Alien terrestrial crustaceans ( Isopods and Amphipods ) Chapter 7 . 1

A total of 17 terrestrial crustacean species aliens to Europe of which 13 isopods (woodlice) and 4 amphipods (lawn shrimps) have established on the continent. In addition, 21 species native to Europe were introduced in a European region to which they are not native. Th e establishment of alien crustacean species in Europe slowly increased during the 20th century without any marked changes during the recent decades. Almost all species alien to Europe originate from sub-tropical or tropical areas. Most of the initial introductions were recorded in greenhouses, botanical gardens and urban parks, probably associated with passive transport of soil, plants or compost. Alien woodlice are still confi ned to urban habitats. Natural habitats have only been colonized by three amphipod species in the family Talitridae.


Introduction
Th e orders in the arthropod subphylum Crustacea are mainly composed of aquaticliving species, at least during part of their life-cycle.Most alien terrestrial crustaceans belong to the order Isopoda, suborder Oniscidea, commonly named woodlice.But several species recorded in Europe belong to the order Amphipoda, and are commonly known as "lawn shrimps" or "landhoppers".
In 2004, the total number of valid Isopod species worldwide was 3637 (Schmalfuss 2003).Woodlice are adapted to various terrestrial environments from sea shores to deserts and have established on all continents.As decomposers of organic plant material, isopods play an important role in ecosystems (Holthuis et al. 1987, Zimmer 2002).Most European species prefer humid and moist micro-habitats (Vandel 1960) like soil, leaf litter, mosses and decaying wood.Several species are known for their myrmecophylic nature.
Amphipods are generally marine or limnicolous, and only a few species can live permanently on land (mainly in the family Talitridae).Some live near the sea, on beaches where they hide under logs and dead algae and vegetation.Th e true terrestrial amphipods live on the surface of mulch and moist ground (Fasulo 2008).Many of the habitat features of terrestrial amphipods are similar to those of isopods.Th ese little animals are most commonly noticed by their strong, rapid jumps upon being disturbed.

Taxonomy of alien terrestrial crustaceans
Th irty-eight species belonging to ten diff erent families were recorded during this study.Th e four most commonly represented families (all belonging to Isopoda) are Trichoniscidae (seven species), Porcellionidae (fi ve species), Philosciidae and Armadillidiidae, both with fi ve species (Figure 7.1.1.).Two main categories were considered: • Aliens to Europe, including 17 crustacean species originating from other continents (  Some of the species above have proved to be very successful colonizers and are currently considered as part of the native fauna in parts of Europe, e.g. in Hungary.However, their synanthropic nature and their extremely wide distribution range suggest a long colonisation history as it is the case for Armadillidium vulgare.
In the remainder of this chapter, we will focus mainly on the species alien to Europe.

Temporal trends of introduction in Europe of alien terrestrial crustaceans
Th e total number of crustaceans alien to Europe has slowly increased during the 20 th and the early 21 st centuries, but without any acceleration in the rate of arrival.Two alien species were fi rst discovered in Europe in the 19 th century, about nine species in the fi rst half of the 20 th century and only fi ve species since then.Th e majority of these alien species have been found in several other countries after their discovery in Europe.However, the number of occupied countries over time has grown steadily rather than exhibiting exponential growth.A similar pattern is apparent for woodlice species alien to Europe.However, because of sparcer information on this group, the date for the fi rst introduction is roughly known for only approximately 50% of species.To our knowledge, at least six species of woodlice classifi ed as aliens of Europe were noticed in the fi rst half of the 20 th century and only fi ve more species since then.
Th us, unlike many other invertebrate phyla, the temporal trend in alien crustaceans (both intra-European and alien) has shown no marked changes during recent decades.As "silent invaders" (Hornung et al. 2007) no terrestrial crustaceans are classifi ed as pests in Europe; they are elusive animals.We suspect frequently a large gap between the date of introduction and "discovery" of alien woodlice species.For example, during an intense eight year survey of the isopod fauna in a large region representing 15% of Hungary, three new alien species for this country were found (Farkas 2007).
To conclude, the atypically gradual trend in the number of alien terrestrial Crustacea in Europe could be an artefact of incomplete knowledge.Because of both the increasing worldwide trade in ornamental plants and the general ecology of terrestrial crustaceans (i.e.often hidden in soils), it is more realistic to expect a future exponential increase in the number of alien species (especially intra-European aliens).

Origin of the alien species
Species alien to Europe almost all originate from sub-tropical or tropical areas (Table 7.1.1.).Only one species -Protracheoniscus major (Dollfus, 1903)-is likely to be native from Central Asia.For several species, their ranges are poorly known (they are also often introduced in other tropical areas).However, several species do have a precise origin.Th e most widely distributed alien woodlouse in Europe is the tropical American Trichorhina tomentosa (Budde-Lund, 1893), while the most widely distributed amphipod is Talitroides alluaudi Chrevreux, 1901.It should be noted that a least six of the seventeen alien species were originally described from Europe (Great Britain, France and Germany) after their introduction.
Th e crustaceans alien in Europe generally originate from the Mediterranean basin (seven species), from western and south-western Europe (fi ve species).

Distribution of the alien species in Europe
Within Europe, Crustaceans of alien origin have mainly been recorded in western countries, where they appeared fi rst.Th e four countries with most species are Great Britain (11 species), the Netherlands (10 species) and Germany (nine species) (Figure 7.1.2).Comparatively few alien species have been recorded in central and eastern Europe to date (e.g.only four species in Hungary).In this part of Europe, the Central-Asian P. major is one of the most widespread alien crustaceans.Th e high number of aliens in western European countries may be linked to the high number of scientists and the intensity of soil research (Hornung 2009).
Th ere are only very few records of alien crustaceans on European islands.Trichoniscus pusillus has been reported from the Azores and Madeira, T. provisorius and A. assimile from the Azores but these species are native of Continental Europe.To our knowledge, the only alien aliens recorded on islands are talitrids, Arcitalitrus dorrieni (Hunt, 1925) in Scilly and Guernsey, Talitroides topitotum (Burt, 1934) in the Azores and Madeira, and T. alluaudi in the Azores and the Canaries.All these species occur outdoors and are therefore considered as naturalised.Th e rarity of alien terrestrial crustaceans on European islands is likely to be due to the primarily introduction route being major greenhouses in large metropolitan cities (see below).
Crustaceans classifi ed as aliens of Europe are typically species which have expanded their range approximately northwards and eastwards.Th e eastern and central countries have a higher number of these species than more westerly countries of Europe.For example, Germany and the Czech Republic, taken together, have nine species of alien woodlice of European origin, about 45% of the total in this category.A striking example of successful colonization and establishment of such species is given by A. nasatum.Th is woodlouse is believed to be native to Italy, southern France and Spain (Vandel 1962).Since the start of the 20 th century, it has been introduced into greenhouses in a number of additional countries of Northern and Central Europe (e.g.Denmark, Finland, Germany, Hungary, Poland, Slovakia, Sweden), making this species one of the most widely distributed alien woodlice of Europe.Moreover, numerous reports highlight the successful establishment of outdoor populations in several western and central European countries (e.g. the Netherlands, Czech Republic, Romania, Slovenia) (Berg et al. 2008, Giurginca 2006, Navrátil 2007, Vilisics and Lapanje 2005).
Some of the aliens of Europe have also invaded other continents and can be considered as very successful invaders.Th e most notable ones are A. vulgare, P. scaber and P. pruinosus.Armadillidium vulgare and P. pruinosus are probably native from Mediterranean regions.In northern temperate parts of Europe, these species are restricted to synanthropic habitats (e.g.gardens, cellars, compost heaps).P. pruinosus is one of the woodlice that has been spread most by man across the world (Vandel 1962) and can now be considered as "synanthropically cosmopolitan" (Schmalfuss 2003).
A consequence of the dominance of Mediterranean origin for species classifi ed as aliens of Europe is their decreasing number towards the north of the continent (Vilisics et al. 2007).In the northernmost countries of Europe (e.g.Finland (Vilisics and Terhivuo 2009)) only the most tolerant habitat-generalists, as well as intra-European aliens, are able to become successfully established.

. Pathways of introduction of alien terrestrial Crustaceans
Because a great majority of the fi rst isopod introductions were recorded in greenhouses, botanical gardens or urban parks, it is clear that many were associated with passive transport of soil, plants or compost.With few visible eff ects in such biotopes, terrestrial crustaceans colonize and spread as undetected "silent invaders" (Hornung et al. 2007).Th us, most introductions were unintentional.Th e one known exception is the spreading of T. tomentosa, commonly sold as pet food, triggered by trading activity in Europe.Th is probably explains why, among all the alien crustaceans, T. tomentosa is the most widespread species in Europe.
Another interesting case is the Mediterranean species P. schoblii.Th is myrmecophylous woodlouse is a commensal of the ant Lasius neglectus Van Loon, Boomsma & Andrásfalvy, 1990 and was fi rst recorded in Hungary in 2001, a few years after the introduction of the ant.P. schoblii was probably introduced at the same time as its ant host (Tartally et al. 2004).It has since been found regularly (Hornung et al. 2005, Tartally et al. 2004, Vilisics 2007, Vilisics et al. 2007) and is now considered established, as is L. neglectus.

Ecosystems and habitats invaded in Europe by alien terrestrial Crustaceans
To our knowledge, the only alien crustaceans invading natural habitats are three talitrid species.Arcitalitrus dorrieni has invaded leaf litter understoreys of deciduous woodlands in Great Britain and Ireland (Cowling et al. 2003, Vader 1972).Talitroides alluaudi is known outdoors in the Canary Islands, and T. topitotum in the Madeira Islands, both species in the Azores (Vader 1972).All other species are generally limited to highly artifi cial habitats and artifi cial ecosystems: mostly greenhouses, urban parks and houses (especially cellars).Th e proportion of introduced isopods can be very high in urban areas.A study in Budapest revealed that 35% of the total species (n = 28) were introduced (Vilisics and Hornung 2009).Th e major settlements of Hungary were characterised as "hotspot for non-native species" (Hornung et al. 2008).Th is could certainly be applied to many major cities in other European countries.
For the tropical species, especially those recorded only once or twice in Europe, they may not be considered as established (Table 7.1.1.)since their survival is completely dependent on warm greenhouses.Among all alien woodlice, none have spread to more natural habitats.However, the situation is diff erent for intra-European woodlice native to southern or Mediterranean Europe.Th ese established aliens can successfully expand by dispersal from very disturbed areas (where they were originally introduced) to more semi-natural habitats in rural-suburban zones (Vilisics and Hornung 2009).With global warming and the large-scale disturbance of biomes in Europe, that trend could increase, especially for the species with large ecological spectra.

Ecological and economic impact of alien terrestrial Crustaceans
Alien crustaceans in Europe are not known to carry diseases or to have an impact on native species and natural habitats.Further, they have no economical impact.Based on existing literature, the occurrence of alien woodlice is strictly bound to the urban environment (e.g.greenhouses, botanical and private gardens); alien terrestrial isopods do not yet seem able to survive or to expand to more natural ecosystems.
Th e case of the alien amphipod A. dorrieni is quite diff erent.Terrestrial amphipods are known to have many eff ects on the soil and leaf litter (Friend and Richardson 1986).Arcitalitrus dorrieni has invaded deciduous and coniferous woodlands in western parts of Great Britain.In Ireland, a study showed that 24.7% of annual litter fall in a coniferous woodland was ingested by this species.It is suggested that "this introduced species plays a more important role than native macrofaunal species in nutrient turnover in this particular woodland habitat" (O'Hanlon and Bolger 1999).It is possible that other, as yet undetected, ecological impacts are likely.
Terrestrial crustaceans can represent a large percentage of biomass and abundance in the soil macrofauna (Gongalsky et al. 2005).Th us any successful invasion by a terrestrial alien crustacean could induce some disturbance if it established in relatively natural habitats.For example, in a forested area of Florida, a study on the introduced European woodlouse A. vulgare showed that this species' activity "had a strong eff ect on the chemistry of the mineral layer" (Frouz et al. 2008) and concluded that in some cases it may signifi cantly alter soil conditions".

Figure 7
Figure 7.1.1.Taxonomic overview of the Isopoda and Amphipoda species alien to and Alien in Europe.

Figure 7
Figure 7.1.2.Colonization of continental European countries and main European islands by myriapod species alien to Europe.Archipelago: 1 Azores 2 Madeira 3 Canary islands.

Table 7 .1.1.
List and main characteristics of the Crustacean species alien to Europe.Country codes abbreviations refer to ISO 3166 (see appendix I).Habitat abbreviations refer to EUNIS (see appendix II).Only selected references are given.Last update 16/10/2009.