ives et al. Loss of habitat and the spread of human populations will force an increasing number of species into these expanding urban environments ( Secretariat of the Convention on Biological Diversity 2012). This fact combined with the accelerating human population growth will lead to a significant expansion of urban environments worldwide ( Ellis and Ramankutty 2008 Grimm et al. Over 50% of the world population already lives in cities and by 2030 that figure is estimated to approach 70% ( Secretariat of the Convention on Biological Diversity 2012). More research is needed on the ability of tardigrades and other taxa to inhabit highly unusual and disturbed urban substrates effectively, and the adaptations that may take place when animals colonise such substrates. We conclude that even though urbanisation may restrict tardigrade abundance, this group of organisms can successfully colonise alternative urban substrates. In fact, the highest tardigrade abundances in urban samples were found in abiotic material accumulated in wall crevices. However, in urban sites we found no differences between the number of tardigrades present in natural and anthropogenic substrates. Among natural substrates, we found fewer tardigrades in Cork city than in rural sites. Our aims were 1, to determine if tardigrade abundance in natural substrates (moss, lichen and leaf litter) differs between rural and urban sites and 2, to assess if tardigrades have successfully colonised urban substrates that are highly anthropogenic (road sediment, and material accumulated under cars and in wall crevices). Tardigrades are thus a promising model to investigate the challenges and opportunities encountered by urban colonisers. Tardigrades are cosmopolitan invertebrates present in all types of ecosystems, including urban environments and they can survive under extreme conditions, including periods of desiccation, thus allowing them to colonise novel harsh habitats. Animals colonising urban environments are exposed to a series of novel stressors and ecological challenges, which can result in adaptations to alternative urban niches.
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