Corresponding author: Katalin Szlavecz (
Academic editor: S. Taiti
In an increasingly urbanized world scientific research has shifted towards the understanding of cities as unique ecosystems. Urban land use change results in rapid and drastic changes in physical and biological properties, including that of biodiversity and community composition. Soil biodiversity research often lags behind the more charismatic groups such as vertebrates and plants. This paper attempts to fill this gap and provides an overview on urban isopod research. First, a brief overview on urban land use change is given, specifically on the major alterations on surface soils. Historical studies on urban isopods is summarized, followed by the status of current knowledge on diversity, distribution, and function of urban isopod species and communities. A review of more than 100 publications revealed that worldwide 50 cities and towns have some record of terrestrial isopod species, but only a few of those are city-scale explorations of urban fauna. A total of 110 isopod species has been recorded although the majority of them only once. The ten most frequently occurring isopods are widely distributed synanthropic species. Knowledge gaps and future research needs call for a better global dataset, long term monitoring of urban populations, multi-scale analyses of landscape properties as potential drivers of isopod diversity, and molecular studies to detect evolutionary changes.
Szlavecz K, Vilisics F, Tóth Z, Hornung E (2018) Terrestrial isopods in urban environments: an overview. In: Hornung E, Taiti S, Szlavecz K (Eds) Isopods in a Changing World. ZooKeys 801: 97–126.
In 2008 humans reached a major milestone: more than 50% of the global population now lives in cities (
In the urban-suburban landscape the habitat unit is often the parcel, a piece of land owned by private citizens, organizations (e.g., neighborhood associations), companies, or other groups of landowners. Decisions about land management happen at this scale further increasing spatial heterogeneity. Individual homeowners or small groups decide on planting, mulching, irrigation, and usage of fertilizers and pesticides. Large amounts of soil and other landscaping material are moved around, overcoming natural or man-made barriers. All these accidental or deliberate actions affect distribution and abundance of soil organisms including that of terrestrial isopods.
While urban land conversion often destroys existing habitats, it also creates new ones. Urban habitat patches range from remnants of the ’natural’ community through more or less disturbed and/or managed habitats (e.g., parks, backyards, industrial grounds) to entirely novel habitats, such as green roofs, greenhouses, or even soilless environments such as basements and underground pipe systems. This wide spectrum of habitat types may result in overall higher species richness than expected. On the one hand, remnant patches of the native vegetation can sustain populations of the regional soil fauna. On the other hand, the physical environment in the novel habitats often allow the existence of species that otherwise would not survive under the normal climatic conditions. Cities are often viewed as hot-spots for non-native species introduction due to high traffic and trade (
Major restructuring of the surface-subsurface affects not only biogeochemical processes, but the biota mediating these processes as well. Soil fauna and microorganisms are key players in regulating pathways and rates of decomposition, thereby affecting storage and release of carbon, nitrogen and other nutrients. Endogeic fauna can modify porosity, which affects water holding capacity, infiltration, and gas diffusivity. Thus, urban soils provide the same ecosystem services that naturally developing or agricultural soils do (
The relative importance of natural vs. anthropogenic drivers in structuring urban communities and controlling ecosystem functions is variable, but generally the latter is viewed as a dominant force (
In this review we summarize past and current research on terrestrial isopods in urban environments. First, we provide an overview on the history of urban isopod research. We then summarize major findings, and highlight research gaps. For species names we follow the nomenclature by
Early studies on urban isopods were mostly zoological surveys in the neighboring parks, backyards or as part of regional fauna assessments (e.g.,
Ecological studies on urban isopods coincide with the rise of urban ecology as a discipline in the 1980–90s (e.g.,
The development of better theoretical framework and methodological approaches (
With the growing interest in cities as ecological systems, the number of publications focusing in urban soil fauna grew, and isopod literature followed this trend (Figure
Publications on urban isopods in the past three decades. Source: Web of Science using the following keywords: terrestrial isopods, woodlice, oniscid, urban, anthropogenic
Cities with records on urban isopod diversity. Names of cities with references are listed in Suppl. material
In any urban area species richness consists of two components: a subset of the regional pool of native species, and a group of accidentally or intentionally introduced species.
Local microhabitat characteristics are one major determinant in the survival of species. For instance, ruins of medieval castles or other historical buildings, stone walls or large slabs of concrete provide suitable shelter for
The review of urban isopod records revealed a list of over 100 species belonging to 15 families (Suppl. material
List of the ten most common
Species | Family | Percentage of records (N = 50) |
---|---|---|
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38 |
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72 |
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44 |
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38 |
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42 |
|
|
42 |
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48 |
|
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68 |
|
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38 |
|
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56 |
What fraction of the regional species pool survives in a city depends on the type and strength of environmental filtering, and tolerance and adaptability of the native species. Both are related to geological and land use history of the region. Cities can harbor a large fraction of the regional native
Many exotic species do not establish successfully in the new environment, although this is difficult to demonstrate. Others sustain populations on the long-term, but are still restricted to man-made environments.
Many cities have been built in regions where the climatic conditions are outside of the tolerance limit of most isopods. These settlements may still harbor high abundance of organisms because the way humans modify the landscape and microclimate, shifts habitat conditions towards more optimal. In metropolitan Phoenix, Arizona, where the natural biome is desert, changes in vegetation cover and regular irrigation shifted residential yards from xeric to mesic. The abundance of isopods increased by a factor of 50, and they became the second most abundant macroarthropod group following ants (
Regardless of diversity, urban habitats support large abundances of isopods. Isopods have been shown one of the most numerous group compared to other epigeic arthropods (
In the above mentioned GLOBENET project, pitfall trap material was analyzed in two cities: Sorø, Denmark, and Debrecen, Hungary. Isopod abundance was consistently higher in the urban core that in the rural habitats (
Responses of three synanthropic isopod species to urbanization gradient. Each data point is percentage of total number of individuals (N) of a given species caught in pitfall traps.
Relative abundance of terrestrial isopods in the urban and suburban landscape. Numbers are expressed as percentage of isopods in pitfall trap materials, thus reflecting their abundance in relation to epigeic arthropods.
Location† | Land use/cover type | Percentage of isopods | Reference |
---|---|---|---|
San Diego CA, USA | Various suburban | 48 |
|
Toledo OH, USA | Various | 59 |
|
Sheffield, UK | Gardens | 45 | Smith et al. 2006 |
Yorkshire, UK | Urban agriculture | 51 |
|
Baltimore MD, USA | Vacant lots | 52 | Szlavecz, unpubl. |
Chicago IL, USA | Woodland fragments | 34/55‡ | McCary et al. 2017 |
Melbourne, Australia | Grass with variable cover | <1 |
|
Osaka, Japan | Variable | 88 |
|
Phoenix, AZ, USA | Variable, mesic-xeric | 1.9/12‡ |
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Phoenix, AZ, USA | Irrigated residential yards | 7/37‡ |
|
† City might include greater metropolitan area ‡ First number was reported in publication and includes all arthropods. Second number is isopod percentage after removing
In the 1980–90s the main focus of urban research was contamination, especially heavy metal pollution (
Fluctuating asymmetry (
Isopod assemblages can also reflect the level of disturbance and/or ‘naturalness’ of an area. Simple species numbers or diversity indices do not inform us about community composition. In cities, where non-native, synanthropic, and/or common species mix with native and/or rare ones, species diversity can be high, yet from a conservation point of view, the quality of the habitat is poor (
The fact that isopods reach high population densities indicates that while species level differences clearly exists, as a group they can successfully colonize urban and suburban habitats. Multiple factors contribute to their success. First, as detritivores, they are food generalists capable of living on leaf litter, garden and kitchen refuse, and even on pet food. In some countries, it is common landscaping practice to cover tree bases, planting beds and bare soil surfaces with a thick layer of mulch. Mulch is often shredded woody material that retains moisture and, by slowly decomposing, adds organic matter to the soil surface (
Isopods are macro-decomposers feeding primarily on plant detritus but complementing it with other nitrogen rich resources, such as roots and tubers, vegetables, occasional green leaves, dead animal tissue and animal droppings. Together with other soil invertebrates, they accelerate plant matter decomposition and promote microbial access to organic carbon, thereby affecting nutrient turnover and soil formation. In urban parks and remnant forests they function similarly to wildland ecosystems. However, in residential areas they may be viewed differently. Some species, such as
Urban agriculture is a fast growing phenomenon worldwide with over 800 million people practicing some form of farming or animal husbandry (FAO 2017). Small community gardens, farms spanning over several blocks, rooftop farms, and other forms of farming provide access to better quality food for inner city residents, build social cohesion in the neighborhood, and utilize abandoned land. Urban farms use large amounts of organic mulch and produce green refuse for composting, both of which are resources for terrestrial isopods, leading to them becoming permanent features in community and residential gardens. As detritivores, the commonly held view was that they cause only minor damage in vegetable and other crops (
Isopods are food source for many invertebrate and vertebrate predators. Ground feeding birds, such as the European blackbird (
Neighborhood parks, schoolyards, university campuses community gardens and other green spaces serve as ‘living classrooms’ for children and adults. In the shrinking cities of US, low income inner city residents may not have the means to ‘venture out to nature’, they experience plant and animal life through green spaces near their homes. Urban soil biodiversity can be surprisingly high in cities, providing opportunity to demonstrate the variety of life forms, to talk about their functions, and to connect soil health and human health. Isopods are part of this conversation, because most people played with roly-polies as children, and because they are easy to observe, culture and experiment with. Citizen scientists can be actively involved in isopod surveys, add new records of species and localities, or make observations on the life cycle of their local populations, while they themselves learn about global change or conservation issues. For instance, the British Myriapod & Isopod Group (
Research questions in this area
What is the fraction of regional species pool persisting in urban/suburban areas?
Does urban isopod species richness exhibit a latitudinal gradient, and if so, how does it compare to trends in natural habitats?
What is the rate of species turnover of urban isopod fauna as a function of distance?
Huge gaps exist in regional and global scale distribution of isopods not only in cities but on the regional species pool, as well (Figure
Extending geographical scale results in examination of a broader range of climatic conditions, biomes and soil types, different cultures, economies, and human perception and value systems. Only a sufficiently large dataset enables us to examine large scale biogeographical patterns.
Research questions in this area
What is the relationship between city size, age, area of green spaces, and other landscape features in isopod diversity?
What is the human perception of soil macroarthropods in general, and specifically on isopods in different regions and cultures?
To answer the questions in this and the previous section, we need reliable data on isopod species richness and composition at city scale. Most studies reviewed here targeted a particular set of urban habitats, such as forest fragments, gardens or urban parks, and their species lists are likely incomplete. Many species are undocumented because they are rare, i.e. their abundance is low, and/or present only in a few, specialized landscape patches, such nearby historical ruins, greenhouses, or sewer drains (
Research questions in this area
What is the relative contribution of local environmental factors (soil type, vegetation, microclimate) and management (irrigation, amendments, pesticide use, litter collection) in determining composition and abundance of isopod assemblages?
What is the role of corridors in dispersal and exchange of individuals among local populations? Do isopods use grey infrastructure (buildings, underground conduits) to disperse?
What are the key landscape properties ensuring long term persistence of isopod populations, and how do these vary with climatic conditions?
The urbanized landscape is highly fragmented leading to isolation of communities. Species richness of these isolated patches can be highly variable from one species to ten or more. Priority effect can play a major role in which species get established. Isopods are being moved around with soil, plants, mulch and other landscaping materials. Consequently, species presence in a local patch might be determined by ‘who gets there first’. Soil invertebrate surveys in cities are often campaign-like, such as the above mentioned Bioblitz efforts, which provide a snapshot of the local community. We know essentially nothing about the persistence of these populations. Long term field monitoring is needed to reveal how patterns of alpha and beta diversity change over time and how stable urban isopod communities are. For arthropods in general, local landscape features have been shown to drive local diversity, although the specific drivers are taxon dependent (
Research questions in this area
Given their often high abundances, what is the role of terrestrial isopods in nutrient turnover, especially in regions lacking earthworms?
Under what circumstances can isopod become pests in urban gardens, local crop fields, and greenhouses?
The urban landscape is dominated by highly manipulated, engineered, and built components designed by humans to serve a given purpose. Soils are often engineered to support buildings and roads, to plant street trees, to establish green roofs, or to intercept and retain water. A particular example of the latter is natural water treatment systems such as rain gardens and bioswales. These novel ecosystems are readily colonized by soil fauna, including isopods (
In general, the urban soil food web is highly altered, because the resource base is altered. On the one hand, leaf litter, a major food source for terrestrial isopods, is removed from lawns and impervious surfaces. On the other hand compost, mulch, manure and other landscaping material create a concentrated abundant resource supply. How does this affect spatio-temporal abundance of terrestrial isopods? A related issue is the relative role of urban isopods in decomposition and nutrient turnover. Grass/lawn is the dominating land cover type in temperate urban/suburban areas.
Research questions in this area
What environmental factors in an urban environment can act as selective forces?
What are the key biological traits contributing to the success of some urban isopod species?
Are life history characteristics of urban and corresponding rural populations of isopods different?
Are urban and rural isopod populations genetically different?
Humans are major drivers of both adaptive and non-adaptive evolutionary change. Urban evolution is an emerging field focusing on individual and population responses to urban selective forces, and the underlying micro-evolutionary changes. So far research has been heavily biased towards vertebrates, and plants (
Terrestrial isopods are ubiquitous members of the epigeic soil fauna in cities. They are well established in the built environment and in all types of urban green spaces including remnant habitat patches, parks, residential yards, vacant lots, and greenhouses. Urban isopod assemblages are a mixture of a few cosmopolitan species that thrive in human dominated landscapes, a subset of the native fauna, and more recently introduced species. The urbanized landscape is highly fragmented, leading to isolation of communities. Alpha diversity of these isolated patches varies, but comparable to species richness in more natural areas. At the same time, species turnover among the habitat patches can be high resulting in high species richness at city scale. These observations are highly biased, because the overwhelming majority of data were collected in the temperate zone. Globally, distribution of isopods is limited at higher latitudes due to cold temperatures. However, current warming trends coupled with urbanization that provides refuges from extreme conditions, are pushing these boundaries northward.
Synanthropic species thrive in the city, often dominating the detritivore macrofauna, but special habitats can be refuges for native species, as well. The urban setting provides an excellent opportunity to study the dynamics of spatially isolated communities, the underlying mechanisms of local extinction, colonization, and dispersal, and the role of human perception, disturbance and management plays in these processes. As abundant, often dominant detritivores, their role in decomposition and nutrient release needs to be studied especially in engineered ecosystems. Future research should also include eco-evolutionary changes, preferably in the rapidly urbanizing regions in the world.
Andreas Allspach provided important historical references on isopod fauna in greenhouses and botanical gardens. Paula B. Araujo (Porto Alegre Rio Grande do Sul, Brasil), Andrzej Antoł and Maciej Bonk (Krakow, Poland), Łukasz Sługocki (Szczecin, Poland), Mateusz Rybak (Rzeszów, Poland), Matty Berg (Amsterdam, The Netherlands), Libor Dvorak (Marianske Lazne, Czech Republic), and Konstantin Gongalsky (Moscow, Russia), graciously shared unpublished isopod records. Helmut Schmalfuss and Stefano Taiti helped with clarifying species uncertainties in Suppl. material
List of cities with terrestrial isopod records
Data type: occurrence
Explanation note: Publications older than 70 years are not included. Localities listed here are mapped on Fig.
List of terrestrial isopod species records in urban areas
Data type: occurrence
Explanation note: Species name have been cross-checked with the world catalog by